ATMOSPHERIC PRESSURE ION SOURCES
Thomas R. Covey,*,{ Bruce A. Thomson,{ and Bradley B. Schneider{
MDS Analytical Technologies, Sciex, Concord, Ontario, Canada L4K 4V8
Received 15 October 2008; received (revised) 15 January 2009; accepted 15 January 2009
Published online 22 July 2009 in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/mas.20246
This review of atmospheric pressure ion sources discusses
major developments that have occurred since 1991. Advances
in the instrumentation and understanding of the key physical
principles are the primary focus. Developments with electrospray and atmospheric pressure chemical ionization and
variations encompassing adaptations for surface analysis,
ambient air analysis, high throughput, and modification of the
ionization mechanism are covered. An important and limiting
consequence of atmospheric pressure chemical ionization,
chemical noise, is discussed as is techniques being employed
to ameliorate the problem. Ion transfer and transport from
atmospheric pressure into deep vacuum is an area undergoing
constant improvement and refinement so is given considerable
consideration in this review. # 2009 Wiley Periodicals, Inc.,
Mass Spec Rev 28:870–897, 2009
Keywords: atmospheric pressure ion source; atmospheric
pressure chemical ionization; electrospray; ion transport; ion
guides; ion focusing; liquid chromatography; mass spectrometry
I. INTRODUCTION
This overview begins where the last one published on this topic
left off. Titled ‘‘Mass Spectrometry with Ion Sources Operating
at Atmospheric Pressure.’’ Bruins (1991) summarized the understanding and developments in this field at a point in time very
close to the first commercial introduction of atmospheric
pressure LC/MS/MS instrumentation. Figure 1 is a histomap
illustrating the expansion of the utilization of atmospheric
pressure ionization techniques soon after this occurred. The map
also provides a perspective on the rise and fall of most LC/MS ion
sources developed over this course of history, their relative
importance at any point in time, and the relationship between
the different research groups. Reference is given to some of the
historical reviews of those LC/MS developments from which
information for this histomap was derived, as well as the authors
personal experience (Thomson, 1998b; Abian, 1999; Gelpi,
2002; Willoughby, Sheehan, & Mitrovich, 2002). Since 1989 the
literature has exploded with applications in areas as diverse as
elemental speciation, pharmaceutical drug discovery and development, and protein sequencing. It is not the intent of this
overview to image this vast landscape of applications that have
come to light over the past 18 years. However, several of the
papers in this series of MS Reviews as well as some additional
————
{
Principal Scientist.
Research Scientist.
*Correspondence to: Thomas R. Covey, MDS Analytical Technologies,
Sciex, 71 Four Valley Drive, Concord, Ontario, Canada L4K 4V8.
E-mail: tom.covey@sciex.com
{
Mass Spectrometry Reviews, 2009, 28, 870– 897
# 2009 by Wiley Periodicals, Inc.
books and reviews could serve as a starting point for the
interested reader (Snyder, 1995; Cole, 1997; Lee, 2002, 2005;
Pramanik, Ganguly, & Gross, 2002; Hopfgartner & Bourgogne,
2003; Boyd, Basic, & Bethem, 2008). It is the intent to focus on
the important instrumentation developments and understanding
of the physical principles that have occurred since the Bruins
review 18 years ago.
Atmospheric pressure ionization (API) instrumentation has
evolved since 1991 in conjunction with a deeper understanding of
the physical processes involved with ionization and ion transport.
Never the less it is fair to say that the foundation principles as
summarized by Bruins have not changed substantially. Many
variations to atmospheric pressure chemical ionization and
electrospray ionization have been spawned from the original
concepts, resulting in improvements to better meet the needs of
specific applications. Improvements in ionization efficiencies
under certain conditions, as well as more effective means for
transferring and transporting gas phase ions from the atmospheric
region into the deep vacuum of the mass analyzer, have occurred.
However, radically new modes of ionization have not yet been
discovered and some of the key limitations to ion transfer and
transport remain as obstinate barriers.
The vast majority of the developments in atmospheric
pressure ion sources that have achieved wide acceptance and
commercial importance have been focused on systems where
the samples are introduced in a liquid stream with the primary
motivation being the coupling to liquid chromatographs.
Improvements to liquid introduction atmospheric pressure
chemical ionization (APCI) and electrospray ionization (ESI)
sources have dominated the developments over the past 2 decades
and will be the first topics considered before delving into issues
involving the transport of ions into the vacuum chamber and
transfer into the mass analyzer. Also reviewed are the plethora
of ion source developments that have occurred during this period,
many of which deal with surface analysis, solid sample introduction, or adaptations to achieve high throughput. Fundamentally these concepts can be viewed as variations on the original
APCI and ESI themes.
II. ELECTROSPRAY ION SOURCES
The most widely used ionization technique in mass spectrometry
today is ESI wherein ions are created by electrically charging a
flowing stream of liquid at atmospheric pressure, resulting in the
emission of molecular ions from the droplets in the subsequent
spray. Since the works attempting to adapt electrostatic spray
techniques to mass spectrometry in the early 1980s, the field has
diverged in two general directions, high and low flow rate liquid
introduction systems. The foundation principles of ion evaporation (Thomson & Iribarne, 1978, 1979) and electrospray
ATMOSPHERIC PRESSURE ION SOURCES
&
FIGURE 1. A history of the development of LC/MS instrumentation as viewed in histomap format. The
‘‘y’’ axis is year and the ‘‘x’’ axis is the relative adoption of the technique at that point in time estimated by
the extent of commercial adaptation as well as publications and activity in academic research institutions.
The concept of a histomap was borrowed from a Rand McNally publication (Sparks, 1952). The year 1989
marks the time of the first commercial API ESI mass spectrometer (Sciex API 3). Names and dates refer to
specific citations in the Reference section.
(Whitehouse et al., 1985) set the stage for the diverging paths.
The electrospray work served as the nucleus for developments
in the submicroliter per minute flow regime referred to as
nanoelectrospray (nano-ESI) (Wilm & Mann, 1994). The ion
evaporation work pointed the way forward into the milliliter
per minute regime which eventually gave way to the now
ubiquitous pneumatically assisted electrospray or ion spray
(Bruins, Covey, & Henion, 1987) which is a nomenclature
resulting from a condensation of the ion evaporation and
electrospray terms. The historical timelines of the development
and relationships between these techniques are captured in
Figure 1.
The motivation for operating in a pure electrospray mode at
low liquid flow rates is to achieve as high absolute sensitivity
as possible (Wilm & Mann, 1994). At low liquid flow rate the
efficiency of ion creation and transfer into the vacuum system of a
mass spectrometer is optimal because small droplets are easier to
evaporate than large droplets, it is easier to create and charge
small droplets from low liquid flows than from high liquid flows,
and it is easier to direct the resulting ions and solvent vapors into
the vacuum system of a mass spectrometer. The charging of the
liquid and the process of droplet generation are accomplished in
a single step with the electrospray process. This involves the
formation of a Taylor cone by high electric fields that emit
Mass Spectrometry Reviews DOI 10.1002/mas
droplets from the apex where the field is high enough to overcome
the liquid surface tension forces. The droplets initially produced
can be readily evaporated with gentle heat over short distances at
atmospheric pressure. The entire plume of ions generated from
the evaporating spray can be inhaled by the mass spectrometer
with the viscous drag forces of the vacuum system so that few ions
or clusters are lost to collisions with the walls in the source or
vacuum interface. Under carefully controlled conditions, at flows
in the tens to hundreds of nanoliters per minute range, the transfer
of ions from the solution phase to the first vacuum stage has been
observed to be greater than 50% but can vary over two orders of
magnitude depending on the liquid emitter geometry, flow rates,
solvent surface tension, and physical positioning of the sprayer.
With all parameters properly balanced, sensitivities can be very
high and this approach can serve as an interface to LC under the
narrowly defined stability conditions required at these flow rates.
The motivation for exploring the pneumatic variants of
electrospray or ion spray was born out of a desire to develop
a general purpose interface to liquid chromatography where
operation over a liquid flow range of tens to hundreds of
microliters per minute, and a tolerance to changes in mobile
phase composition, are important. Most commercial HPLC
instrumentation is geared toward this flow regime because of
the simplification of construction, fluid flow control, and user
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COVEY, THOMSON, AND SCHNEIDER
operation compared to instrumentation designed for nanoliters/
min flows. Given that, in the majority of cases sample
concentration detection limits are the most important analytical
metric and sample volumes are not limited, HPLC systems with
large sample injection capacity are desirable. Since sample
injection capacity scales with the square of the column diameter,
the absolute sensitivity losses obtained at higher flows can be
compensated for by injecting more sample, often resulting in
lower sample concentration detection limits with high flow
systems. To minimize the ion losses and stabilize operation over a
wide range of mobile phase conditions the formation of droplets
by the combination of gas shear and electrical forces was
borrowed from the ion evaporation work. The process of ion
emission from cloud droplets was the original incentive to study
ion evaporation where droplets are charged by statistical or shear
forces. Nebulizers were implemented to create an experimental
platform mimicking natural droplet creation such as in cloud
formations or in the cascade of a waterfall. The natural statistical
charging of droplets was enhanced with an induction electrode
which did not make physical contact with the liquid. During the
course of these studies the observation of labile organic ions,
some multiply charged, indicated that the technique had potential
as an analytical tool (Thomson, Iribarne, & Dziedzic, 1982).
However, the relatively low sensitivity demonstrated from this
configuration stifled interest from the general community.
Nonetheless it provided the basis for the concept of separating
the charging from the droplet generation process which is a key
element of the high flow sources. Incorporating direct electrical
charging of the liquid with the proper voltage isolation resulted
in significant sensitivity improvements over induction based
approaches. This would form the basis of the ion spray interface
where the initial pneumatically driven droplet generation step
is decoupled from the charging step and the careful balance
between voltages, flow rates, mobile phase compositions,
aperture dimensions, and physical position becomes relaxed.
What both the electrospray and ion evaporation techniques
demonstrated was that gas phase ions of very labile organic
compounds could be created from charged droplets at atmospheric pressure. The appearance of these atmospheric gas phase
ions, as indicated by the presence of multiple charges (Covey
et al., 1988; Fenn et al., 1989), reflect their solution state
chemistry at the time of emission. What they both have in
common is a requirement to create droplets that have a net charge,
and the diameter of those droplets must be rapidly reduced to the
Rayleigh limit where coulomb explosions will lead to the final
cascade of droplet diameter reduction and ultimate release of ions
and clusters. The process of going from macroscopic droplets
to fully desolvated ions is not understood in all details. The main
theories that attempt to explain the creation of single ions from
droplets that contain hundreds to thousands of analyte ions have
been well described and debated in the literature (Kebarle &
Tang, 1993; Analytica Chimica Acta, 2000, entire volume;
Gamero-Castano & Fernandez de la Mora, 2000). The main
theories (the ion evaporation theory and the charged residue
theory) end with ions in the gas phase that presumably contain a
number of attached solvent molecules. The important point is that
the high and low flow techniques share the same mechanism of
ionization, share the same means of droplet charging, but they
differ in the means by which the droplets are initially created.
872
Some of the directions that these two approaches have taken over
nearly the past 2 decades will be described. As we will see,
significant effort has been directed at evaporating as much of the
spray as possible, and then sampling as many of the ions as
possible into the vacuum chamber.
A. Nanoelectrospray: Low Flow Rates
(1–1,000 nL/min)
The starting point of nano-ESI developments can be traced to
the seminal works of Wilm and Mann (1996) in the mid 1990s,
who described the physical principles underlying electrospray
operating at nanoliters/min flows and demonstrated the practicality of the technique for protein identification after 2-D gel
electrophoresis separations (Wilm et al., 1996). This original
work used a commercially available triple-quadrupole instrument (API 3) with an unmodified API source. The interface
on this instrument was unique by today’s standards in that the
transition from atmosphere to deep analyzer vacuum was
conducted in a single stage through a 125 mm pinhole aperture.
This required pumping speeds on the order of 100,000 L/sec
which could only be achieved by compressed helium cryogenic
pumps that froze the incoming gases on cryopanels surrounding
the ion optics, and thus required periodic thawing and recycling.
Their work on this system indicated that there were two critical
parameters to control which are as important today on modern
multistage turbomolecular pumped instruments as they were
back then. The first is regarding sensitivity and the full utilization
of vacuum drag forces to inhale most if not all of the ions
produced at atmospheric pressure. This aspect will be covered
in Desolvation and Declustering Section. The second critical
parameter they identified was that the nano-ESI emitter geometry
is critical for achieving stable ion currents at low liquid flow rates.
An explosion of research in academic and industrial institutions
occurred around emitter geometries, materials, and fabrication
techniques. Today this field has largely matured such that nanoESI emitter design and fabrication has advanced into the realm
of commercial mass production resulting in reproducible and
high performance sprayers. Short of an exhaustive review of all
emitter geometries reported, the key principles that provided the
foundations for current popularized designs will be discussed.
1. Relationship Between Optimal Flow Rate
and Emitter Geometry
The essence of the nanoflow methodology is to reduce the flow
rate of the sprayed sample to the sub microliter per minute regime
so that the entire vaporized spray can be inhaled into the mass
spectrometer. The efficiency of the ionization and ion transfer
process improves approximately proportional to the flow rate
reduction. The flow at which maximum efficiency is reached
depends on the characteristics of the atmosphere to vacuum
interface and the sprayer geometry. The former will be discussed
in the ion transport section and the latter in this section. Factors
affecting the formation and stabilization of the Taylor Cone are of
paramount concern and are greatly influenced by the relationship
between the electrospray tip geometry and liquid flow rate as well
as other factors such as solvent viscosity and conductivity.
Mass Spectrometry Reviews DOI 10.1002/mas
ATMOSPHERIC PRESSURE ION SOURCES
The theoretical derivations by Wilm and Mann for nanoESI, as well as the studies published in the general physics
literature on Taylor Cone properties (Ganan-Calvo, Davila, &
Barrero, 1997; Ku et al., 2001), showed a relationship between
the liquid flow rate and the diameter of the droplets emitting from
a Taylor Cone. At low nanoliters/min flows, these droplets were
calculated to be in the low to sub micron range, small enough to
result in ion emission with very little time or distance required for
further desolvation. This insight into the effects of low liquid flow
rates led them to construct electrospray needles with 1–3 mm exit
apertures that delivered flows on the order of 10–50 nL/min, and
could thus be positioned in close proximity to the entrance
aperture of the mass spectrometer maximizing the collection of
ions, and posing no danger of overwhelming the vacuum system
with liquid vapors. However, the theory provided little guidance
as to what these electrospray needles should look like. Inspired
intuition directed this and other groups working on the same
problem, to experiment with glass and fused silica capillaries
drawn to different dimensions and methods for delivering the
electrical charge. Fortunately a large body of literature already
existed describing techniques for making similar electrodes
for use in measuring electrical potentials across cell membranes.
For an in-depth treatise on the science and history behind
microelectrode technology see Brown and Flaming (1992).
A large number of publications began to emerge rapidly,
optimizing tip configurations for specific applications such as
infusion, capillary electrophoresis/MS (CE/MS), and capillary
HPLC/MS. A common theme among them was the investigation
of the various aspects of the physical architecture of these
nanoflow electrodes and the effect that it has on their operational
behavior, of primary interest being the lowest flow rate a
particular size and shape electrode could support without
compromising stability of the ion beam. The picture has
developed into a complex interrelationship between tip inner
diameter, outer diameter, channel taper to the tip, and solvent
composition, all exerting some influence on the lowest attainable
flow with a particular nanoflow electrode. The various researchers in this area derived their preferred combination of these
parameters to achieve the operational results they desired for
their application. Some guidelines will be put forth below, based
on these authors’ experience and the collective considerations of
the literature studies regarding one crucially important feature of
electrode geometry as it relates to flow rate, the diameter of the
emitter at the exit. A comprehensive list of the many emitter
geometries and configurations adapted for various applications
and flow rates is provided here (Emmett & Caprioli, 1994; Wahl,
Gale, & Smith, 1994; Valaskovic & McLafferty, 1995, 1996a,b;
Kriger, Cook, & Ramsey, 1995; Davis, Stahl, & Lee, 1995;
Figeys et al., 1996; Valaskovic, Kelleher, & McLafferty, 1996;
Bateman, White, & Thibault, 1997; Cao & Moini, 1997; Davis &
Lee, 1997; Kelly, Ramaley, & Thibault, 1997; Vanhoutte et al.,
1997; Gatlin et al., 1998; Geromanos et al., 1998; Hannis &
Muddimann, 1998; Vanhoutte, Van Dongen, & Esmans, 1998;
Wang & Hackett, 1998; Fong & Chan, 1999; Juraschek, Dulcks,
& Karas,1999; Barnidge, Nilsson, & Markides, 1999; Barnidge
et al., 1999; Barroso & de Jong, 1999; Hsich et al., 1999; Feng
& Smith, 2000; Gatlin et al., 2000; Geromanos, Freckleton, &
Tempst, 2000; Covey & Pinto, 2002; Schmidt, Karas, & Dulicks,
2003).
Mass Spectrometry Reviews DOI 10.1002/mas
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It has been generally observed that, as both the inner
diameter (i.d.) and outer diameter (o.d.) of the tip at the exit get
smaller, stable sprays can be maintained at lower flows. Amongst
the lowest flows published were in the 0.5–5 nL/min range using
0.8 mm i.d. apertures (Geromanos et al., 1998). Figure 2 shows
some results from a flow rate versus aperture i.d. experiment
that depicts what is generally observed. A strong relationship
between tip inner diameter and lowest sustainable flow is seen
(Covey & Pinto, 2002). The inflection point in the graphs is
referred to as the ‘‘optimum flow.’’ This optimum flow is the
highest flow where ionization efficiency remains at its peak
for that electrode. At flows below this the ion current begins
to decrease with flow exhibiting a mass-flux sensitive response
(i.e., the ion abundance varies with the rate at which sample
molecules are introduced into the ion source). Here the ionization
efficiency remains optimal but the mass flux into the sprayer
decreases resulting in a proportionally decreasing signal. This
linear response range occurs over a very narrow range of flows
and is soon disrupted when the spray becomes unstable at flows
that are too low. At flows above the optimal flow the ionization
efficiency decreases but the signal remains relatively constant as
the mass flux into the sprayer increases. In this range the response
appears to be similar to a concentration sensitive detector, such
as a UV detector, but this is not a true concentration sensitive
response. It is an anomaly resulting from the changing ionization
efficiency over this part of the flow range and is better described
as a pseudo concentration sensitive response.
Several possibilities can be proposed to explain this
observed relationship between aperture size and optimum flow
rate. Central to this understanding is a consideration of all factors
affecting the size, shape, and stability of the Taylor cone, which
has been studied in great detail (De la Mora & Loscertales, 1994;
Cloupeau & Prunet-Foch, 1990). An explanation of this specific
case, which reflects general observations from the field of nanoESI, is that optimal flow is the minimum flow where the base
of the Taylor cone just bridges the emitter i.d. and maintains
stability firmly anchored to the walls of the electrode. At flows
much below this the cone becomes unstable as it begins to
collapse inside the lumen of the emitter. This is the mass sensitive
region which occurs over a very small flow range before
instability takes over. At flows above the optimum the Taylor
cone can remain stable over a considerable flow range as the base
of the cone spreads over the outside and along the length of the
electrode. With higher flows the growing Taylor cone emits larger
droplets that decrease the ionization efficiency. This is the pseudo
concentration sensitive response region. With increasing flow in
the overflow state pneumatic nebulization is eventually required
to sustain stable droplet formation. All high flow rate ESI
sources operate in this regime. In this regime heat transfer and
evaporation rates become increasingly important as will be
discussed in more detail in the following section.
Figure 2 is a reasonable portrayal of the general trends
to expect with this relationship between i.d., flow rate, and
ionization efficiency. However, it should be kept in mind that
there are other electrode architectural features involved in
establishing the optimum flow rate. Among these is the outer
diameter of the tip at the exit of the electrode, the taper of the
channel leading up to the exit aperture, and the method for
delivering the voltage. Superimposed on electrode geometry are
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COVEY, THOMSON, AND SCHNEIDER
FIGURE 2. Relationship between flow rate and emitter diameter. The ‘‘y’’ axis is intensity of the peptide
ion signal (monitored by selected ion monitoring) expressed in relative terms and normalized for each
flow rate. Emitters were made from a 1 cm length of 150 mm o.d. fused silica having six different i.d.’s.
Samples were infused with a syringe pump to control the flow and the voltages were varied to maintain
maximum sensitivity and stability. The solvent composition and sample concentration remained the same
throughout. The arrows point to the ‘‘optimal flow’’ for each electrode. Reprinted with permission from
Covey and Pinto (2002), copyright 2002, Marcel Dekker and Thomson (2007b), copyright 2007, Elsevier.
solvent viscosity and conductivity, all of which will have an
affect on determining the precise optimal flow for a particular
emitter. Nevertheless the data in Figure 2 are a reasonable first
approximation for most electrodes used today.
2. Electrospray Emitters: Three Basic Configurations
The construction of electrospray emitters is guided by the desired
application of the device. They fall into three general categories;
those designed for bulk flow sample introduction, those designed
for coupling to continuous flow sources such as liquid chromatographs, and those constructed using micromachining principles
with applications for both bulk and continuous flow sample
introduction. Electrodes in all three categories are commercially
available. Bulk and continuous flow emitters are available from
companies such as New Objective, Woburn, MA; Proxeon,
Odense, Denmark; and Phoenix S&T, Chester, PA. Various
configurations of micromachined devices are available from
Advion Biosciences, Ithaca, NY; Agilent Technologies, Santa
Clara, CA; and Waters Corporation, Milford, MA.
a. Bulk flow electrodes. Bulk flow or discrete sample introduction electrodes (i.e., those not involving chromatographic
separations) are typically drawn from a borosilicate glass tube
of approximately 1 mm o.d 0.8 mm i.d. with apertures ranging
from 1 to 5 mm. Figure 3 is a series of photos taken with various
magnification of such an electrode made by New Objective with
dimensional control of the aperture by an HF etching procedure.
874
In this case electrical contact is made through vapor deposited
metal layers stabilized with overcoatings of SiOx. Another means
of securing the vapor deposited metallic coating uses a bifunctional organosilane undercoating (3-mercaptopropyl) (Kriger,
Cook, & Ramsey, 1995). Yet another approach for establishing a
permanent voltage contact is called the ‘‘fairy dust’’ technique
(Barnidge, Nilsson, & Markides, 1999) created by gluing gold
particles to the electrode. For applications where the emitters are
FIGURE 3. Photograph of a bulk flow emitter at four different
magnifications. Reprinted with permission from Covey and Pinto
(2002), copyright 2002, Marcel Dekker, and New Objective, Inc.
Mass Spectrometry Reviews DOI 10.1002/mas
ATMOSPHERIC PRESSURE ION SOURCES
disposable the fragile metal layer is sometimes not secured. A
non-metalized electrode has also been reported using a tungsten
wire inserted into the back of the capillary (Van Berkel, Asano,
& Schnier, 2001), and this was primarily used to study the
electrochemical properties of ESI (Van Berkel and Kertesz,
2007b). These studies showed that unwanted analyte oxidation
and reduction reactions can occur if there is an excessive
residence time in the vicinity of the high voltage electrode
surface.
b. Continuous flow electrodes. The use of fused silica tubing
available with a wide variety of precise inner bores has become
popular for coupling to, or integrating with, capillary HPLC or
CE columns. Exit i.d.’s are tailored to a desired flow rate range by
drawing the fused silica with a laser-heated pulling apparatus.
Electrical contact is made with similar metalizing techniques
as used for the bulk flow electrodes or through the use of a
conductive union. Figure 4A is a picture of an integrated single
piece column/emitter system. Alternatively, specialized zero
dead volume unions where voltage can be applied at the point of
column/emitter junction have been developed commercially.
One example of this is shown in Figure 4B. Continuous flow
emitters have also been implemented with pneumatic nebulization to extend the flow range capability to the low microliters/min
regime (Schneider et al., 2005).
c. Micromachined electrodes. Microfluidic systems, generally
defined as those where channels and other structures are etched in
bulk substrates have demonstrated utility for nano-ESI applications. Arrays of emitter nozzles have been created in silicon
FIGURE 4. Continuous flow emitters for nano-LC coupling. A: Pulled
fused silica emitter with integrated LC stationary phase and frit. B: Zero
dead volume fitting to couple a 75 mm i.d. capillary LC column and
electrospray emitter. Voltage can be applied at the junction with an
embedded wire (wire not shown). Reprinted with permission from New
Objective, Inc. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
Mass Spectrometry Reviews DOI 10.1002/mas
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substrates with reactive plasma etching techniques to enable
automation of the emitter tip replacement process. Figure 5A is a
photo of such a device made by Advion using reactive plasma
etching in a silicon substrate. This technique is used to produce
arrays of nozzles in chip format to enable rapid tip replacement
(Schultz et al., 2000). These emitters have been used for both bulk
flow sample introduction or for coupling to chromatographic
systems. Chip based integrated column/sprayer devices have also
been developed commercially by Agilent and Waters.
B. Pneumatic Electrospray: High Flow Rates
(1–1,000 m L/min)
Since the 1991 Bruins review on atmospheric pressure sources
the ion spray interface has grown widely in popularity and
dominates the application field of LC/MS. Pneumatic nebulization of a charged liquid stream will generate a stable ion current
over a broad flow rate range and provide a response dynamic
range of 103 –104 (Kostiainen & Bruins, 1994) while remaining
relatively insensitive to changes in voltage, mobile phase
composition, sprayer position, and electrode dimensions. Other
mechanical means for droplet creation, such as ultrasonic
nebulization, have also been demonstrated (Banks, Quinn, &
Whitehouse, 1994) but the pneumatic approach remains the most
widely used because of its simplicity and performance. It has also
found use as a supercritical fluid chromatography interface
(Baker & Pinkston, 1998).
1. Nebulizers
For some insight into the mechanism of initial droplet production
with a pneumatic nebulizer for electrospray devices, a first
approximation of the physical force available to disperse the
liquid is in order. The majority of the nebulizers used today for
this purpose achieve gas velocities from 100 to 300 m/sec (Mach
1) over distances of less than 3 mm from the exit of the gas nozzle.
It is worth noting that to accelerate from zero to 125 m/sec in
this distance implies an average acceleration of 340,000 g. The
corresponding turbulent aerodynamic force at the liquid gas
interface in the first few millimeters of distance is more than
sufficient to shatter any liquid at these flows into populations of
charged drops ranging in dimensions from submicron to tens of
microns in diameter. A photograph of this highly energetic region
in the first few millimeters from the exit of a pneumatic nebulizer
is shown in Figure 6. After the mechanical disruption, columbic
forces take over the droplet size reduction process for the smaller
droplets and rapid heating is required to assist this process for
the larger ones. Further details of this nebulization process and
its operating characteristics have been described elsewhere
(Covey, 2007).
This decoupling of the charging and initial droplet
generation process with common pneumatic nebulizers leads to
a usable voltage range that is quite broad. It can be so broad
that at high liquid flow rates (1 mL/min) and maximum gas
flows essentially no voltage needs to be applied, a condition
reminiscent of the ion evaporation device. The high gas shear
velocities create a situation where statistical charging of the
liquid provides sufficient charge. A low voltage, on the order
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COVEY, THOMSON, AND SCHNEIDER
FIGURE 5. Micromachined electrospray nozzles. A: Electron micrograph of a nozzle in a silicon substrate
with a 10 mm inner diameter aperture. Reprinted with permission from Advion and reference Thomson
(2007b), copyright 2007, Elsevier. B, C, D: Successive magnifications nozzles. The inner diameter of the
nozzle in D is 10 microns. [Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
of 50–100 V instead of several kV, is useful to establish an ion
drift potential toward the inlet aperture but does little to effect
charging. This operational regime has been observed for
several years on systems with heated pneumatically nebulized
electrosprays and employed for some drug assays (Bi et al., 2000)
but is often overlooked because it provides no particular
sensitivity or operational advantage over the use of high voltage
for these types of applications. Recently it has been observed that
operating a nebulizer in this fashion leads to reduction in protein
charge states and will produce atmospheric gas phase ions of
both polarities simultaneously which has interesting utility in
some areas (Hirabayashi et al., 1994; Takats et al., 2004). Similar
phenomena appear to be operating with other low voltage, high
shear systems (Cristoni et al., 2005).
2. Jet Dynamics and Heat Transfer
While the early focus on these systems was on the importance of
high velocity gases, later attention during the mid 1990s began
to be focused on understanding the processes involved in the
slowing down of these jets and how to take advantage of
these phenomena. Laser Doppler anemometry and analytical
modeling were the primary investigative tools. The peak
FIGURE 6. Schlieren photograph of a jet of air issuing from a nebulizer
nozzle at sonic velocity, showing the Mach structure extending a few mm
from the nozzle. Reproduced with permission from Covey (2007),
copyright 2007, Elsevier, and from Issac (1994), copyright 1994, with
permission of the American Institute of Aeoronautics and Astronautics.
876
velocities mentioned above are achieved within the first
few millimeters of the expansion nozzle. After this the jet
continuously decelerates to speeds of approximately 10–30 m/
sec 2–3 cm from the nozzle. The velocity profile across the jet is
Gaussian, with higher speeds and larger droplet diameters near
the center line and a large population of small micron and
submicron sized droplets in the periphery (Covey, 2007).
The region close to the jet boundaries is of particular
importance. This is where droplet size is smallest and density of
these small droplets is the highest. It is also at this point that the
momentum transfer from the surrounding air to the jet occurs, the
overall system conserving momentum. As the jet slows down
there is an increase in the mass of external air being drawn in or
entrained at a lower velocity. The trajectory of the gas flow
streamlines are approximated in Figure 7 from the results of the
analytical models. This phenomenon is commonly encountered
in a shower stall with a flexible plastic curtain. When the shower
spray is turned on, the curtain is annoyingly drawn inward toward
the bather. Analytical models of the gas dynamics of these jets,
verified by measurements of the velocities of the gas at the jet
boundary, indicate that the entrainment ratios are greater than
20:1. For every liter per minute of nebulizer gas consumption,
>20 L/min of external air is drawn into the body of the jet. This
presents an excellent opportunity to transfer a large amount of
heat to the droplets for efficient and rapid desolvation and flow
rate extension. With 10–20 L/min of 8008C entrainment air
injected into the primary jet, 1 mL/min of water will completely
desolvate within a few cm. Injection of clean air will
also minimize the inclusion of background contaminants,
eliminate recirculation effects that occur in closed chambers,
and reduce the radial expansion of the plume.
At first consideration temperatures of 8008C seem alarming,
with the assumption that thermal degradation will occur.
However this is not observed to be excessive for good reason.
Analytical models have been developed that predict that the
temperature of the liquid in the droplet under these conditions
does not rise much above ambient conditions (French, Etkin, &
Jong, 1994). Evaporative cooling balances the rate of heat
transfer into the drop. In a typical calculation a droplet in 7508C
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ATMOSPHERIC PRESSURE ION SOURCES
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FIGURE 7. Entrainment gas streamlines with a fully developed, unconfined nebulizer jet. Reprinted with
permission from Covey (2007), copyright 2007, Elsevier.
air will reach a steady state temperature of 448C. Upon reaching
emission diameters, typically considered to be around 10 nm or
less, the ion will leave the droplet as a cluster or with a small
solvation sphere. It will reside in the hot gas for a short period
of time before being drawn into the vacuum system, typically
a few tens to hundreds of microseconds. Thermally labile
compounds, such as nucleotide phosphates, generate predominantly molecular ions under these circumstances. In a heated
nebulizer APCI source, where vaporization occurs as a result of
heat transfer during droplet impact with a hot surface at similar
temperatures, these same compounds are completely thermally
degraded highlighting the radically different mechanisms
involved in the desolvation and analyte liberation processes with
these two different high temperature techniques.
3. Sampling Efficiencies
One means of quantifying the sensitivity of an ion source is to
measure the sampling efficiency (El-Faramawy, Siu, & Thomson, 2005; Schneider, Javaheri, & Covey, 2006). The sampling
efficiency is the fraction of analyte molecules introduced in
solution into the ion source that are captured as ions by the initial
ion optics in the deep vacuum of the mass spectrometer. It is
the product of the ionization efficiency (efficiency of ion creation
at atmospheric pressure) and transfer efficiency (efficiency of
transferring ions from atmosphere into vacuum). Figure 8
pictorially depicts this concept for the high and low flow
electrospray systems described.
When this measurement of sampling efficiency was applied
across the flow range on a high sensitivity commercially available
triple quadrupole mass spectrometer, the results in Figure 9
were obtained (Covey et al., 2009). The difference in sampling
Mass Spectrometry Reviews DOI 10.1002/mas
efficiency between highest and lowest flows is approximately
100-fold. Referring back to the earlier comments on absolute
versus sample concentration detection limits, in situations where
the absolute amount of material to be analyzed is limited, such
as sequencing peptides extracted from 2-D gels, nano-ESI has a
large advantage. In cases of trace analysis where the sample
volume or mass is not limited, such as drug quantitation in blood
plasma, the advantage lies with the high flow system where the
injection capacity of the HPLC system compensates for the loss
in efficiency and more routine operation is afforded.
Another important observation to be made from these
data is that sensitivity improvements with nano-ESI systems
are approaching their theoretical limits. As mentioned in the
Transport of Ions From Atmosphere to Vacuum Section, with
slight modifications to commercial instrumentation efficiencies
as high as 80% have been achieved and reproduced. Significant
gains remain to be harnessed with high flow systems, however the
physics of this region of the mass spectrometer is highly complex,
dominated by multiple high velocity gas jets, space charge, and a
medium (atmospheric pressure) where electrostatic focusing is
difficult (see Focusing Ions at Atmospheric Pressure Section).
The underlying predictions of the behavior of multiple intersecting fluids and gases in the turbulent regime lie in the solutions to
the Navier–Stokes equations for fluid flow, which is one of the
seven Millennium Problems in mathematics resisting a general
solution over the years despite its massive practical importance
to many fields (Girvan, 2003). Computational fluid dynamics
working in close cohort with empirical experimentation may
bring important advances. Experiments with an air amplifier on
focusing low flow rate electrospray plumes (Zhou et al., 2003)
have shown some initial indications that further improvements
in focusing droplets at atmospheric pressure with gas flow may
be possible.
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COVEY, THOMSON, AND SCHNEIDER
FIGURE 8. Sampling efficiency comparison. The sampling efficiency is the ratio of the number of analyte
molecules present in solution in the sprayer to the number of ions that enter the RF multipole ion guide in
the vacuum. It is the product of the ionization and transfer efficiency. A: Corona discharge needle with APCI.
B: Low flow ESI system.
FIGURE 9. Sampling efficiency versus flow rate. The data in the upper dashed trace (black), read from the
right Y axis, is an expansion of the data in the lower solid trace (red) read from the left Y axis. For each flow
the optimum source and interface configuration was used for those conditions. For high flow rates above
1 mL/min a heated TurboIonSpray1 source with a conical pinhole aperture interface (standard interface) on
an API 5000TM triple quadrupole instrument. For points below 1 mL/min a nanospray source with a 15 mm
aperture fused silica capillary and an interface optimized for nanoflow introduction (PDI interface, See Ion
Transport Section, Declustering, PDI interface) was used.
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III. ATMOSPHERIC PRESSURE CHEMICAL
IONIZATION SOURCE
APCI mass spectrometry was commercially developed many
years before electrospray ionization (ESI), but remained a niche
technique until the explosion of interest in LC/MS for biological
analysis. APCI/MS was used primarily for air analysis because of
the high sensitivity afforded toward polar volatile and semivolatile species (French et al., 1984). The analysis of airborne
substances and thermally desorbed surfaces and solids with
hot gases was the primary application of atmospheric pressure
chemical ionization sources in the 1980s. A common demonstration of this method of sample introduction was the direct
detection of volatiles emanating from solid substances as
shown in Figure 10. A carrier gas sweeps over the sample
bringing ambient volatiles into the source. The instrument is a
triple-quadrupole called the trace atmospheric gas analyzer
(TAGA), some of which were installed in vans as mobile
laboratories for real-time monitoring and tracking of industrial
pollutants (see Lane & Thomson, 1981 for a report on an
emergency-response application using an earlier single-quadrupole version). This technology also was the foundation for
the AROMIC system, a real-time cargo examination system
developed in the mid-1980s that used APCI with MS/MS for
contraband detection. While these systems had some commercial
success, there was no single large application for APCI to carry it
into the mainstream of analytical mass spectrometry which was
largely GC/MS based By the time of the Bruins review in 1991,
the interest in atmospheric pressure ionization had shifted largely
toward liquid introduction systems, driven by electrospray rather
than APCI as the ionization method. Since that time, however,
interest in APCI and direct analysis has re-emerged with a focus
on the rapid analysis of substances thermally desorbed from solid
surfaces. This will be discussed further in the section of this
article describing variations on the APCI theme.
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A. Ionization
The foundation principles of APCI can be found in the early
literature on low pressure chemical ionization and are well
understood (Harrison, 1992; Lias & Bartness, 2008). APCI is a
gas phase ionization process as opposed to the liquid phase
ionization process of ESI. Ionization of the volatilized neutral
analyte will only occur if it has sufficient gas phase basicity or
acidity to extract or donate a proton from the reagent ion
population present in great excess (French et al., 1984; Bruins,
1991). In the case of LC/MS the solvents are the dominant
reagent ions but they can be carefully selected to enhance either
positive or negative ionization (Schaefer & Dixon, 1996) Adduct
ion formation is also a possible channel for ionization with ion
molecule reactions. Other processes such as charge transfer and
electron capture can be forced to occur given the correct analyte
and reagent ion population conditions to yield very high
sensitivities (Singh et al., 2000). One glaring difference between
liquid phase and gas phase ionization is the absence of multiply
charged ions. It is both thermodynamically and kinetically
unfavorable for two or more reagent ions to react with the same
neutral molecule within a short period of time. Both charge
repulsion and collision probabilities prevent this from occurring.
One possible exception would be chemical ionization of massive
molecules such as proteins where this becomes a possibility. The
occasional observation of minor doubly or triply charged protein
ions in a MALDI spectrum may be the result of a multiple-event
chemical ionization reaction.
Ionization by APCI is an indirect process where an initial
source of ions is used to create ions from the background gas. At
atmospheric pressure the mean free path between collisions is
short so a large number of gas phase reactions occur very quickly
and come to an equilibrium state where the most stable species
predominate and serve as the reagent ions. The high density of
reagent ions results in very high ionization efficiencies, nearly
FIGURE 10. APCI ambient air monitoring interface. A carrier gas sweeps over the sample in the glass inlet
transporting the volatiles into the APCI source of a TAGA 6000 triple quad instrument. This photograph
was taken in 1984. [Color figure can be viewed in the online issue, which is available at www.
interscience.wiley.com.]
Mass Spectrometry Reviews DOI 10.1002/mas
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COVEY, THOMSON, AND SCHNEIDER
all analyte molecules are ionized if the reaction chemistry is
favorable. These final reagent ions are typically protonated
(positive ions) or deprotonated (negative ions) resulting in
(M þ H)þ or (M H) analyte molecular ions or the formation of
adducts. If great care is taken to exclude all proton donating or
abstracting species (ubiquitous water being the main source of
these), electron donating or abstracting reagent ions can be used
resulting in radical cations or anions, similar to those that would
be observed from a direct ionization source such as a low pressure
electron ionization source. On occasion analyte ions will be
directly ionized by the primary source of ions, such as the corona
discharge itself, but at atmospheric pressure those ions are lost in
subsequent reactions so the final products are dominated by the
rules of gas phase ion chemistry. It is very difficult to achieve and
maintain direct ionization at atmospheric pressure and at the
same time have high efficiency. The most widely used source for
the initial ionization event is the direct current corona discharge
needle because of its simplicity, robustness, and high efficiency.
Many other ionizing devices have been used for different reasons,
such as the 63Ni beta emitter commonly used today for field
portable instruments (Horning et al., 1974), photons from
vacuum UV lamps which operate without high electric fields
(Robb, Covey, & Bruins, 2000; Syage, Hanning-Lee, & Hanold,
2000), RF plasma’s (Ratcliffe et al., 2007), crystal discharges
for miniaturization (Neidholdt & Beauchamp, 2007), microwave
plasmas (Shen & Satzger, 1991; Moini et al., 1998), and lasers
(Constapel et al., 2005).
The most important sample inlet device for APCI is the
heated nebulizer which vaporizes the LC solvent and creates a
reagent ion population from this gas with a high voltage corona
discharge (Thomson, 2007a). The nebulizer is designed to
cause pneumatically generated droplets to collide with hot
surfaces to transition the solvent and analyte molecules to neutral
gas phase species. The basic principles have not changed
radically over the last 20 years, but improvements have been
made to enhance performance. These improvements have been
primarily centered on improving droplet trajectories, impact
dynamics, and desolvation rates.
B. Desolvation: Nucleate Boiling
The primary problem is one of achieving complete evaporation of
the liquid and vaporizing the analyte in as short a time frame as
possible, <one millisecond considering spray is emitting from a
near-sonic nozzle and traversing only a few centimeters. To
achieve this, nebulized droplets are driven to impact hot surfaces.
The droplets are evaporated rapidly by conductive heat transfer
whereby the droplet spreads on a heated surface, and the analyte
is flash vaporized by a process referred to as nucleate boiling
(Bernardin, Stebbins, & Mudawari, 1997). This is an efficient
means of heat transfer but can lead to thermal degradation as a
result of a temporarily dried analyte residing on a hot surface.
It can also lead to sample memory effects as a result of high
boiling point materials slowly desorbing. The heated nebulizers
produced during the 1990’s were of this vintage.
C. Nebulization and Desolvation: Vapor Film Boiling
More advanced designs have evolved that take advantage of
nebulizer jet properties to drive the droplets toward the heated
walls of the desolvation chamber. Figure 7 shows the fully
developed unconfined nebulizer jet of a heated pneumatic
electrospray nebulizer where as much entrainment gas is
supplied as the nebulizer demands, allowing an unperturbed
forward velocity spray to result. With optimized APCI interfaces
the nebulizers are choked by enclosing them in a small finite
space, so the only source of gas to supply the entrainment
originates from the gas within the space and from the expanding
gas jet. The average streamlines then take the form of a closed
loop or recirculation eddy. In this region highly turbulent flow
FIGURE 11. Drawing of a heated nebulizer desolvation chamber showing the vapor film boiling process
and the zoned heaters. Reprinted with permission from Thomson (2007a), copyright 2007, Elsevier. [Color
figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
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ATMOSPHERIC PRESSURE ION SOURCES
predominates and the droplets are driven to the surface of the
chamber in a short region downstream of the tip as diagrammatically portrayed in Figure 11.
Improvements to the vaporization step have also been
implemented by utilizing a vapor film boiling process, instead
of a nucleate boiling process, which takes advantage of the
Leidenfrost effect (Chandra & Avedisian, 1991; Hatta et al.,
1995). Heated nebulizers have evolved to maximize this
effect, minimizing direct sample contact with the hot surface.
As the droplet approaches a surface of sufficiently high temperature and surface polish a vapor film will grow between them. The
vapor pressure will eventually exceed the droplet momentum
toward the surface and rebound the droplet before contact, in the
process losing approximately 50% of its volume. Depending on
the surface roughness, the droplet impact energy, and the impact
angle, the droplet will rebound and break up into smaller droplets.
Below this temperature the droplet and sample will stick and
spread, undergoing nucleate boiling. The design concept of
heated nebulizers taking advantage of this effect is also shown in
Figure 11. Photographic images of droplets undergoing nucleate
and vapor film boiling are shown in Figure 12A,B.
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After several repetitive rebounds the initial droplet volume
will reduce by several orders of magnitude, resulting in a dry
vapor. By maximizing this effect vaporization can occur without
sample directly depositing on the surface. Polished ceramic tubes
with embedded zoned heaters have been widely employed to
serve this purpose. The flexibility to zone or gradient the heat
with such heaters allows for temperatures approaching 8008C to
be located in the region of initial droplet impact and cooler
temperatures during the final stages of vapor drying where the
fine droplets have lost their momentum, and are no longer
impacting the heater surfaces. Excessive convective heat transfer
during this final stage of drying will induce thermal degradation.
The Leidenfrost effect can be observed in the kitchen when
water droplets are sprinkled on a hot pan (dancing drops). In the
case of a near-sonic nebulized spray the Weber numbers
(measure of droplet impact energy) are much higher than for
droplets impinging by gravitational acceleration alone. Thus
heated nebulizer surface temperatures need to be much higher.
Despite these efforts, thermal degradation occurs to a
significantly greater extent with these interfaces than with
thermally assisted electrospray. This is because the neutral
FIGURE 12. Time course photographs of nucleate and vapor film boiling starting with initial droplet
contact with a surface at time 0.2 msec with final frame at 8 msec. Reprinted with permission from Chandra
and Avedisian (1991), copyright 1991, Royal Society.
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COVEY, THOMSON, AND SCHNEIDER
molecules are driven from the liquid phase to the gas phase
entirely by thermal processes. With electrospray the final phase
transfer is field induced which is a more mild process. The
energetics of the chemical ionization event may also contribute.
However this approach using very rapid heating of samples
dissolved in droplets produces much less thermal degradation
then techniques that thermally desorb solids or samples dried on
surfaces.
D. APCI and Liquid Flow Rates
One of the main attributes of APCI interfaces is the excellent
stability of the ion beam that results. This makes it popular for
designing targeted quantitative assays for chemical species that
are known not to suffer thermal degradation effects. Heated
nebulizers readily accommodate conventional LC flow rates
in the milliliters per minute range, which enable large injection
volumes to be used resulting in low sample concentration
detection limits. Matrix suppression of ionization also tends to
be more easily controlled than for electrospray based assays
(King et al., 2000). For these reasons APCI has maintained a very
high adoption rate over the years, as depicted in Figure 1 with
the first demonstration of high speed quantitative drug analyses
to appear in 1986 (Covey, Lee, & Henion, 1986). It is not
uncommon, in laboratories dedicated to quantifying drugs in
biological samples, to conduct 20–30% of their analyses by
APCI with the remainder by high flow ESI.
Sampling efficiencies in the milliliters per minute flow range
are very similar to those obtained for the high flow ESI interfaces
at the same flows as the data shown in Figure 9. The reason for
the ion losses at these high flow rates is similar to the high flow
rate electrospray situation. Ions are created with very high
efficiency but are distributed throughout the API source and not
subject to the efficient focusing provided by the vacuum drag into
the mass spectrometer. APCI has not been adapted to nano flows
largely because there has been no impetus from a methodology
point of view for reasons cited above. However it is possible that
low volumes of liquid and samples can be ionized by APCI
directly in front of the vacuum aperture, and thus achieve similar
sampling efficiencies as nano-ESI, by taking advantage of the
vacuum drag at that point. Corona discharge sources are not ideal
for this because the space charge that they induce will repel ions
out of this critical vacuum drag region. However other field-free
means of ionizing the reagent gas can be used such as 63Ni beta
emitters or photons. Some steps have been made to construct
micro-heated nebulizers to accommodate the nanoflow regime
where all of the ions and solvent vapors could be inhaled by the
MS (Ostman et al., 2004).
IV. OTHER ATMOSPHERIC PRESSURE
ION SOURCES: VARIATIONS ON THE
ORIGINAL THEME
A myriad of other atmospheric pressure ionization sources have
developed over the past decade. Some of them are based on liquid
sample introduction systems and multiplexed versions of these,
others have moved back to the earlier concepts of solid sample
introduction reminiscent of API in the 1980s. So far they all share
882
in common the final method of sample ionization, either ESI
or APCI, or some slight variation on these themes. A variety of
different methods for transferring the sample from the solid to the
gas phase have emerged often for the explicit purpose of surface
analysis or chemical imaging (Van Berkel, Pasilis, & Ovchinnikova, 2008). Below we will briefly mention some of the more
popular techniques that have emerged to date, many of which
have not yet reached their full potential.
A. Variants of ESI
1. Electrochemical Cells
Avery large body of work studying the electrochemical processes
involved in the electrospray process has been conducted by Van
Berkel (2007a) and Van Berkel and Kertesz (2007b). During
these studies it was observed that one can take advantage of these
processes to drive oxidation and reduction reactions to extend
the ionization capabilities of ESI, as well as to conveniently
create by-product species that mimic biological metabolism.
Specialized porous graphite flow-through electrochemical cells
were integrated into the ESI emitters to maximize this additional
ionization capability (Van Berkel & Kertesz, 2005).
2. Sonic Spray
The origins of sonic spray hearken back to the original research in
ion evaporation, the creation of charged droplets by frictional
forces rather than electrochemical means driven by an external
power supply. It occurs when a solution in a capillary is sprayed
with a high velocity gas flowing co-axially to the capillary, and
approaches optimal conditions when the gas flow approaches the
speed of sound (Hirabayashi et al., 1994). A similar technique is
Electrosonic Spray Ionization, however in this case a bias voltage
is used similar to the ion spray technique as described earlier
(Bi et al., 2000). Protein ions tend to have fewer charges than
those produced by nano-ESI, suggesting that protein folding is
conserved during ionization (Takats et al., 2004). Another variant
of this is Cold Spray ionization which sprays into a liquid
nitrogen cooled chamber to try to preserve non-covalent
complexes (Yamaguchi, 2003).
3. DESI
Desorption Electrospray Ionization (Cooks et al., 2006) is a
surface ionization technique that operates by impacting highvelocity charged droplets with the surface to be analyzed,
extracting the surface chemicals by the liquid that makes
momentary contact (Costa & Cooks, 2007). The sensitivity is
primarily dictated by the extraction efficiency that occurs during
the impact event. The nebulizers that are used are similar to the
pneumatically assisted electrospray and sonic spray devices
described earlier. A similar approach, referred to as DeSSI,
utilizes a sonic spray nozzle without the high voltage (Haddad,
Sparrapan, & Eberlin, 2006).
4. Surface Sampling Probe
The surface sampling probe (SSP) is a direct liquid extraction
device for surface analysis and imaging (Van Berkel et al., 2008).
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It is coupled to the ESI source utilizing the self aspirating
capabilities of pneumatic nebulizers to deliver and draw
sampling liquid from a surface. Because of the high extraction
efficiency it has very high sensitivity for use as a surface analysis
device and can be adapted to chromatographic separations if
needed (Kertesz, Ford, & Van Berkel, 2005). It is also readily
adapted to APCI sources (Asano et al., 2005).
5. Fused Droplet ESI
Work to ionize the eluent from a gas chromatograph (Lee &
Shiea, 1998) or flow pyrolyzer (Hong, Tsai, & Shiea, 2000),
through intersection with charged electrosprays, eventually
evolved into a new variation of electrospray. Early fused droplet
electrospray sources involved intersection of an ultrasonically
nebulized neutral liquid stream containing the analytes with
multiple electrospray plumes to ionize various components of
the stream (Shiea et al., 2001). The use of a simple nebulizer
and single electrospray probe, oriented to ensure charged plume
and neutral droplet intersection (Shieh, Lee, & Shiea, 2005), are
common between this and a similar technique referred to as
extractive electrospray ionization (Chen, Venter, & Cooks,
2006).
6. ELDI, MALDESI, and LAESI
Electrospray-assisted laser desorption/ionization mass spectrometry (ELDI) is another surface analysis technique whereby
a nitrogen laser is used to directly desorb neutral analyte
molecules by thermal processes. The neutral vapor is intersected
with electrosprayed droplets that incorporate the gas phase
molecules which then ionize by ESI (Shiea et al., 2005). With
matrix assisted laser desorption ESI (MALDESI) the sample is
indirectly desorbed via a transfer of energy through a MALDI
matrix to the sample using a similar laser to ELDI (Sampson,
Hawkridge, & Muddiman, 2006). The laser ablation electrospray
ionization technique (LAESI) uses a mid-IR laser to directly
desorb the sample and has the ability to accommodate aqueous
samples (Nemes & Vertes, 2007). Both MALDIESI and LAESI
also utilize electrosprayed droplets intersecting the vaporized
sample for the final ionization event.
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eventually results in transfer of charge to the analyte ions through
gas phase reactions (Robb, Covey, & Bruins, 2000). In many
instances APPI has shown a sensitivity advantage over the corona
discharge source likely because of the field-free nature of the
photon source. Space charge repulsion of the high intensity
corona can lead to some scattering of ions. The technique has
found further application as a component of field deployable and
air analysis instruments (Syage, Hanning-Lee, & Hanold, 2000)
and as a component of multipurpose sources combined with
APCI (Syage et al., 2004).
2. DART
The direct analysis in real time device (DART) was developed
primarily as a surface ionization source (Cody & Laramee, 2005).
Initial ions are created with a discharge in a flowing stream of a
hot noble gas such as argon or helium, forming metastable
excited state atoms by Penning ionization which transfer charge
to the background gas in an API source by chemical ionization
processes. Analyte molecules are desorbed from surfaces by the
heated gas.
3. ASAP
Similar to DART the atmospheric pressure solids analysis probe
(ASAP) was developed as a rapid and convenient device to
analyze solid sample surfaces and liquids (McEwen, McKay, &
Larsen, 2005). Vaporization of the sample is achieved with a hot
nitrogen gas stream and ionization occurs by APCI initiated with
a corona discharge.
4. PADI
Plasma assisted desorption/ionization (PADI) is a surface
analysis technique which involves the direct contact of a sample
on a surface with a plasma generated from a radio-frequency
discharge needle. The ionization in the plasma is thought to
include a mixture of metastable Penning ionization, electron
ionization, and ion molecule reactions. Since the resulting
spectra are dominated by products of ion molecule reactions
(proton transfer and proton abstraction) chemical ionization
appears to dominate (Ratcliffe et al., 2007).
B. Variants of APCI
1. Photoionization
5. Laser Desorption Techniques: LDTD,
AP-LD/CI, LIAD
The motivation for the research in atmospheric pressure photoionization (APPI), as with many of the others cited below, was to
try to find a means of direct ionization of the analyte molecules so
that substances that could not be ionized efficiently with chemical
ionization processes could be accessed with an API source.
In general this goal has met with limited success to date with
any of the approaches described in this review, but other benefits
have accrued from these developments. With APPI the sample is
vaporized with a conventional heated nebulizer and the initial
ionization event is with photons from vacuum ultraviolet lamps
having variable photon energies around 10 eV. Often dopants
such as toluene are used to create the first ionization event, which
Several laser based techniques are used to desorb samples
from surfaces and subsequently ionize by APCI. One technique
indirectly desorbs ions using conductive heat transfer of samples
through a metal surface using IR lasers. This technique is referred
to as laser diode thermal desorption or LDTD and was presented
at the 54th ASMS conference in 2005 by P. Picard. Atmospheric
pressure laser desorption/ chemical ionization (AP-LD/CI)
uses direct laser heating of the sample followed by chemical
ionization (Coon, McHale, & Harrison, 2002). Laser-Induced
Acoustic Desorption (LIAD) uses a Nd:YAG laser to generate
acoustic waves in a metal foil to desorb the sample (Shea et al.,
2007) followed by the chemical ionization process.
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6. Laser Ionization Techniques
a. APLI. Atmospheric pressure laser ionization (APLI) is a laser
based technique but is different from the others in that the sample
is not desorbed by the laser but rather ionized by it. It is a spray
technique where a heated nebulizer is used to vaporize a solvent
containing the sample creating a dry, droplet free vapor.
Ionization is performed by selective resonance enhanced multiphoton ionization (REMPI) where the laser energy appears to be
directly ionizing the sample molecules as indicated by formation
of the radical cation molecular ions from aromatic hydrocarbons
(Constapel et al., 2005). Similar spectra have been observed
with photoionization and aprotic dopants as a result of electron
transfer processes from the reagent ions, and it is yet to be
established to what degree APLI avoids ion molecule reaction
chemistry.
b. AP MALDI. While MALDI has been widely used in vacuum
for many years, only recently has it been applied to API sources
(Wolfender et al., 1999; Laiko, Baldwin, & Burlingame, 2000).
Ion sampling considerations are much the same as they are with
ESI based sources. Although there are no liquids to desolvate per
se, clusters and particles are formed in the atmospheric pressure
ablation plume that require significant quantities of heat to
liberate the ions. The heated transport tubes or chambers
typically used to help decluster ions for ESI tend to require
higher temperatures for AP/MALDI to achieve maximum ion
transmission. This would seem reasonable given the lower vapor
pressure of ablated solid matrix crystals and clusters versus liquid
solvents. High ion transport efficiencies, similar to those with
nano-ESI, can be obtained by taking advantage of the vacuum
drag effect (Wang et al., 2006). The mechanism of the desorption
and ionization event is a topic of intense study, with a large
component of the process appearing to be the formation of
matrix/analyte clusters and the subsequent charge separation of
these species particularly with protein and peptide like molecules
(Karas & Kruger, 2003).
The behavior of low molecular weight drug like substances
(<1,200 Da) with MALDI empirically bears a striking resemblance to the thermal desorption APCI process. A study of low
molecular weight compounds (120–700 Da), covering a broad
range of chemical space including strong and weak acids and
bases, neutrals, quaternary ammonium compounds, and amphoteric compounds, comparing MALDI (alpha-cyano matrix),
APCI, and high flow ESI on a triple-quadrupole instrument,
showed a strong correlation between MALDI and APCI with
very little correlation of either of them with ESI (Covey et al.,
2009). Compounds that thermally degraded with flow injection
APCI and showed low molecular ion sensitivity also did so with
MALDI. ESI showed strong molecular ion response on these
types of labile molecules (drug conjugates for example) and,
from this comprehensive compound set, roughly 35% of the
substances fell into this category, that is, strong molecular ion
response with ESI but little response with APCI or MALDI. Both
mono- and bi-functional pre-formed ions (quaternary ammonium
compounds such as succinyl choline) showed strong response
with ESI for the solution state ion, (M)2þ in the case of
bifunctional quaternary ammonium compounds, supporting a
direct desorption mechanism, but no substantial response (at least
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1,000-fold lower) from either MALDI or APCI indicating that
thermal degradation or gas phase neutralization processes
dominate. Neutral molecules behaved for all three sources
according to the expectations from gas phase ion/molecule
reaction thermodynamics which correlates closely with solution
state polarity. In our experience a good first indication of whether
or not a compound will efficiently produce molecular ions by
MALDI is to flow inject the analyte into a heated nebulizer APCI
source. Recently new matrices designed to optimize gas phase
proton transfer reactions have been developed which improves
the MALDI ionization of low proton affinity substances
compared to the alpha-cyano matrix (Jaskolla, Lehmann, &
Karas, 2008).
Different controlled biological matrices that suppressed
ionization were also compared in this same study with MALDI,
APCI, and ESI, again showing a close similarity between APCI
and MALDI (Covey et al., 2009). Biological extracts that were
high in lipids and surfactants were devastating to electrospray
ionization efficiency, consistent with a desorption mechanism
from a droplet surface. These same matrices were not nearly
as suppressing for APCI or MALDI, indicating a gas phase
ionization mechanism although they were not entirely immune
from all suppression effects. High gas phase proton affinity
substances, which tend to be low molecular weight and polar in
biological samples, strongly suppressed APCI and MALDI
which supports a gas phase reagent ion depletion process. These
same substances (urea for example), because of their polarity,
tend to elute in the void volume of reversed phase LC separations,
creating a situation where APCI in practical analytical scenarios
demonstrates fewer examples of ion suppression problems
than ESI since lipids elute during the chromatographic process
sometimes at the same time as the analytes. It is to be expected
that LC/MALDI would demonstrate the same advantage as APCI
in this regard.
C. Multisprayer Sources
Because neither vacuum consideration nor physical space are
limitations with API sources, they are particularly convenient
for mounting multiple ESI and APCI inlets, and sometimes
combinations of the two. This has found application in such areas
as accurate mass measurement, ion-ion reactions, sensitivity
improvement at higher liquid flow rates, and multiplexing to
achieve high throughput analysis. Combinations of any of the
various atmospheric pressure ion sources are possible; however,
multiple sprayer electrospray ion sources and dual sources based
upon ESI, APCI, and APPI are the dominant approaches in the
literature. A recent publication reviewed the general and patent
literature relating to ion sources containing multiple electrospray
probes (Schneider & Covey, 2007). In general, these types of
sources can be categorized by the presence or lack of indexing,
which is the means by which the mass spectrometer rapidly turns
on and off the signal coming from each of the multiple inlets and
assigns the resulting signal to a particular inlet channel. The
literature contains a number of examples of non-indexed multiple
sprayer approaches for purposes such as mass calibration (Jiang
& Moini, 2000) ion/ion reactions (Ogorzalek Loo, Udseth, &
Smith, 1991, 1992), ion/molecule reactions (Ogorzalek Loo
et al., 1992), and throughput enhancement (Hiller et al., 2000).
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Indexed multiple sprayer systems require a means for
enabling and disabling each of the sprayers, as well as the ability
to assign a signal to a particular sprayer. The various means can be
grouped into four main categories: fluid selectors (A), spray
controllers (B), spray blockers (C), and ion beam selectors (D), as
shown in Figure 13. Fluid selectors operate by enabling and
disabling the liquid flow to a sprayer with valves (Schneider &
Covey, 2007). Spray controllers operate by turning on and off the
strong electric field at the electrospray tip (Schneider, Douglas, &
Chen, 2002). Spray blockers interrupt the spray between the
emitter and vacuum entrance by mechanical means such as a
blocking plate (Wang et al., 1999) or by moving the sprayers
relative to the entrance aperture (Chan et al., 2002; Park, Kim, &
Kim, 2004). Multiple API vacuum apertures, each with an
independent sprayer, can be used to create independent ion beams
in the vacuum system that can be deflected with lenses. Although
this presents the fastest means of switching, it would require
higher vacuum pumping speeds that may not be practical.
Efforts to improve compound coverage have led to the
combination of ESI and APCI inlets. Agilent and Waters have
accomplished this by electrospraying into a region containing
chemical ionization reagent ions. The use of APCI and APPI has
been combined into a single ion source in an attempt to extend
ionization capabilities on Thermo instruments (Syage et al.,
2004). Alternatively, a switching valve may be used to direct a
single flow stream sequentially to separate APCI and ESI probes
to generate mass spectra based upon each ionization method as
commercialized by AB/Sciex.
FIGURE 13. Four different means to index a multiple sprayer ion source
to a mass spectrometer. A: Fluid selectors gate the signal by interrupting
the fluid on the way to the sprayer. B: Spray controllers gate the signal by
controlling the electric field at the sprayer tip. C: Spray blockers
interrupts the spray between the sprayer tip and the mass spectrometer
entrance. D: Ion beam selectors gate the signal inside the vacuum system
with a lens where the beams originate from multiple atmosphere-tovacuum apertures. Reprinted with permission from Schneider, 2007,
copyright 2007, Elsevier.
Mass Spectrometry Reviews DOI 10.1002/mas
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Operation of arrays of nano-ESI emitters has been studied
for several years as a means of increasing the ion sampling
efficiency at liquid flow rates beyond the nano-ESI regime
(Rulison & Flagan, 1993; Deng et al., 2006). Multiple emitters in
front of multiple atmosphere to vacuum sampling apertures have
shown gains over a single emitter/aperture arrangement at 1 mL/
min flow (Kelly et al., 2008). While promising, this approach
introduces a high degree of complexity and simpler single emitter
systems have achieved 80% sampling efficiency at 500 nL/min
(Schneider, Javahari, & Covey, 2006) leaving little room left for
improvement. There are, as mentioned earlier, significant
opportunities for improvements in the high flow regime and it
is here where the multiple sprayer approach might make gains in
the future.
V. CHEMICAL NOISE: AN OUTCOME OF
ATMOSPHERIC PRESSURE IONIZATION
TECHNIQUES
Chemical noise is a special problem of the atmospheric pressure
ionization techniques which is often responsible for raising
detection limits in both quantitative and qualitative analyses.
Many substances are ionized by API techniques such that trace
contaminants in solvents, apparatus, and air, in combination with
clusters, generate ions across the m/z range. Compounds such as
phthalates, silicones, phenyl phosphates, sebacates, adipates, and
clusters of these ions with HPLC solvents and additives, are
ubiquitous and unavoidable. Despite dramatic efforts to improve
ion source hardware with pure materials such as stainless steel
and ceramics, judicious use of heat, positive source pressure to
exclude ambient air contaminants, off-axis sprays, curtain gases,
declustering fields, off-axis and curved entrance ion optics, and
software noise filtration techniques, this problem has remained
persistent. Meticulous laboratory practices utilizing the purest
available solvents and gases have also not solved the problem in a
general sense. At low mass ions exist at nearly every nominal
mass at different intensities, many of which have been identified
(Guo, Bruins, & Covey, 2006; Keller et al., 2008). With the
improved sensitivity of API sources chemical noise has appeared
even under tandem MS conditions, confounding spectra interpretation and increasing baseline noise under multiple reaction
monitoring conditions.
Since chemical noise is a consequence of atmospheric
pressure ion sources, approaches for dealing with it have been
emerging in recent years and this line of development will
undoubtedly continue on into the future with increasing
emphasis. For this reason some of the recently investigated
techniques will be briefly mentioned here whether or not they are
an integral part of the API source. They tend to fall into two
general categories. In the first case extrinsic properties of
molecules and their interactions with other species in their
immediate environment are leveraged, that is, chemical properties. In the second case we are dealing with intrinsic properties of
molecules that are independent of their immediate surroundings
such as nuclear composition (mass) or internal bond energies
(molecular fragmentation).
One successful example of a chemical means to remove
background ions is the dynamic reaction cell developed for
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inductively coupled plasma mass spectrometry, an atmospheric
pressure ion source designed for elemental analysis. Gas phase
ion/molecule reactions in cohort with high and low mass band
pass filters can be used to effectively separate isobaric elemental
ions. The separation of argon and calcium ions with ammonia
is an immediate example (Tanner, Baranov, & Bandura, 2002).
This principle has been applied to electrospray instruments and
organic analyses, utilizing dimethyl disulfide to selectively
separate clusters and sodiated ions from protonated molecules
(Guo, Bruins, & Covey, 2007). Another approach that presents
promise for development is the use of ion/ion reactions to induce
charge inversion of targeted analyte ions (Min, Emory, &
McLuckey, 2005). This borrows from concepts in accelerator
mass spectrometry but accomplishes the charge inversion in
a much different way. A third approach is the use of mobility
separations prior to mass analysis. Of particular interest is the
implementation of FAIMS (Purves & Guevremont, 1998) or
Differential Mobility spectrometry (Levin et al., 2006) which
separate ions on the basis of the difference between their high and
low field mobilities. The reason these mobility approaches are
classified as chemical techniques is because the selectivity
obtained by such devices is highly influenced by the type of
carrier gases that are used and can be adjusted by altering this
parameter. This behavior bears analogy with the mobile phase in
HPLC.
The second category of approaches to noise reduction is an
extension of mass spectrometry development that has been
undergoing continued activity since the earliest days. These
include means of obtaining higher resolution and serial fragmentation of molecules with collisions and multiple stages of mass
analysis. The field is currently focused on trapping type analyzers
such as FTMS and Orbitraps as well as the pulsed beam approach
of time-of-flight. They all can achieve resolution in the tens of
thousands or greater, and each has their own particular attributes
such as relative speed and dynamic range. The important question
is how much resolution is required to eliminate all possible
scenarios where chemical noise is limiting. Some early empirical
studies presented by Bateman at the 2008 ASMS in Denver with
an Orbitrap mass spectrometer indicated that a resolution of
30,000 may provide sufficient selectivity for routine drug
discovery assays, but these are early days and the number of
potential scenarios where noise is limiting is almost infinite.
Multiple stages of MS/MS is another way to gain selectivity
from intrinsic properties of molecules. Three-dimensional ion
traps have demonstrated the capability to perform MSn but suffer
from duty cycle and ion current loss limitations. Combinations of
linear ion traps with other mass analyzers such as Orbitraps,
quadrupoles, and TOFs are beginning to show promise in this
area, where the particular noise reduction application will be for
targeted quantitative analysis (Leuthold et al., 2004; Collings,
2007).
What is emerging from this field of active research is that no
one technique will solve all chemical noise problems. Techniques
that rely on intrinsic properties of molecules will likely be more
generally applicable and easier to control, requiring less understanding of chemical selectivity. However, they will never be able
to distinguish between substances that have the same exact mass
which is well within the realm of the techniques that use chemical
interactions and reactions (Eberlin, 2006).
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VI. ION TRANSPORT
A. Transport of Ions From Atmosphere to Vacuum
1. Desolvation and Declustering
Declustering is the term used to describe the process of removing
the last layer of solvent molecules from gas-phase ions, while
desolvation is a broader term that can include the process of
evaporating macroscopic droplets. All atmospheric pressure ion
sources and interfaces must accommodate means for both
desolvating and declustering, and these techniques can overlap.
The problem is similar for the high and low flow sources,
differing only in the magnitude of the amount of solvent that has
to be vaporized.
Declustering must be carried out before the ion enters the
mass analyzer. The general techniques used for declustering
(Bruins, 1991) have not significantly changed in the last 15 years.
However there have been some modifications tailored to the
nanoflow regime that will be discussed below. On the atmospheric pressure side of the aperture declustering can be carried
out by drying the ions with either heat or dry gas, or a
combination of both. On the vacuum side of the aperture an
electric field can be used to decluster by means of collisioninduced dissociation. A counterflow of dry curtain gas in front of
the orifice acts to shift the cluster equilibrium to smaller clusters
(and eventually the bare ion), and also keeps solvent from
entering the free jet expansion where it can re-cluster with the
ions. Heating the gas or the region in front of the aperture assists
further in declustering. One way to achieve this is to use a heated
capillary tube to sample the ions from atmosphere into vacuum.
The ions become declustered in their passage down the heated
tube, which may be 0.3–0.5 mm diameter and up to 10 cm long.
Today all methods described above are used widely, and often
together.
Special efforts to increase the residence time of droplets in
hot zones in front of the aperture have been undertaken to
improve nano-ESI operation and attempt to maintain nano-ESI
efficiencies at flow rates as high as possible, extending into the
low microliters per minute range. Since the first description of
nano-ESI it was apparent that operation in the nanoflow regime
offered the possibility for very high sampling efficiency. A
number of research groups demonstrated nano-ESI sampling
efficiencies on the order of approximately 1–10% for electrospray infusion at solvent flows ranging from 2 nL/min up to tens
of nanoliters per minute (Wilm & Mann, 1994; Geromanos,
Freckleton, & Tempst, 2000; El-Faramawy, Siu, & Thomson,
2005). The desire to improve nano-ESI efficiency further, and
increase the flow rate to couple it more readily with liquid
chromatography approaches, has led to the development of new
types of interfaces capable of high performance operation at flow
rates in the hundreds of nanoliters per minute. As the flow rate
increases from the traditional nano-ESI regime (<100 nL/min)
up to the nano-LC regime (100–1,000 nL/min), maintaining
high efficiency is challenging due to the increased desolvation
requirements as well as the increased tendency for the spray to
diverge over a larger area. High sampling efficiency requires
capturing the majority of the electrospray plume and providing
sufficient desolvation means for efficient ion liberation.
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ATMOSPHERIC PRESSURE ION SOURCES
Figure 8B shows an illustration of a particle discriminator
interface (PDI) specifically optimized to provide high efficiency
for electrospray operation up to approximately 1 mL/min
(Schneider et al., 2003, 2005). It comprises a gas conductance
limiting orifice separating the atmospheric pressure source
region from the first vacuum chamber of the mass spectrometer.
Sealed onto the orifice is a short laminar flow chamber of larger
diameter than the orifice that can be heated to temperatures on the
order of 3008C to ensure complete desolvation over a variety of
flows. The interface configuration also includes a counter-current
gas flow to aid in removal of solvent molecules prior to entrance
into the vacuum system. Since the orifice controls the rate of gas
flow into the vacuum system of the mass spectrometer, the
diameter of the heated laminar flow chamber can be varied
substantially with no effect on the downstream pressures. The
vacuum draw into the orifice establishes laminar flow conditions
through the heated chamber, and generates a suction effect at the
entrance. With suitable sizing of the laminar flow chamber
diameter, electrospray plumes for solvent flow rates up to
approximately 1 mL/min can be entirely consumed into the inlet.
The charged droplets and ions within the laminar flow chamber
are entrained in the net gas flow to the orifice as shown in
Figure 14.
In general, the laminar flow chamber is designed such that
the ratio of volume (dependent upon radius and length) to gas
flow rate provides approximately 1 msec residence time for ions.
Increasing the residence time further can improve desolvation
efficiency for a given temperature setting, but diffusional ion
losses can become significant. A fully optimized version of this
interface has been shown to maintain electrospray sampling
efficiency for a favorable sample in the range of 70–80% at
flows up to approximately 500 nL/min, making it amenable to
nanoflow LC/MS (Schneider, Javaheri, & Covey, 2006). The
necessary temperature to achieve these high efficiencies was
directly related to both the solvent flow rate and the gas flow
rate (dictated by the size of the orifice). Similar experiments
conducted using a traditional heated capillary inlet demonstrated
similar plume consumption phenomena with >90% of the total
ion current consumed in to the inlet at flows up to approximately
1 mL/min. However, diffusion losses and insufficient heat for
desolvation limited the observed sampling efficiency to approximately 20% (Page et al., 2007). These experiments highlight the
critical importance of both capturing the electrospray plume and
achieving sufficient desolvation as the main drivers for very high
sampling efficiency in electrospray ionization.
Complete declustering is not usually achieved on the
atmospheric pressure side of the aperture. In addition, neutral
solvent molecules that enter the vacuum system will re-cluster
with molecular ions in the supersonic expansion, requiring the
use of additional declustering on the vacuum side of the aperture.
In the most common sampling configuration, a skimmer is
located with the tip 2–10 mm downstream of the aperture.
The background pressure in the vacuum region between the
aperture and skimmer is of the order of 1–3 Torr (Bruins, 1991),
the skimmer tip can either be within the Mach disc of the
supersonic expansion, or it can be downstream of the Mach disc.
More details of the gas flow and pressure patterns associated
with the supersonic expansion are provided later, but here it is
sufficient to note that the gas density between the orifice and the
Mass Spectrometry Reviews DOI 10.1002/mas
&
skimmer is highly inhomogeneous, changing by several orders
of magnitude in a few mm. A voltage difference between the
aperture and the skimmer of 10–200 V accelerates the ions to
collide with the background gas (air or nitrogen). The voltage is
usually optimized to produce declustering without fragmentation
(although the ability to induce fragmentation by increasing the
voltage is an important feature of this geometry). A second and
equally important function of the voltage difference is to focus
the ions, as described later. The declustering voltage can be
compound dependent (due to different tendencies to fragment),
m/z dependent and solvent dependent. The actual voltages used
depend on the distance and the pressure (or gas density) between
the orifice and skimmer, which varies among different instruments. The voltage is normally considered to be a user-adjustable
parameter, to be optimized as needed.
B. Transport of Ions From the Ion Source Into
the Vacuum Chamber
1. Focusing Ions at Atmospheric Pressure
Ions created in the ion source must be transported from
atmospheric pressure into high vacuum where the pressure is
typically 3 105 Torr or lower, requiring small apertures to
separate the ion source from the vacuum. This suggests
immediately that focusing the ions in the source toward the
orifice should improve the transmission efficiency and sensitivity.
However, at the most basic level, it can be shown that any attempt
to increase the concentration of ions by using DC focusing fields
at atmospheric pressure where ion motion is governed by ion
mobility is frustrated by Maxwell’s equations (specifically
Gauss’ Law).
Considering the issue in more detail, the first aperture
separating the source from the ion optics is typically 0.2–1 mm in
diameter, most commonly 0.3–0.7 mm. In some cases the
aperture is a tube with a length-to-diameter (L/D) ratio of >20,
in other cases a thin plate with L/D ratio of approximately 1.
Ultimately it is gas drag that carries ions from atmospheric
pressure into the first vacuum stage, which is normally pumped
by a mechanical vacuum pump to a pressure of 0.5–10 Torr. Gas
drag (or suction) in the region close to the orifice pulls ions from
an area that is larger than the actual area of the hole. The effective
size of the aperture due to the gas drag (i.e., the area from which
ions are pulled through into vacuum), relative to the physical
aperture size, depends on the electric field strength on the high
pressure side of the aperture. Strong focusing fields can and
usually are applied to direct ions toward the orifice. At
atmospheric pressure ion velocity is directly proportional to the
field strength (with minor higher order terms as the field strength
becomes very high), and ions effectively follow the lines of
electric flux. Flux lines can be made to converge toward a small
region (the orifice), focusing ions toward the orifice. As
suggested above, it is often not appreciated that the volume
concentration of ions does not increase, due to Gauss’ law: as the
flux lines converge toward a point, the magnitude of the electric
field increases, as does the ion velocity. Therefore the ions
converge toward a smaller area in two dimensions, but their
velocity in the 3rd dimension (toward the hole) increases so that
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FIGURE 14. Computational fluid dynamics (CFD) models of gas entrainment streamlines with PDI
interface with a 1 mm i.d. laminar flow chamber attached to a 0.25 mm orifice with a 1 mm spacer of
wider i.d.
FIGURE 16. Computational fluid dynamics (CFD) model of the free jet expansion, and calculated ion
trajectories. Upper half of figure shows gas flow (purple lines) and ion trajectories for m/z 228 (black lines).
Lower half of figure shows ion trajectories for mass 16,950 Da (9þ charge state at m/z 1,883.4). Color key
shows gas velocity. Upper half shows a velocity range from 0 to 3,150 m/sec; lower figure half shows an
expanded range of 0–720 m/sec.
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Mass Spectrometry Reviews DOI 10.1002/mas
ATMOSPHERIC PRESSURE ION SOURCES
the ion concentration is unchanged. Because the gas volume
flowing through the hole is constant, the ion flux through the hole
cannot be increased if the ion volume concentration remains
unchanged. Of course, if the ions could be focused toward a small
hole and carried through the hole into the ion optics, and if the gas
could be pumped away while the ions are captured by the optics,
then the intended increase in ion concentration and hence ion
sampling efficiency would be achieved. To first order, the real
gain in ion transmission due to atmospheric pressure focusing
with static (DC) fields is given by the ratio of the electric field
strength in the vacuum region downstream of the aperture to the
electric field strength on the atmospheric pressure side of the
aperture (assuming that all ions could be captured and focused by
the optics). However, the high electric field strength that focuses
the ions toward the orifice cannot be carried through into vacuum
because of electrical breakdown. The weaker field strength that
must be maintained on the vacuum side causes the ion trajectories
to diverge, and the focusing gain is lost as ion trajectories diverge
and ions are lost to the wall of the aperture as they enter the
vacuum system.
In spite of this rather negative view of the benefit of focusing
ions toward the hole, some successes can be and have been
achieved. For example, a small conical-shaped orifice was
employed on the API III triple quadrupole system. This conical
shape produced some focusing of the ions toward the tip of
the cone, and produced a twofold improvement in sensitivity
compared to a flat disc with the same orifice diameter. A saving
factor is that as the ions come close to the hole, the gas drag begins
to dominate over the electric field. As the electric field starts to
weaken close to and through the plane of the orifice, and the ion
flow starts to diverge, the gas drag force overcomes the field
strength and ions are pulled through. This dominance of the
gas drag over the electric field near the aperture is the vacuum
suction effect. In a typical source with field strengths of 1,000–
3,000 V/cm on the upstream side (and lower fields on the vacuum
side) the effective area of the orifice can be three to four times
larger than the actual area. This applies to thin apertures as well as
tubes (where the field strength inside the tube drops to zero). In
principle, a weaker field on the upstream (high pressure) side
results in a larger effective sampling area. In the limit, if ions
could be generated in a large field free volume, and the only exit
was suction through the vacuum orifice, then all ions could be
sampled (limited by diffusional losses to the walls which are not
considered here). This issue has been recognized by Willoughby
and Sheehan and presented at the 2008 ASMS conference in
Denver. Methods were proposed for reducing electric fields in the
high pressure ion source, as well as for using laminated apertures
and tubes to prevent the field divergence into vacuum. However,
practical implementation in an electrospray source has not been
fully demonstrated.
In reality, most API sources use significant gas flow to
nebulize and dry ions, and most of the gas, droplets and charge
exit through a waste port. Any attempt to focus the ions from this
flow toward the orifice must overcome the gas flow velocity in the
source, extracting ions from the flow and moving them toward the
orifice. The large field required for this purpose limits the ability
to focus ions through the hole for the reasons described above.
This catch-22 situation frustrates the simple focusing schemes
that have been proposed and tried in the past. Some early attempts
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to focus ions from a corona discharge point (by reconverging the
field lines from the discharge point toward an orifice in a
‘‘mirror’’ configuration for the electric field lines) were able to
achieve almost complete focusing of the 5 microamp current on
the region close to the orifice (or on a small collector plate
approximately the size of the orifice), but required field strengths
approaching the breakdown limit to do so (the mirror image of the
corona discharge itself). Even staying below the breakdown
limit, attempts to get the highly focused beam through the
hole were not very successful. Compared to the normal, rather
unfocused geometry, a factor of three increase in ion current was
the best that could be achieved (unpublished data).
In recent years, ion focusing at atmospheric pressure has
been accomplished using RF fields in specific geometries. The
most successful configuration has probably been achieved with
a FAIMS source. While FAIMS has been developed as an ion
separation method at atmospheric pressure, it can also act to
focus or concentrate the ions into a smaller region of space
(Guevremont & Purves, 1999). The method has been used to both
focus (while separating ions) and to trap ions in a small region of
space in front of the orifice. It is important to note that the
technique does not focus all ions together, but rather focuses
ions of a specific mobility and configuration, while other ions
are defocused or lost, so it is not a general solution to the
high-pressure focusing problem.
It has also been proposed to use RF ion guides to focus ions
at atmospheric pressure (Willoughby & Sheehan, 1984). RF ion
guides are often used at low or intermediate pressure to focus ions
(described later, and also in the review article by Douglas, this
volume). While the same technique can in principle be applied at
atmospheric pressure, the focusing effect is very slow (in time)
at high pressure, so either a small device is required (which makes
it unsuitable for collecting and focusing ions from a large-volume
source) or a very long device is required to give the ions
sufficient time to focus to the center.
C. Sampling Ions Into High Vacuum
1. The Orifice-Skimmer Geometry
The vacuum aperture consists of either a thin plate with an orifice
or a tube that can be several centimetres long. The orifice
produces a well-defined supersonic expansion through the hole,
with ions and gas accelerated to supersonic velocities in a jet
structure that has been thoroughly studied and characterized in
the field of rarified gas dynamics (Campargue, 1984). Figure 15
shows the basic structure of the barrel-shaped expanding jet,
terminating in a shock wave (often referred to as the Mach disc)
where the supersonic gas velocity becomes sub-sonic through a
thin re-compression region.
Ions carried by the expanding gas are usually sampled into
the next vacuum stage through a skimmer. The skimmer can be
located with the tip upstream of the Mach disc (point A in Fig. 1)
so that ions and gas are sampled inside the so-called ‘‘zone of
silence,’’ or downstream of the Mach disc (point B of Fig. 15), a
position that allows ions to pass through the shock wave before
being sampled. There are several considerations when deciding
where to sample with the skimmer. Upstream of the Mach disc the
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FIGURE 15. Structure of the supersonic free jet. (1) Orifice plate.
(2) Barrel shock defining the radial boundary of the free jet expansion.
(3) Mach disc, defining the axial boundary of the supersonic portion of
the free jet expansion. (4) Skimmer. A: Position upstream of the Mach
disc. B: Position downstream of the Mach disc.
velocity of ions and gas is high and primarily directed parallel to
the axis. A skimmer at this location samples a higher gas and ion
flow than a skimmer located further downstream. Additionally,
the gas density is lower within the zone of silence, so that ions can
be more readily focused toward the skimmer, using for example a
small ring electrode as shown in Figure 15. Outside of the Mach
disc, the pressure is higher, but the gas velocity is lower. Because
of the larger distance from the orifice, the gas flux into the next
chamber is significantly reduced, which is beneficial when
considering the size of vacuum pump required. However, the ion
flux is also lower and it is more difficult to refocus ions efficiently
through the skimmer at the higher pressure.
These considerations have led to different approaches. In
Sciex mass spectrometer systems the skimmer has always been
located upstream of the Mach disc, providing maximum benefit
for ion focusing and declustering (both leading to higher
sensitivity). Other systems use a skimmer located outside of the
Mach disc and in some cases the skimmer is located slightly offaxis to prevent large droplets from being sampled directly into the
ion optics and subsequently to the detector.
The search for a deeper understanding of the limitations of
ion sampling efficiency in the orifice-skimmer region led to the
application of computational fluid dynamics (CFD) to the ion
sampling region. A sophisticated gas-dynamic model was
developed and combined with an electric field and ion trajectory
calculator to study the neutral gas and ion motion between the
orifice and the skimmer (Jugroot et al., 2004a,b, 2008). This
modeling effort provided clear insight into the structure of the
expansion which was numerically modeled in detail for the
890
first time on this small scale. It also provided information on the
ion motion due to the electric fields that are applied to both focus
and decluster the ions. The simplistic view has been represented
in the past as an axi-symmetric expansion with a Mach disc that is
penetrated by the skimmer (as shown in the schematic of Fig. 15).
The nature of the flow in the region downstream of the skimmer
(into the next vacuum chamber) has never been clear. The
numerical model provided details that showed that the skimmer
disrupts the Mach disc so that it is never visible, and the flow
persists through the skimmer and into the next chamber as shown
in Figure 16. Figure 16 shows the computed ion trajectories
(black lines) and neutral gas flow (purple lines) for a 0.25 mm
orifice followed by a 2.3 mm diameter skimmer located 2 mm
downstream. The upper part of the figure shows the trajectories of
ions of m/z 228 (typical of a small singly charge ion) and the lower
part shows ion trajectories for mass 16,951 amu with a charge
state of 9þ (m/z 1,859), representative of a large biomolecule.
The focusing effect is provided by a potential difference of 80 V
between the orifice and skimmer. For this case the focusing ring
shown in Figure 15 was not included.
It is clear that the smaller ions are more efficiently focused
through the skimmer, but some are lost to the inner wall of the
skimmer due to the field divergence. The larger ions are less
affected by the field and are less well focused. The model
predicted sampling efficiencies of 68% and 38% for these two
extremes of m/z, values that agree roughly with our measurements of transmission efficiency (unpublished).
The model solves the Navier–Stokes equation for the gas
flow, and the Laplace equation for the electric field, and then uses
a drag model for ion motion through the gas (with measured or
estimated ion-neutral cross-sections). The model also allows an
assessment of ion temperature by calculating the heating of the
ion due to its drag through the gas. While probably only
qualitatively correct, the results clearly show that the regions of
strong heating are concentrated near the edge of the skimmer,
with much less heating in the center of the gas flow. It is this
heating, controlled by the voltage difference between the orifice
and the skimmer, that provides for declustering and even
fragmentation in this region. The obvious inhomogeneity in the
ion temperature shown by the model suggests that orificeskimmer fragmentation is not a very precise technique for
controlling the degree of ion heating and fragmentation.
2. The High Pressure RF Ion Guide
To increase the sampling efficiency the atmosphere-to-vacuum
orifice diameter can be increased. This either requires using a
larger mechanical pump to maintain the interface pressure at
about 1 Torr, or else it requires focusing ions at a higher pressure.
Focusing ions into the skimmer becomes more difficult when
the gas density is higher, so the efficiency decreases when
the pressure increases. Additionally, increasing the aperture
diameter causes the gas flow through the skimmer to become
higher, resulting in a pressure rise in the next chamber.
Consideration of these issues led to the development of an
rf-only quadrupole (the QJet1) to focus ions from the orifice into
the next vacuum stage. An orifice diameter of 0.62 mm results in a
vacuum pressure of 3.4 Torr for a reasonable sized mechanical
Mass Spectrometry Reviews DOI 10.1002/mas
ATMOSPHERIC PRESSURE ION SOURCES
pump with a pumping speed of 11 L/sec. The high pressure
requires that RF voltages between the rods be kept at a reasonably
low value (<300 V) to avoid the possibility of electrical
breakdown. The CFD gas flow and ion trajectory model
described above was used to show that ions are largely entrained
in the free jet and therefore confined within the boundaries of the
barrel shock during the initial expansion. The diameter of the rfonly quadrupole was therefore selected to be slightly larger than
the diameter of the barrel shock. Ions expanding into the volume
of the ion guide are captured by the rf field after the initial
pressure decrease in the jet expansion, while the neutral gas is
allowed to escape between the rods. The ions are eventually
collisionally focused by the action of the gas pressure and rf field
(see review by Douglas in this volume for a description of
collisional focusing). This decreases the diameter of the ion beam
so that it efficiently passes through a small aperture into the next
vacuum chamber.
Figure 17a shows an example of the neutral gas flow
expanding from the orifice into the chamber with an rf-only ion
&
guide at a pressure of 2.3 Torr. Although the rods disturb the
development of the barrel shock and Mach disc, the supersonic
portion of the jet can be observed by the red color in the figure.
The yellow and green colors indicate sub-sonic gas velocity
along the axis until the end of the rods where most of the
flow diverges, with some flow continuing into the next (lowerpressure) chamber. Not visible in this view is the structure of the
gas flow between the rods (to the pumping port located on the side
wall). Figure 17b shows the ion trajectories calculated for a singly
charged ion of m/z 609 under the action of the gas flow and rf field
with 150 V peak-to-peak applied between the rods that are 4 mm
apart. This view shows the plane between rods, where the gas can
exit toward the pumping port. While some ions from the edge of
the expansion are lost by being pulled by the gas flow in between
the rods, it is clear that a large proportion of ions are collisionally
focused toward the axis by the rf field. We have measured
efficiencies of 50% for the ion transport though a 1.5 mm
aperture at the end of the ion guide, comparing the ion current
entering through the orifice to the ion current measured in the
following chamber. This geometry therefore provides significant
advantage over the orifice/skimmer combination when the gas
flow and pressure in the first vacuum chamber are high. Some
additional focusing and declustering can usually be achieved by
applying a voltage difference of 100–200 V (dc) between the
rods and the orifice, providing a similar function as the voltage
difference between the orifice and skimmer in previous geometries. This voltage is empirically optimized (either manually
or automatically under computer control) to provide the best
sensitivity and/or signal to noise ratio.
3. The Ion Funnel and Other Ring Guides
FIGURE 17. a: CFD model of 5 cm-long QJet1 with free jet expanding
through 0.6 mm orifice into a pressure of 2.3 Torr. The red color
indicates the region of fastest flow. b: Calculated trajectories of ions of
m/z 609 through the QJet1 with an RF amplitude of 150 V on the rods.
Mass Spectrometry Reviews DOI 10.1002/mas
Focusing and confinement of ions in rf-only devices is not limited
to the use of a quadrupole. Hexapoles and octapoles are also
widely used as both collision cells and ion guides (see Douglas
review in this volume). Compared to a quadrupole, these higher
order multipoles provide stronger containment fields for the same
voltage and frequency applied to the rods, but a broader pseudopotential well that results in a less tightly focused beam.
Therefore they can be advantageous for confining ions that
have high radial energies due either to the energy distribution of
the incoming beam or to scattering from collisions inside the
multipole.
The principle of rf-focusing can be broadly described
(Gerlich, 1992) as due to the action of a spatially inhomogeneous
oscillating field. It can be shown mathematically that the rf field
produces a net force on ions that is directed toward regions where
the rf-fields are weakest (the force being identical in direction for
both positive and negative ions). This force can be thought of as
due to the action of a pseudo-potential created by the gradient of
the rf-field. In the case of a multipole the rf-field strength is
highest at large radial distances (but within the boundaries of the
rod set) and weakest on the axis (where the field is always zero in
a perfect quadrupole field). To first order the pseudo-potential has
the form of a harmonic potential that causes ions to oscillate in the
x- and y-directions as they move along the axis of the guide, with
the combined x-and y-motions acting to produce rotational
motion. Superimposed on this secular motion is the micromotion
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COVEY, THOMSON, AND SCHNEIDER
at the rf frequency (Thomson, 1998a). The general principle,
however, allows the possibility of a variety of electrode structures
that can function as rf ion guides. Gerlich (1992) pioneered the
use of rf-ring guides as containment devices with a pseudopotential that is very steep near the electrodes but very flat in the
center. The ring guide consists of a series of plates with apertures
that form the walls of the ring guide, with opposite rf phases
applied to alternate rings. The device can be thought of as a tunnel
with corrugated walls. If the rings are widely spaced, small
trapping regions are formed between the rings. With the correct
ring spacing, the walls appear relatively smooth and the trapping
regions are very small.
The rf-ring guide was adapted by the Smith group at Pacific
Northwest National Laboratories to form a high-pressure ion
focusing device by arranging decreasing ring aperture diameters
along the axis, forming a funnel structure (Schaffer et al., 1998)
as shown in Figure 18. Termed an ion funnel, this structure has
been used in a variety of applications to accept a widely dispersed
ion beam and focus it to a smaller diameter, usually to introduce
the ions through a small aperture into the next vacuum chamber.
Because reasonably small rf voltages are required when the rings
are spaced closely together, the device has been successfully used
to efficiently focus ions at pressures up to at least 5 Torr (Kim
et al., 2000). The structure is ideally arranged to allow an axial
electric field to be applied, simply by providing a resistive divider
network between the rings and a single voltage source to provide
decreasing DC voltages on each ring. For a fixed rf voltage, a
wide mass range of ions is stably confined. The device does have
the disadvantage of being more mechanically and electrically
complex than a multipole, but its advantages (primarily being
more efficient at focusing ions at higher pressures) have been
significant enough to see adoption in some instrument configurations. For example, it has been used to effect by the Smith
group to refocus ions at the end of an ion mobility drift tube that
operates at a few torr pressure, after the ion beam has expanded
due to diffusion (Tang et al., 2005). Multiple ion funnels have
been used between stages of tandem ion mobility cells, and
ultimately to focus ions into the aperture of the mass spectrometer
vacuum chamber.
Another adaptation of the ring guide has been developed
and commercialized by Waters (Pringle et al., 2006). They
used a straight ring guide, but applied a traveling wave to
the plates to move ions through the ring guide. Instead of a
dc axial field, the traveling wave uses a moving potential
front to push ions along. The ring guide prevents ions from
escaping radially. The traveling wave allows ions to ‘‘surf,’’
moving on the wave front or falling into the next trough to be
pushed by the next wave front. This novel design has found
use as a low-resolution mobility separation device between
the quadrupole and TOF section of a QTOF, providing
additional separation that is a function of charge state and
collision cross-section.
Other rf-only ion guide structures have been proposed
and tested, but all provide the same basic function of confining
ions radially while allowing them to move axially (or pushing
them axially with axial electric fields), and they have all
proven to be efficient at pressures of several torr. This makes
them useful as devices to efficiently transfer ions from a high
pressure ion source into vacuum. Many instruments employ
multiple rf ion guides in series, usually at different pressures.
Segmented rf multipoles have been used as collision cells,
ion guides, ion mobility drift cells (Dodonov et al., 1997;
Javahery & Thomson, 1997). Mass spectrometers with an
atmospheric pressure ion source almost always use one or more
rf ion guides to transfer ions through the higher pressure vacuum
stages.
FIGURE 18. The ion funnel configuration in cross-section. Each plate is a disc with a central hole as shown.
Two phases of rf voltage are applied to adjacent rings.
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Mass Spectrometry Reviews DOI 10.1002/mas
ATMOSPHERIC PRESSURE ION SOURCES
VII. CONCLUSION
The number of topics addressed in this review attests to the
growth in interest in atmospheric pressure ionization techniques
since 1991, and the increase in the number of academic and
industrial researchers that are now actively working to further the
development and application of atmospheric pressure ionization
mass spectrometry instrumentation and methods. The techniques
of electron and chemical ionization that were still dominant in the
early 1990s have become routine tools and commodity items with
little growth or active development. The early predictions about
the utility of API due to the seeming simplicity of having the ‘‘ion
source in the laboratory’’ are now true, and the growth is
continuing. Of course, having the ion source at atmospheric
pressure where the samples originate solves some problems but
raises others that we have touched on here—issues to do with the
extreme sensitivity toward ubiquitous impurities that contribute
to background chemical noise and raise detection limits; issues of
how to efficiently transport ions into the vacuum system; issues of
how best to ionize the enormous diversity of compounds that are
of interest to the biological, environmental and industrial
communities. These challenges will continue to drive the
development of new and novel instruments and techniques in
the foreseeable future, hopefully with a ‘‘change-the-game’’
outcome that will rival that experienced in the last 20 years.
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