MS Detectors
Just as laser eye surgery has restored fading
human vision, new technologies are needed to
improve ion “chemical vision” detection.
David W. Koppenaal
Charles J. Barinaga
Pacific Northwest National Laboratory
M. Bonner Denton
Roger P. Sperline
University of Arizona
Gary M. Hieftje
Gregory D. Schilling
Francisco J. Andrade
Indiana University
James H. Barnes, IV
Los Alamos National Laboratory
I
n MS, detectors are the “eyes” of the instrument. New detectors and technologies
are needed to correct, improve, and extend
ion detection and, hence, our “chemical vision”. This report reviews current MS detector
technology and provides a glimpse of what we
hope future detectors will be capable of.
The technology of MS can be divided into
three processes: ion generation, ion separation, and ion detection. Ion generation and
separation have received significant and focused attention. The first prevalent MS ionization source, the electron impact (EI) source,
has given way to a variety of other specialized
ionization sources, for example, spark source
(SS), thermal ionization, chemical ionization,
fast-atom bombardment, secondary ion, glow
discharge, inductively coupled plasma (ICP),
ESI, and MALDI. Similarly, a variety of ion
(mass/charge) separators have been developed, including the seminal sector-field or
magnetic MS, electrodynamic techniques (2D quadrupoles, 3-D quadrupole ion traps),
combined magnetic and electrodynamic tech-
© 2005 AMERICAN CHEMICAL SOCIETY
niques (FT ion cyclotron resonance [ICR]),
and drift-tube or TOF devices.
Ion detection technology, by contrast, has received less attention. Traditional analog (Faraday) and electron multiplier (EM) detectors have
been used for decades. Advances in this technology have occurred, but they have been incremental rather than revolutionary. Very few new
detection approaches have been devised. In addition, detector technology is often given brief or
no mention in MS texts and reviews. Thus, it has
remained an area of rather understated importance, despite the fundamental need to better
“see” ions. Only a few groups have recognized
the demand for new MS detectors, and only one
lone special journal issue on MS detectors has appeared (1–4). Some initially promising developments, including work on electro-optical arrays,
never quite took hold in the MS community (5,
6). With continued advances in ionization and
separation techniques and development of evermore-sensitive methods, detector technology
will need to keep pace, and revolutionary new
approaches will be required.
N O V E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
419 A
Considerations and needs
Several varieties of MS detectors currently exist; they vary primarily according to spectrometer design and/or analytical application requirements. The characteristics of an ideal MS detector are given in the box on page 424 A. Certain of these
characteristics are common to all detectors, such as high sensitivity and linear, quantitative response. However, some detectors are designed for very specific functions or applications.
Many MS detectors must be able to perform at very high count
rates (>106 counts/s) with a minimal recovery time. TOF instruments require detectors with very rapid readout and response. Isotope-ratio mass spectrometers require highly stable,
multiple, intercalibrated detectors that count or measure several
ion masses simultaneously to provide extremely high precision
ratio measurements (<0.01% RSD).
Of course, low noise is required for good limits of detection,
sensitivity, accuracy, and precision. Unfortunately, several sources
of noise exist, and they may vary for the different types of detectors. These can arise from within the detector itself, its supporting electronics, the instrument, and the ever-present external
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sources. Attention to electronic design and shielding, simultaneous detection arrays, signal filtering, and time averaging can reduce, but not eliminate, noise. Alas, no detector simultaneously
meets all of the ideal characteristics.
Biological MS demands an ability to uniformly detect ions in
the range 102–105 m/z. With conventional secondary-electron
MS detectors, mass response falls off dramatically for larger ions,
resulting in poorer detection and analytical performance for
many macromolecules. This limitation is particularly severe for
MALDI MS, in which intact, singly charged biomolecules are
often measured. A new type of detector, discussed later, offers
some intriguing promise in this regard.
Another distinction among detector types is the ability to
count single or multiple masses. Many MS detectors are fixedpoint detectors that only monitor single ion masses or sequentially scanned masses. This is akin to watching just one player at
a time in a football game. One sees some action but not the entire field of play. Multiple cameras can provide segmented views
of several players, but nothing beats seeing all the action, all the
time, via a wide-view camera or being in the stadium. A detector
FIGURE 1. Conventional MS detector schematics.
HV, high vacuum; ITMS, ion trap MS; QMS, quadrupole MS; TIMS, thermal ionization MS.
that monitors all ions all the time would maximize data retrieval
and enable enhanced or complete compositional information to
be obtained. Imagine being able to obtain a mass spectral snapshot of all elements and isotopes, 0–250 Da, in a sample with a
single elemental MS run. A focal-plane array detector has this
type of capability (7–9). The earliest such detector was a photoplate located at the MS focal plane. This detector is effectively
obsolete today, but an electronic analogy of this detector would
provide truly remarkable capability for modern MS.
A parallel development in optical spectrometry could provide
inspiration for the development of a modern focal-plane array
detector. For years, the photon detector of choice was the photomultiplier tube (PMT), which was selected on the basis of
spectral response and efficiency. Single PMTs were used in scanning spectrometers, and multiple PMTs were used with dispersive spectrometers. During 1985–1995, more efficient and uniformly responsive CCDs began to replace PMTs in analytical
spectroscopy. Today, CCDs are used almost exclusively to monitor wide spectral regions that cover multiple wavelengths and
analytes (10, 11). Selected, single PMTs thus gave way to com-
mon, versatile CCD-based photodetector arrays. A similar sort of
disruptive technology is needed for MS detectors.
The venerable Faraday and EM detectors remain the most
common. These detectors are reviewed in approximate historical
order. Figure 1 illustrates these detector designs, and Table 1
(found in Supporting Information online) lists their specifications and features.
Photoplate detector
The photoplate MS detector dates back to J. J. Thompson’s
work in which photographs of luminescent screens were first
used to visualize and display mass spectra. Later, ion-sensitive
photoplates were placed in the vacuum region of the MS instrument for direct ion detection. Various designs of the photographic plate concept were used for many years. New photographic plate instruments (e.g., SS MS) were still being produced
into the 1980s, and some are still in use today. The 2 10 in.
Ilford Q2 photoplate was a staple SS MS detector for many years.
Not only was the photographic plate among the earliest ion
detectors, it was and still is the largest ion detector. In typical SS
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421 A
MS instruments, the ions are dispersed along a rather lengthy
focal plane (~25 mm); this provides adequate unit mass resolution
across the entire spectral region. The photoplate thus offers simultaneous detection and signal integration features. Figure 1a illustrates a photoplate with multiple, vertically stacked exposures.
Disadvantages are many, however, including poor-to-moderate
sensitivity, a short linear dynamic range, off-line image development (in a photo lab or darkroom), off-line calibration (with an
optical densitometer), poor precision and quantitation, fragility,
and (these days) a shortage of commercial photoplate suppliers.
Faraday detector
Rugged, easy detection of ions is accomplished by direct charge
(current) measurement with a conducting electrode acting as
input to the detection electronics. Faraday detection was extensively used in the early days of MS. High-aspect-ratio (depth/
width) cup geometries are prevalent (Figure 1b) and serve to
efficiently capture ions while minimizing scattering losses. The
preferred cup lining material is carbon because it produces few
secondary ions. Because MS ion beams can have currents in the
subfemto- to nanoampere range (1 fA = 6242 ions/s), significant electronic amplification (109–1013) is necessary when current is converted to voltage for signal handling by typical electronic circuits. This amplification is achieved with high input
impedance and large feedback resistance, which result in high
gain, increased noise, and slow but stable response. Even with
such high gain, these detectors are relatively insensitive compared with pulse-counting EM mode of detection techniques.
In addition, the Johnson (thermal) noise and shot noise associated with the feedback resistance significantly restrict the limits of detection. Because separate amplifiers have individual resistors, this noise is not reduced by multiple, simultaneous
detection. The high degree of electronic amplification used with such detectors requires particular attention to minimizing electronic noise sources in
the detector’s circuitry.
Today, these detectors are
generally used for isotope-ratio
MS for which high signal stability is
important for precise ratio measurements. Typically, 6–9 matched detectors
are spatially adjusted to monitor a suite of element ratios; the
cups must be stable and intercalibrated. Other uses include monitoring combined high- and low-intensity signals—for example,
the accelerator MS monitoring of a high-concentration spiked or
natural-abundance isotope in the presence of a low-abundance
(<10−12) radioisotope.
EM detectors
The EM or secondary EM (SEM) is the most common ion detector today, with a variety of designs illustrated in Figure 1d–g.
The early SEM detectors were derived from photon detection or
PMT technology. A SEM detector can be thought of as a windowless PMT with an ion-sensitive first dynode. Ions striking
this conversion dynode produce secondary electrons, and subse422 A
A N A LY T I C A L C H E M I S T R Y / N O V E M B E R 1 , 2 0 0 5
quent dynodes generate an electron cascade, effectively multiplying the single incident ion by 106 or more. The degree of multiplication depends on the composition of the individual dynode
surface (work function), acceleration per stage (bias voltage),
number of dynodes (6 –20), and bias current circuit design.
When individual dynodes are used, the device is known as a discrete-dynode SEM (Figure 1d). In “analog” counting mode, the
raw, unamplified detector secondary electron current is processed via a current-to-voltage converting amplifier with subsequent digitization. In addition to Johnson, shot, and flicker
noise, other noise sources are the variation in the number of secondary electrons emitted per charged particle hit, thermionic
emission of electrons from the dynode surface, and EI ionization
of background gases.
Alternatively, the resulting individual electron pulses from each
primary ion are electronically buffered, shaped, and counted as
discrete events in a process known as pulse counting mode detection. The same electronic noise and events occur as in analog
mode detection, but the smaller pulses resulting from thermionic
emission and EI ionization can be discriminated against. The result is highly sensitive single-ion detection capability. The limitation for the pulse counting mode, however, is that its linearity falls
off at >106 counts/s, due to pulse “pile up” effects. Thus, an EM
detector can perform in either a pulse-counting or an analog detection mode; the former is used when sensitivity is at a premium,
the latter when the ion count (or signal) is high.
A single, continuous, electron-emissive dynode can be made
from a resistive film on an insulating surface in the form of a
curved tube or convoluted channel; this dynode can also serve as
an SEM detector. The bias voltage across the insulating surface
produces a continuous accelerating field. Ions striking the surface at the detector entrance produce secondary electrons that,
in turn, cascade down the channel, causing additional electron
emission (Figure 1e). The surface film composition is often proprietary but is formulated with high secondary-emission and
durability characteristics. These types of detectors are called
continuous-dynode EMs (CDEMs) and are compact,
rugged, and available in a wide variety of custom geometries and sizes.
A variant of the CDEM is the multichannel plate (MCP)
detector, in which the amplifying volume is a series of microchannels in a plate or disk-shaped device (Figure 1f). In this
format, the individual channels are a few tens of micrometers in
diameter and a few millimeters in length. An ion strikes the emissive surface near the microchannel entrance and initiates a burst
of electrons, as in the CDEM. The cloud of ions exiting the microchannel can be directed onto an anode for electronic detection of the current or charge pulse, as in other SEMs. Passage of
the emitted ion current to a fluorescent surface is also used for
ion-beam imaging. Two or more MCPs can be stacked to provide additional gains that are typically 102–104/plate. Because of
the very short electron path for these multipliers, very short electron pulse widths (~1 ns) with low jitter are obtained. This makes
the MCP, either individually or as a component in a hybrid device, the detector of choice for TOFMS, which requires precise
arrival times and narrow pulse widths. One disadvantage is that
(a)
(b)
Superconducting film
Inner insulating film
Superconducting film
SC film
SC film
Tunnel barrier
Energy
these plates can be quite fragile and sensitive to environmental conditions.
A novel variation on the
Tunneling
MCP design is the microQuasiparticles
current
sphere plate (MSP), in which
(signal)
~50-µm-diam coated glass
Quasiparticle
beads are fused together as a
tunneling
Incident ion
current
thin, porous plate (12–14).
As with an MCP, a potential
eVbias
Energy gap
is applied across the plate; as
ions strike the face, secondLattice heating;
Cooper
Cooper e– pair breaking
ary electrons are emitted and
pair
B
accelerated into and through
the tortuous interstitial pores,
striking other bead surfaces
along the way and multiplying the electron pulse (Figure 1g). A cloud of electrons
is emitted out of the back FIGURE 2. (a) Schematic and (b) energy-level diagram of an STJ cryodetector.
face of the plate. One advantage of the MSP is that nearly all the emitted secondary electrons mass resolution, for TOFMS. A significant advantage of this apwill be accelerated onto and through the plate because of the proach is that the phosphor screen can also serve as a vacuum
rounded topography of the face presented to the incoming ions. window; this allows the PMT detector to be located outside the
In contrast, MCPs have a significant amount of “dead”, flat sur- MS vacuum chamber. In addition, with a properly designed conface between the microchannels. Ions not striking pores will pro- version dynode, the Daly detector minimizes mass bias effects
duce stray secondary ions that do not fall into a channel and do because photons are what is ultimately detected. It can also be
not get multiplied.
operated in analog or pulse-counting mode.
A common addition to an SEM is a conversion dynode that
EOID. The electro-optical imaging detector (EOID) comcan be separately biased positively or negatively versus the source bines MCP and Daly design principles into a 2-D device (Figure
to attract negative or positive ions, respectively, and direct sec- 1h). An MCP is coupled with a photodiode array or CCD deondary electrons to the actual SEM. At high voltages, this dy- tector; the result provides a useful means for position-sensitive
node decreases the mass bias effect otherwise exhibited by ions imaging, ion-beam profiling, and small-mass-range or focalimpinging directly onto an SEM surface. Conversion dynodes plane detection. With this device, multiple conversion (ion to
are thus most useful for detecting high-m/z organic or biomo- electron to photon to electron) steps are required; these take
lecular ions.
place in differing MCP and photon-detector channels. When the
Characteristics that make the SEM an excellent MS detector EOID was used adjacent to magnetic fields, fringe-field effects
in the current (analog) or pulse-counting modes are the high on the MCP output caused spectral blurring, loss of detector resgain (103–107), relatively low noise (>1 count/s), and relatively olution, and generally poor performance in elemental MS (15).
large linear dynamic range (104–106). SEMs can also be operat- Although EOIDs have been made commercially available, hopes
ed in a dual analog and pulse-counting mode, thereby achieving that this detector would achieve electronic photoplate perforan operative dynamic range of >108. Commercial improvements mance have not yet fully materialized (16, 17).
Image-current detection. Detecting ions that are trapped yet
have produced devices with narrow pulse widths and pulse width
distributions, good storage and operational lifetimes, and many still in motion is possible by observing induced currents on adjasizes and configurations for use in a wide variety of applications cent electrodes or pickup antennas (Figure 1i). In the case of
FTICR MS, the trapped or resonating ions are detected by the
and instrument designs.
image currents they induce on a pair of detection plates (two of the
six sides) on a cubic ICR cell. In a given magnetic field, ions with
Other common detectors
Daly detector. The Daly detector, an early example of an electro- different m/z values orbit at different radii and frequencies, thus
optical ion detector, is a combined ion and photon detection de- generating an rf signal whose frequency is related to the m/z of
vice (Figure 1c). Ions are accelerated toward a high-voltage con- the ion. Because the electronic determination of frequency can be
version dynode, and secondary electrons are accelerated in the very accurate, m/z can be determined with high resolution and
opposite direction toward a scintillation or phosphor screen by high mass accuracy, especially with re-measurement techniques.
the same potential field. A conventional PMT detects the intense These techniques can be used because detection of image currents
photon flashes. The relatively fast response time (narrow pulse is nondestructive (uniquely so among MS detectors) and detected
width for a single ion) produces good time resolution, and thus ions remain in the cell; the ions can thus be re-excited and re-deN O V E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
423 A
The ideal MS detector
Analytical attributes
Unity ion-detection efficiency
tected multiple times
until the desired measurement parameters
Simultaneous detection
are met (18). These deWide mass-range response
tectors can actually
Mass-independent response
measure 10–100 ions
Wide dynamic range
with long acquisition
Fast response
and re-measurement
Short recovery time
techniques, especially
High saturation level
when few other ions
are present. Image-curOperational attributes
rent detection has also
Long life
been used with other
Low maintenance
3-D, Paul-type ion trap
Easy to replace
mass spectrometers,
Low replacement cost
and more recently with
Kingdon trap-type systems (19–23).
We now turn our attention to the new types and configurations of MS detectors currently under investigation by several research groups. A few of these efforts hold significant promise.
High stability
Cryodetectors
Cryogenic detectors operate by detecting lattice excitations in superconducting (SC) thin films. When a particle or ion impinges
onto a surface, energy is deposited; heat is generated; and neutrals, ions, electrons, and photons are sputtered. With a conventional detector at a normal temperature, the temperature rise associated with this process is indiscernible. However, with a small
detector at a very low temperature (<3 K), the instantaneous temperature rise after particle impact can be measured and can provide information about the arrival rate and the ion energy. All incident ions deposit some energy in the form of lattice disruptions
and heat. Thus, the appeal of cryodetectors is that the response is
both 100% efficient and mass-independent, with no fundamental
upper mass limit. In contrast to EM or MCP ionization detectors,
which are markedly less efficient at generating secondary electrons
from larger, complex, biomolecular ions, cryodetectors exhibit no
falloff in response at high mass. The recent interest in using cryodetectors for MS has been strongly motivated by the desire to
improve measurement detection, sensitivity, and uniformity of
response for very large biomolecules (2, 3, 24).
Several types of cryodetectors (also known as calorimetric detectors) have been developed for particle physics and astrophysics
applications. The SC tunnel junction (STJ) type of cryodetector
is discussed here (Figure 2). The device consists of two SC metal
films separated by a thin semiconducting film (the junction). In
typical metal films held at low temperatures, most valence electrons are weakly tethered as Cooper pairs, which are electrons that
are spatially paired in the metal lattice. The two electrons, normally mutually repulsive, have a small attraction brought about by
traveling positive charges generated as a result of lattice vibrations.
This positive charge acts as a glue for the two ever-moving electrons, effectively binding them in a lower-energy, SC state.
When a pulse of energy is deposited in the film, some Cooper pairs are broken, and this allows the electrons to move more
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freely. This results in electronic excitations known as quasiparticle excitations or, more intuitively, in the generation of quasiparticles. Quasiparticles can quantum-mechanically tunnel through
the insulating barrier that separates the two SC films. The tunneling current, although small, can be accurately measured. The
physics community has used this effect for several decades to detect particles and photons (2). More recently, several groups have
pursued the use of STJ devices as ion detectors for TOFMS. As
Figure 2 indicates, an ion incident upon the upper (left) SC film
will deposit energy, causing lattice vibrations, heating, and Cooper-pair breaking. The energy released causes tunneling current
to flow to the lower (right) SC film. In this case, the pulse is correlated to the ion arrival time (and thus to m/z); measurement
of pulse height can be used to distinguish ion energy and, hence,
charge state or other features. Cooper pairs are subsequently reformed in the SC film on the back side.
Although cryodetectors were used for neutral, molecularbeam spectroscopy and diagnostics many years ago, Twerenbold’s 1996 suggestion sparked renewed interest in applying
these detectors to MS (3). Twerenbold clearly recognized the
sensitivity and mass-range
limitations of then-current
TOFMS instruments, particularly for biological MS.
(a)
110
90
70
50
30
10
25
50
75
125
150
100
m/z (kDa)
175
200
(b)
20
Energy (keV)
Low or no noise
M3+
15
M2+
10
M+
0
25
50
75
100
125 150
m/z (kDa)
175
200
FIGURE 3. MS detection of human IgG on a MALDI TOFMS with an
STJ cryodetector.
(a) Detection of 1 fmol IgG (150 kDa; 1:1 with sinapinic acid). (b) Ion energy
vs mass plot showing M+, M2+, and M3+ at 5, 10, and 15 keV, respectively.
(Courtesy of www.comet.ch/index.php?id=270.)
Reset
Field-effect transistor
C = 10 fF
+
He proposed using cryodetector technology as the
basis for a cryogenic mass spectrometer to sequence
DNA, although proteomics appears to be the more
V in
Amp
topical area now. An empirical confirmation of the
utility of an STJ detector used with MALDI TOFMS
followed shortly—it exhibited a high detection efficiency for 14-kDa ions (sinapinic acid matrix,
lysozyme clusters) compared with EM detection (25) .
128-channel
Other researchers confirmed these encouraging reamplifier – multiplexer
Electrodes
50-µm pitch
sults when they used MALDI TOFMS and a different
type of STJ detector to measure human serum albumin (66 kDa; 26). The STJ’s energy resolution was
successfully able to distinguish singly and doubly
charged albumin ions. This unique and inherent ionenergy measurement feature also offered new insights
into vaporization and ionization processes. Twerenbold et al. have recently compared the relative responses of traditional EM detectors with an STJ detector in proteomic applications; the STJ detector
performed better than the EM detectors (27).
On the basis of these demonstrations, a MALDI
TOF instrument featuring cryodetector technology,
called the “macromizer”, became commercially available in 2002. The macromizer offers near-uniform,
femtomolar sensitivity for ions with masses as high as
400 kDa. The detector is composed of a 16-element FIGURE 4. Integrated 128-channel focal-plane MS detector prototype.
STJ array that operates at 0.3 K. The STJ array allows (bottom) Electronics and idel/multiplexer mounting. (upper right) Micrograph of a portion of the
individual STJs to remain small (~100 µm2) to enable ion-detection element array. (upper left) Schematic of the amplifier and multiplexer channels.
single-ion detection on any array element (the total V in is input voltage.
detector area is ~1 mm2). The detector cryostat is
based on closed-circuit, regenerative 3He cryosorption technol- storage capacity, and an abundance of ions is usually deleterious to
ogy, which is controlled automatically and can operate for 20 h detecting trace-level ions; this effectively negates the simultaneous
between regeneration cycles. It can detect 1 fmol of IgG (150 detection capability. Current multicollector MS instruments, used
kDa); differentiate charge-state information from ion-energy for isotope-ratio determinations in the geological and nuclear
measurements (Figure 3); and measure megadalton-sized, singly fields, generally offer <9 discrete detectors. For wide spectral covcharged ions of immunoglobulin M as well as von Willebrand erage, only the focal-plane detection concept is practical, in which
electrostatic and magnetic fields spatially separate ions across a
factor proteins and their multimers (28).
plane. Our research group is endeavoring to develop an all-electronic equivalent of the photoplate focal-plane detector.
Focal-plane array detectors
The concept of an electronic photoplate is certainly not new
Simultaneous detection, which has several advantages, remains a
holy grail of analytical chemistry in general and MS in particular. (5). The Birkinshaw and Langstaff group conceived and develSimultaneous detection maximizes data retrieval and data quali- oped a focal-plane detector system made of an MCP and an
ty; noise is reduced or eliminated by using ratioing techniques. anode array (4, 29–32). The anode array was a completely chipDiscrete signals from small samples (e.g., particles, laser ablation based complementary metal oxide semiconductor (CMOS) depulses, GC peaks) can be measured without the peak skew anom- vice composed of 18-µm-wide aluminum detection strips and
alies that occur with sequential detection. Many MS techniques circuitry to sense and count current pulses emanating from the
approach simultaneous detection by using selective detection ~12-µm-wide channels of the MCP. Acceptable noise, sensitivity,
(i.e., discrete, multiple ion detectors at fixed or restricted posi- and resolution were obtained, but the disadvantages associated
tions), total ion accumulation (i.e., ion traps, FTICR MS), or fast with an MCP (count-rate limitations, blooming) are also present
scanning and acquisition (i.e., TOFMS), but these methods only in this type of detector system.
Sinha and Wadsworth at the Jet Propulsion Laboratory (JPL)
approximate simultaneous detection.
Of the detector systems already discussed, the only truly simul- have recently reported a miniature MS system that incorporates
taneous detectors are the photoplate, multicollectors, and image- a CCD-based array detector in which the normal photon-senscurrent detectors. Although it is capable of simultaneous detec- ing elements of the CCD are replaced by MOS capacitors that
tion, the image-current detector is compromised by the cell used directly detect ions (33). The detector uses “fill-and-spill” electo capture the ion current. The FTICR MS cell has a finite ion tronics to collect, amplify, and measure the accumulated charge.
N O V E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
425 A
(a)
(b)
160,000
Signal (A/D unit)
Signal (A/D unit)
60,000
40,000
20,000
120,000
80,000
40,000
0
91 92 93 94 95 96 97 98 99 100 101
0
m /z
141
Bad idels
m/z
Signal (A/D unit)
147
90,000
153
159
50,000
165
10,000
171
91 92 93 94 95 96 97 98 99 100 101
m /z
50
100
150
200
Retention time (s)
250
300
FIGURE 5. Focal-plane MS detector results.
(a) Comparison of ICP mass spectra of molybdenum taken with (upper) 32-channel (~3 idels per m/z ) and (lower) 128-channel (10.5 idels per m /z ) detectors.
(b) Reconstructed ion chromatogram and 3-D graph of separation of lanthanide elements. The graph was obtained by combining the results of two separations
with the 32-channel detector.
The researchers ambitiously predict sensitivity of ~5 ions for a
fully prototyped detector. Another miniaturized MS-detector
system, akin to the JPL design, was developed for atmospheric
monitoring applications (34, 35) and is now available from OI
Corp. Fuerstenau, also at JPL, has advocated active pixel sensor
technology, which uses CMOS imaging detectors that have had
their photodiode inputs replaced with simple charge-collecting
metal strips (36). The CMOS approach differs from CCD technology in that each pixel has self-contained amplifiers in the circuitry. But with as many amplifiers as pixels in the device, the determination and control of interpixel gain and offset become
problematic. In addition, the readout times for such devices are
such that only quasi-simultaneous detection is possible.
In our laboratory, we took a “something old, something
new” approach. As in several of the efforts described earlier, we
used direct charge-sensing elements, or Faraday collection strips.
These are referred to as ion detection elements, or “idels”, which
are analogous to pixels or voxels. The new twist is to couple these
charge detectors with modern multiplexer-based electronics developed for optical astronomy. The main feature of our approach
is direct charge amplification by dedicated capacitive transimpedance amplifiers (CTIA) connected to individual, independently
addressable channels of the multiplexer, which selectively transfers the output of the individual CTIAs to an A/D converter for
recording. Incoming ions impact an electrode surface, imparting
their charge to it, and the resulting voltage appears across the
feedback capacitor, which is matched by the output of the CTIA
(Figure 4). This output voltage can then be switched to the A/D
for a single conversion with a read noise of ~10 electrons followed by a reset of the accumulated charge to 0, or for multiple
conversions for better precision. The multiplexer allows independent selection of any channel(s) for conversion and reset.
Thus, channels receiving a more intense beam of ions can be
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singly read and reset often enough to avoid saturation, and channels receiving low-intensity beams can be integrated for longer
periods and read multiple times before they are reset. The result
is a very flexible and fully addressable readout system that can be
operated in either destructive or nondestructive mode.
A first prototype detector was fabricated with a 31-channel,
commercially available CTIA device. Bonding to the CTIA chip
dictated idel geometry and dimensions; thus, rather large idel
widths of 145 µm were used (37 ). This first device successfully
detected ions from both glow-discharge and ICP elemental ion
sources (38, 39). The current-generation prototype has 128
CTIAs and the multiplexer on the same chip, coupled to an idel
array composed of narrower, 50-µm-wide charge-collection strips
and staggered 65-µm-wide bonding pads (Figure 4, upper right).
The performance characteristics of this type of array ion detector with an ICP source in a custom Mattauch–Herzog MS instrument are detection limits of 10–100 ppq (10 –100 fg), dynamic range of at least 7 orders of magnitude, isotope-ratio
accuracy of 5% error, and precision of 0.007% RSD. The detection limits and dynamic range are similar to those of an SEM on
the same instrument, whereas the isotope-ratio accuracy and precision are better. However, with our prototype, all the ion beams
that strike the detector array are detected simultaneously, whereas those in the SEM are sequentially scanned across the single detector. Simultaneous detection of molybdenum isotopes is illustrated in Figure 5a for the 32- and 128-channel devices. Note
that the 128-channel (50-µm) device exhibits improved spectral
fidelity and peak shape compared with the 32-channel (145-µm)
device. Both devices are necessarily small-mass-range prototypes
at this stage of development.
Ultimately, we are interested in capturing an electronic snapshot of all elements and isotopes in the range 2–250 Da/charge
by using an idel array of 1024– 2048 channels with 4– 8 channels
per isotope. The application of this concept
has already been demonstrated for transient
and separated samples, such as individual
laser ablation pulses, GC peaks, ion chromatography peaks (Figure 5b), and LC peaks
—thus, the full elemental or isotopic mass composition of very small, discrete, or time-resolved samples can eventually be ascertained (40 –42). Other applications include biological MS detection and ion mobility spectrometry
(43). The motto for this detector technology is “all the signal, all
the time”. It is anticipated that many, if not all, of the enormous
benefits that array detection has brought to optical spectroscopy
will also occur in MS; however, full utilization of this approach
may also require development of a new generation of static, as opposed to dynamic, dispersion mass spectrometers. We hope that a
final prototype for this detector technology will be available soon.
Recent developments in MS technology, coupled with ever
more stringent application needs, are demanding modern, higher-performance ion detectors. Simultaneous, mass-independent,
and single-ion detection are still the ideal (vs practical) characteristics of most currently used detectors. Innovations are close,
however, and new concepts for ion detection are on the horizon.
With sharper vision, MS will soon see the forest, the trees, and
all the leaves.
Support for this work was provided by the U.S. Department of Energy (DOE),
Office of Nonproliferation Research and Engineering. Pacific Northwest National Laboratory (PNNL) is operated by the Battelle Memorial Institute for
DOE under contract DE-AC06-76RLO-1830. Los Alamos National Laboratory
(LANL) is operated by the University of California under DOE contract W7405-ENG-36.
David W. Koppenaal is a chief scientist and Charles J. Barinaga is a
senior research scientist at PNNL. Koppenaal’s research interests
include atomic MS, MS instrumentation and detectors, and metallomics. Barinaga’s interests include innovation and development of
analytical instrumentation, especially for elemental and isotopic
analysis. M. Bonner Denton is a professor at the University of Arizona. His long-term research focuses on optical and MS detectors,
laboratory automation, ion mobility spectrometry (IMS), and chemical
forensics. Roger P. Sperline is a senior research scientist in Denton’s
research group. His research interests include trace level detection,
IMS, MS, vibrational spectroscopy, statistical analysis, and mathematical modeling. Gary M. Hieftje is a professor at Indiana University.
His research interests encompass atomic spectrometry, MS, chemical instrumentation, spectrochemical analysis, fiber-optic sensors,
IMS, and correlation spectroscopy. Francisco J. Andrade is a postdoc
and Gregory D. Schilling is a graduate student in Hieftje’s group. Andrade’s current research interests include atomic MS and the study
and applications of atmospheric-pressure glow discharge sources.
Schilling’s research interests include development and characterization of new instrumentation for atomic MS. James H. Barnes, IV, is a
postdoc at LANL and is currently investigating novel ionization sources
and instrumentation for DOE-related applications. Address correspondence about this article to Koppenaal at PNNL, P.O. Box 999,
Richland, WA 99352 (david.koppenaal@pnl.gov).
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