Hot laminar flow sampling of droplets and ions using low and high flow rates
for improved S/N LC/MS
Ilia Tomski, Gholamreza Javahery, Charles Jolliffe, Serguei Savtchenko, and
Lisa M. Cousins
IONICS Mass Spectrometry Group
The electrospray ion source is arguably the most complex region in liquidchromatography mass spectrometry. It is the source of both the high sensitivity and also the
chemical noise of a mass spectrometry. The ionization and sampling efficiency into the mass
spectrometer is still very low (much less than 0.1%) and thus there is still much need for
improvement.
ESI gently and efficiently forms adduct ions of organic molecules from solution, with
little fragmentation. Combined with mass spectrometry, it has become extremely useful in the
analysis of drug toxicity and efficacy in animals and humans, pesticide residues in food, and
cancer biomarkers in cells. In most cases liquid flow rates are sufficiently high (by mass
spectrometry standards) that assistance by gas nebulization and heat is required for efficient ion
formation. In ESI, ions are initially formed in solution, and then ejected into the gas phase at
atmospheric pressure, but still “solvated” in droplets. The droplets undergo sequential
evaporation and eventually eject single molecular ions. This process is called desolvation, i.e.
the formation of single molecules from clusters or droplets of molecules. Desolvation at
atmospheric pressure is a complex phenomenon, due to competition between internal energy
dissipation, weak energy pickup in external fields, and the large number of energy-dissipating
collisions with air molecules. This is especially true under the complex conditions of high flow
rate. Molecules in the droplets typically do not completely desolvate (Fig.1), limiting the
number of ions available to be focused into the mass spectrometer and detected (i.e., the
sensitivity).
Figure 1. Desolvation in ESI LC/MS.
Here, we discuss a new approach to sampling ions from atmosphere that improves the
sampling efficiency and reduces chemical noise. Ions are entrained in hot gas and are
orthogonally extracted using a combination of the influences of flow dynamics and electric
fields, and they experience multiple bends prior to extraction into the mass spectrometer. This
approach is suitable for all ranges of electrospray flow, from nanoflow up to several mL/min.
The cross section of such a device is shown in Fig. 2
Figure 2. Cross section of ion sampling
device (called IONICS HSID for Hot Source
Induced Desolvation).
Using a combination of the influences of flow dynamics and electric fields, ions are
orthogonally extracted and transported through various channels and bends entrained in a hot
laminar flow of gas. Efficient desolvation is accomplished via energy transfer from the hot gas as
the ions flow through multiple flow regions. Ions are then extracted at 90o into the low pressure
region of the mass spectrometer. This configuration yields excellent signal stability, substantial
reduction of chemical background and carryover, and excellent long term stability, significantly
improving the continuous operation of the mass spectrometer at high flow rate. Qualitatively, the
gas flow undergoes numerous transitions, from supersonic to shock to turbulent to laminar. The
exact positions of these boundaries, and their impact on desolvation and ion formation, are very
difficult to calculate.
Importantly, a laminar flow interface can yield substantial improvements over a
molecular beam interface for LC/MS/MS. The flow rate through the orifice from atmosphere to
vacuum is commonly given by
Go  0.445no ao Do
2
where no is the number density at the source, ao is the speed of sound Do is the orifice diameter.
For both laminar flow and a free jet expansion, the flow rate Gs into the mass spectrometer is
given by
Gs  ns ao [
Ds 2
]
2
where Ds is the skimmer diameter, ao is the speed of sound, and ns is the density at the skimmer.
In a free jet, the orifice can be opened in order to obtain more sensitivity; however, to gain
substantially more sensitivity in the mass spectrometer, the skimmer diameter is typically
increased to match the geometry of the free jet. Therefore the load Gs into the mass spectrometer
increases, and more turbomolecular pumping in the mass spectrometer is required, incurring
much more expense.
On the other hand, optimizing the geometry and flow in the laminar flow region can yield
substantial mass spectrometer gains. The flow in the laminar flow region is given by
Go  nl vl Al
Go is the flow from atmosphere to vacuum (as above); nl is the downstream number density; vl is
the flow velocity; Al is the cross-sectional area of the flow tube. Therefore
vl 
Go
nl Al
By adjusting parameters such as geometry and gas flow velocity, the ion velocity and flux into
the mass spectrometer can be optimized to yield higher sensitivity. The neutral gas flow into the
mass spectrometer (and corresponding high vacuum pump speed) does not need to be increased.
For example, below are shown MS/MS results for reserpine, monitoring the 609 →195
transition of reserpine in low flow infusion, for three flow velocities. Here the laminar flow
source temperature was set to 300C. It is evident that as the velocity is increased from 10 to 30
to 40 m/s, the signal level increases from about 3 e6 (10 scans) to 6 e6 (10 scans) to 1.6 e7 cps (10
scans).
a)
b)
c)
Figure 2. Monitoring the MS/MS transition for 609→195 of reserpine, as a function of flow
velocity. a) 10 m/s; b) 30 m/s; c) 40 m/s.
Results will be discussed for high and low flow ESI, including some experimental
results to demonstrate performance gains, numerical modeling results to describe the gas flow,
and recent results on using such a device for multiple sprayers for high flow.
FAIMS SELECTIVITY IN BIOANALYSIS
James Kapron, Thermo Electron, Ottawa, ON, Canada K1A 0R6
This presentation will focus on three aspects of FAIMS technology. The “early expressions of
FAIMS technology” section will describe the physical placement of the FAIMS interface and
compare FAIMS with the traditional atmospheric pressure ionization “cluster buster” interface.
A brief description of ion mobility will be presented, in which this technique will be compared
and contrasted with FAIMS. Also to be discussed are the two types of FAIMS electrodes and the
waveform that is critical to the FAIMS effect. The section on applications to drug discovery will
provide examples of the experiments in which FAIMS has proved useful. Beginning with mass
spectral filtration and proceeding to chromatographic detection improvements, the selected ion
monitoring (SIM) and selected reaction monitoring (SRM, MRM) experiments combined with
FAIMS will be highlighted. Also in this section, a description of when to use FAIMS in the daily
workflow of method development will be offered for assessment. For example, chromatographic
interferences due to matrix components may be removed by the addition of FAIMS to the
method. Small drug molecules between molecular weight 150 – 600 Da will be used as
examples. Data from the analysis of prostaglandins in real-world samples will be presented. The
use of FAIMS in one case allowed the elimination of chromatography during the analysis of
acetaminophen for drug discovery purposes. Finally, examples from the combination of FAIMS
with highly-selective reaction monitoring (H-SRM) will be described. The talk will end with a
discussion of future directions.
James Kapron obtained his Ph.D. from the University of Alberta. He has extensive experience in
the application of novel technologies to solve real-world bioanalytical problems.
.