Transversal Modulation Ion Mobility Spectrometry (TM-IMS), A New

Aug 27, 2012 - Correlating Resolving Power, Resolution, and Collision Cross Section: Unifying Cross-Platform Assessment of Separation Efficiency in Io...
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Transversal Modulation Ion Mobility Spectrometry (TM-IMS), A New Mobility Filter Overcoming Turbulence Related Limitations G. Vidal-de-Miguel,*,†,‡ M. Macía,† and J. Cuevas† †

SEADM S.L., Valladolid, Spain Valladolid University, Energy and Fluid Mechanics Engineering Department, Valladolid, Spain

‡

S Supporting Information *

ABSTRACT: The analysis of ions according to their mobility is a technique that is attracting increasing interest. The new technology presented here, which we have termed Transversal Modulation Ion Mobility Spectrometry (TM-IMS), utilizes only electric fields, operates at atmospheric pressure, produces a continuous output of mobility selected ions (according to their true mobility and not to nonlinear effects), and has a very accessible inlet and outlet. These features would make it an ideal choice for tandem IMS-MS analysis in combination with most commercial Atmospheric Pressure Interface MS (APIMS) systems. We modeled and evaluated two different TM-IMS configurations (TM-IMS, and multistage TM-IMS), and we concluded that the most promising configuration would be a two-stage TM-IMS. We developed and tested a TM-IMS, and the measured resolving power is R = 55. The TM-IMS behaves similarly to the planar Differential Mobility Analyzer, but the TMIMS utilizes only electric fields, and no fragile flow with high Reynolds numbers is required. We tested the robustness of the TMIMS, which proves to be a very robust and reliable analyzer: the required voltage accuracy is 5 V in 10 kV, and the mechanical precision is 1 mm in 5 cm.

I

source is also pulsed, but it usually hinders transmission and complicates the interfaces in hyphenated schemes and with other continuous ion sources such as electro-spray (ESI). Overtone Mobility Spectrometry. (OMS)17−20 utilizes on-axis frequency modulated electric fields and solves this problem with an almost continuous output (with a duty cycle of 50%). Traveling Wave IMS. (TW-IMS): the success of the first commercially available IMS-MS system (Waters Synapt12,21−28) indicates the increasing interest in the IMS-MS approach. TWIMS separation mechanism allows true mobility separation, but in practice it also produces pulsed packets of ions and, what is more serious, the reliability of the structural information obtained is unclear because (i) in the intense electrical fields required, ion heating can have a significant effect29 and (ii) drift time is related to the mobility in a complicated way which is still not completely understood.30,31 Trapped IMS. (T-IMS)32 combines an on-axis electric field opposed by a gas flow, and a transversal confinement produced by an RF ion-funnel-like electric field. It also allows the ions to be trapped in different axial positions according to their mobility, and to be later sequentially released for further analysis and detection.

n this paper, we present and demonstrate the viability of a new Ion Mobility Spectrometry technique that we have termed Transversal Modulation Ion Mobility Spectrometry (TM-IMS), the operating principles of which are explained in patent.1 Selection and analysis of ions and charged particles by virtue of their electrical mobility K (defined as the ratio of electric velocity to electric field) is useful for many applications including the detection of explosives, and pharmacologic, environmental and biological analysis.2 Ion Mobility Spectrometry followed by Mass Spectrometry (IMS-MS) analysis is an emerging and very powerful technique that provides extra structural information, and an increased resolving power, both these features being very useful in the fields of -omic and biological analysis, as shown by different studies.3−7 Drift Time IMS. (DT-IMS)8 is one of the best known mobility techniques, perhaps because of its simplicity, robustness, speed, and relatively small size and power consumption. DT-IMS are mostly used for military and security purposes,2 although they are also used in other industries as well as in many new areas of research including proteomics and structural biology.9−12 The resolving power (R) is mainly limited by Brownian diffusion; classic DT-IMS can reach R = 100, but their sensitivity is limited by a low duty cycle. Nevertheless, their transmission can be improved by the use of ion funnels,13 multiplexing and ion accumulation.14 Resolving powers higher than 300, and approaching 400, were achieved with the socalled High-Resolution Ion Cyclotron Mobility.15,16 The pulsed input and output of DT-IMS might be advantageous if the ion © 2012 American Chemical Society

Received: June 2, 2012 Accepted: August 27, 2012 Published: August 27, 2012 7831

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Field Asymmetric IMS. (FAIMS),33−36 more recently termed Differential Mobility Spectrometry (DMS), is an alternative and robust technique that separates ions in space rather than in time, thus producing a continuous flow of selected ions with a 100% duty cycle. FAIMS cells no bigger than a dime37,38 show its miniaturization potential. FAIMS separates ions according to nonlinearities in the mobility arising in strong fields,39−41 and traditionally produced relatively poor resolving powers (near 20). Nevertheless, recent developments42−45 have shown that the separation capability can be dramatically increased by adding polar dopants to the drifting gas. Shvartsburg and Smith46 also reached resolving powers exceeding 200 by increasing the time of residence of ions within the filter. The new generation of DMS-MS commercialized as SelexIon is a powerful tool to reduce background levels,47 and allows mobility selection before ions pass through the Atmospheric Pressure Ionization (API) interface,48 which permits the incorporation of the IMS by a relatively simple upgrade of the MS (if compared with TW-IMS and TIMS that require low pressures), but it does not provide clearly interpretable structural information. Differential Mobility Analysis. (DMA) provides true mobility analysis, and also produces a continuous output of mobility-selected ions. Cylindrical DMAs are widely used for the analysis of submicrometer aerosols.49 Later improvements allowed ion analysis,50−53 and even DMA-MS analysis,54−56 although with very poor transmission. Planar DMAs57 permit coupling with virtually any API-MS57 and provide an improved transmission of ions. In addition, because planar DMAs operate at ambient pressure, mobility is measured at moderated ionic temperatures with little fragmentation, which makes structural interpretation of the data easier.58−61 However, DMAs require a flow with high speed and high Reynolds numbers (Re) that is prone to turbulence.62 The relatively high resolving power (above 100) measured by Martinez Lozano et al.53 is not easy to reproduce because it is strongly limited by tiny and difficult to control mechanical defects and deformations that easily arise at the pressure gradients produced by the high velocity flow, and which initiate turbulence. These effects can be minimized by using very rigid structures, but as a consequence, DMAs become rather bulky. The pump required to produce the fast fluid flow also hinders miniaturization and limits the operating temperature. Moreover, compressibility effects and sound waves traveling upstream through the DMA channel also reduce the resolving power achievable.52 In this article, we present a new technology named Transversal Modulation Ion Mobility Spectrometry (TMIMS), based on a previous patent,1 which separates ions according to their true mobility using only electric fields. The selected ions coalesce at the analyzer outlet, while other ions are deflected away and not transferred. Ions are separated in space and thus a continuous flow of filtered ions with a narrow range of selected mobility ions is produced, as in Differential Mobility Analyzers (DMAs); yet no high fluid velocity field is required, thus avoiding the limitations in DMAs associated with flow unsteadiness, compressibility, and turbulent transition. In the first part of this study, we explore theoretically the expected performances of a TM-IMS and a multistage TM-IMS configuration that utilizes two stages of TM-IMS coupled and synchronized. We also developed and tested experimentally a first prototype based on the single TM-IMS stage configuration. The results of these tests are presented in the second part of this study.

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THEORY TM-IMS technology is based on simple principles already explained in the previous patent1 and summarized here: ions are pushed by an axial electric field at a velocity proportional to their mobility and travel through the TM-IMS during a time inversely proportional to their mobility. When this time resonates with the period of the transversal (here termed deflector) electric field, the trajectories of the selected ions are brought away from the central axis during half the cycle of the deflector electric field, and then they are brought back to the central axis during the reminding of the cycle, and they coalesce at the analyzer outlet, while other ions having different mobilities do not. Figure 1a illustrates schematically the

Figure 1. (a, left) Different types of trajectories of ions through the TM-IMS: ions with the selected mobility (top), overspeeding ions (middle), and lagging ions (bottom). Figure 1b (right) illustrates the theoretical transmission of an ion as a function of the frequency of the deflector electric field. The curves s(ω) represent the transmission of a single stage TM-IMS, while the curve s2(ω) represents the transmission of a two stage TM-IMS.

different trajectories of ions in a TM-IMS including two parallel electrodes (responsible for the axial steady electric field) with aligned inlets and outlets, and two deflector electrodes (responsible for the deflector electric field). There are three types of behavior: (i) the selected ions, whose trajectories coalesce at the outlet; (ii) overspeeding ions with higher mobility than the selected ones; and (iii) lagging ions with lower mobility than the selected ones. The TM-IMS includes two parallel electrodes (an inlet electrode and an outlet electrode), separated at a distance l, that produce an axial steady electric field E0, and two deflector electrodes that produce a transversal and oscillating electric field E1, termed deflector electric field. Ions are continuously introduced into the device through an inlet slit and, after following curved trajectories, only ions of the selected mobility reach the aligned outlet slit and are continuously transmitted. Let us suppose the simplified ideal case for which the electric fields are uniform and the components of the ionic velocity are: u = KE0 and v = KE1 sin(Ωt). The trajectories of the ions entering through the inlet slit can be easily integrated analytically, and their distance to the axis Y when they reach the x = l axial coordinate would be Y=2

KE1 ⎛ Ωl ⎞ ⎛ Ωl ⎞ ·sin⎜ ⎟ ⎟ ·sin⎜Ωt − Ω 2KE0 ⎠ ⎝ 2KE0 ⎠ ⎝

(1)

For the selected ions, Y (of eq 1) equals zero continuously (first sin equals zero), and a continuous output is produced. Note, however, that each type of ions produce multiple peaks (for the 7832

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different function roots of the first sin), and that other ions also reach the outlet slit periodically (when the second sin equals zero) and produce a pulsed output. Two main factors limit the resolving power of the TM-IMS: diffusion, that broadens and mixes the ion trajectories, and the ionic flow (that accounts for the volumetric flow of ions, which move dragged by both the fluid flow and by the electric fields, inputted and outputted through the slits, and which is measured in lpm). To estimate the effect of diffusion, we assume that ions are ideally inputted through an infinitesimally thin stream-tube through the inlet slit, and hypothesize that the instantaneous concentration of ions at the outlet plate is a Gaussian distribution centered in Y (of eq 4) and with a variance σr2 = 2Dτ (where D is the diffusion coefficient, that can be related to the ion mobility through Einstein’s relation, and τ = l/(KE0)).63 The averaged signal produced by an ion of known mobility can be calculated by numerically integrating the instantaneous signal over time, and the resolving power can by estimated by reconstructing a peak and applying the full width at half height (fwhh) algorithm to it. Numerical results show that the timeaveraged resolving power of the TM-IMS is RD ≃ 0.187

E1 E0

V0e kBT

will eventually dominate. Finally, combining eqs 2 and 4, we see that, at the point where its effect is comparable to that of Brownian diffusion, the maximum flow of ions (per unit of slit length) that the TM-IMS can handle is q = 3.77·K VkBT /e

To give an example of the expected performance, a TM-IMS working at V0 = 10 kV, and with E1/E0 = 1, would have a resolving power of R = 117, and would be able to handle an ion flow rate of 0.35 lpm per mm of slit length (for K = 1 cm2/sV, and at room temperature and pressure). These performances could be comparable to that produced by other technologies. However, the TM-IMS also produces an averaged nonzero signal for undesired ions that constitutes a disadvantage. Hopefully, this undesired signal can be drastically reduced by using two stages. Multistage TM-IMS: By using two equal stages of TM-IMS with an offset of 90° in their respective oscillating fields, the pulses produced in the first stage would be synchronized with the trajectories for which Y reaches its maximum value in the second stage, so that they would not be transferred. Ions would be transmitted only when the offsetted pulses produced by each stage overlap. The averaged function s2 corresponding to the overlapping pulses can be easily estimated as a function of s (see Supporting Information Appendix I). The resolving power of a two-stage scheme was also computed numerically, turning out to be 30% higher than the one computed for each single stage. Figure 1b also represents the function s2(ω), showing that the pulsed noise is drastically reduced. Two stages of TM-IMS would together deliver a very high resolving power (limited only by the high voltage power supplies available) with an acceptable ionic sample flow rate, and a duty cycle of 100% that could potentially lead to a theoretical transmission of 100%, and this without the undesired pulsed signals. For these reasons, the Multistage configuration seems to be the most promising. To test the validity of the proposed ideas, we developed a first prototype based on a single stage TM-IMS, and studied its behavior.

(2)

where V0 is the axial voltage between the inlet and outlet electrodes, and the parameters kB, T, and e are the Boltzmann’s constant, the absolute temperature of the gas, and the charge of the ion, respectively, that appear in Einstein’s relation. It is interesting to note here that the resolving power improves with the square root of the applied voltage (V = lE0) as in DT-IMS and DMA, and with the ratio ε = E1/E0. For more details, Supporting Information Appendix I explains the origin of this equation. The ionic flow is here defined as the volumetric flow of ions that are handled by the TM-IMS. It has to be sufficiently high to cope with most commercial MS that typically sample 0.5 to 3 lpm. Basically, the width of the stream tube of ions r0 is related to the ionic flow per length of slits q through the expression q = 2r0KE0, and ions will periodically reach the TM-IMS outlet when |Y| < r0. The pulsed signal can be characterized by a Pulse Repetition Interval (PRI) and a Pulse Width (PW), that can be easily estimated by combining eq 1 and the inequation |Y| < r0. The averaged gain of transferred ions is computed as s = PW/ PRI. Finally, after rearranging equations, the function s can be expressed in terms of the dimensionless parameters k = (l/ r0)(E1/E0))−1 and ω = (Ωl2)/(2KV0) = (Ωl)/(2KE0) yielding s=

⎛ ω ⎞ 2 arcsin⎜k−1 ⎟ π ⎝ sin(ω) ⎠

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METHODS The TM-IMS is composed of an insulator box housing the inlet, outlet and deflector electrodes. Figure 2 shows schematically the architecture of the TM-IMS. The axial voltage (also termed inlet electrode voltage) is supplied with an Applied Kilovolts high voltage amplifier, while the outlet electrode is grounded. Deflector electrode voltages are supplied by two Matsusada high voltage and high speed amplifiers. Ions are produced by means of a Methanol-THABr (Tetra Heptil Ammonium Bromide) electrospray that produces desolvated THA+ ions with mobilities around K = 1 cm2/sV,64 which faces the inlet slit, housed in a leak-proof chamber, and which is commonly used to calibrate atmospheric pressure operated DMA.64 A counterflow gas exits through the inlet slit so as to prevent droplets from entering the analyzer. Ions reaching the outlet slit are carried by means of an outlet flow toward a faraday cup electrometer. The required gas is introduced into the TM-IMS chamber, after passing through a flow meter and a valve, through two lateral inlets equipped with laminarizing meshes designed to prevent turbulence within the TM-IMS chamber that would otherwise disrupt the ion trajectories. Two flow meters and valves placed downstream of the electrospray

(3)

Figure 1b illustrates the function s(ω) for two different values of k, showing the first three peaks corresponding to the first three resonances (for ω = n·π; n = 1, 2, 3, ...). Resolving power for the first peak appearing in figure 1b was numerically computed using the fwhh algorithm and for different values of k, showing that it is linear with k: R Q ≈ 0.353·k = 0.353

E1 2V0Z E0 q

(5)

(4)

At low ionic flows, the resolving power will be limited by Brownian diffusion, but, as the ionic flow is increased, its affect 7833

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and high voltage amplifiers that are responsible for the deflector electrodes. In operation, the Software (SW) commands the signal generator, scans over the frequency, reads the electrometer signal, and plots the resulting spectra, while the operator sets the offsets manually. Safety Considerations. High voltages are applied to the TM-IMS and the electrospray. Care must be taken to avoid electric discharges and injury.

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RESULTS AND DISCUSSION This first prototype achieves resolving powers as high as 55, comparable to the resolving power achieved with high Re DMA that has been developed over the last fifteen years. Figure 3

Figure 2. Schematic illustration of the TM-IMS including an ESI source, an inlet electrode with an inlet slit, the deflector electrodes, the outlet electrode with the outlet slit, and the architecture of the electronics used to control the voltages of the TM-IMS and measure the output of ions.

Figure 3. Resulting spectrum measured with the TM-IMS alone, with the first three resonances evidenced by three equally spaced peaks.

chamber and the electrometer allow us to control and measure the counterflow and the electrometer outlet flows, as well as the pressure inside the TM-IMS. The maximum axial voltage is 10 kV; this value was selected because it is relatively easy to manipulate. According to eqs 2 and 5, this voltage should produce RD = 117 (for E1/E0 = 1 and at room temperature and pressure) and should allow us to handle an ionic flow rate of approximately 3 lpm for a slit 1 cm long. The distance between the inlet and the outlet electrodes is 7 cm, and the time of residence of the ions crossing the TMIMS would be τ= 5 ms for K = 1 cm2/sV (which is very close to THA+64). Varying the working frequency from 50 to 1000 Hz allows us to select most common ions ranging from 2 cm2/sV (near the polarization limit in air at atmospheric pressure65,66) down to 0.2 cm2/sV, that would correspond with an ideal cluster of 2.5 nm in diameter and one single charge.67 In this frequency range, the maximum capacitive load consumes approximately 0.05W when the deflector voltage is 10 kV Vpp. Luckily, although the electric fields are rather strong, they correspond to 7.5 Towsends (approximately) at atmospheric pressure conditions and, although electric fields vary by 50% depending on the phase of the deflector voltage, they produce few deviations from the ideal mobility in the limit of low electric fields,42 and the measured mobility can be assumed to be constant in first approximation. The radius of the deflector electrodes is 2 cm, and the distance between their centers is 9 cm. A Data Acquisition System (DAS) measures the electrometer signals and regulates the inlet electrode high voltage amplifier, while a PC-controlled signal generator produces a wave of known amplitude and frequency. A second inverse wave is produced by means of an inverse operational amplifier, and both the direct and the inverse waves are further biased with two continuous (manually controlled) DC offset voltages. Finally, these two inverse waves of controlled frequency, amplitude, and mean value are used to feed the two high speed

shows a spectrum of positive electrospray of THABr produced with the TM-IMS. The first conclusion resulting from this figure is that the TM-IMS really works. As expected, multiple peaks are produced for each resonance, and the signal does not drop to zero between each peak because the electrometer, that has a time response of approximately 100 ms, automatically averages the pulsed output of ions. We shall focus at this stage of development on the peak that appears first. Resonant peaks and the pulsed background will be dealt with in the future with the multistage configuration. Effect of the Deflector and Axial Voltages. The parameter ε = E1/E0 cannot be defined for the experimental configuration because electric fields are not uniform. However, we can still define the equivalent deflector-to-axial voltage ratio ed = Vd/V0, where Vd is the amplitude of the voltage of the deflector electrodes (peak-to-peak voltage), and V0 is the voltage of the inlet electrode (the outlet electrode is grounded). We measured the resolving power of the spectrum at a fixed V0 (of 9 kV) and varying Vd. Figure 4a shows that resolving power grows linearly with ed in accordance with the theoretical model (see eqs 2 and 4) until it starts falling quickly for ed higher than 1. The reasons for this dramatic change in behavior are still unknown, although we speculate that ions could be approaching and impacting the deflector electrodes, whereupon they would be lost. Perhaps future computational models could explain this behavior. We also measured the resolving power of the spectrum as a function of the working voltage V0, and at a fixed ed and found that it grows with the square root of the voltage as RD = 0.084(V0/(kBT/e))0.5. Theoretical and experimental results agree qualitatively well (see eq 2) in that resolving power varies with the square-root of the voltage, but the measured resolving power is approximately half the expected theoretical value. This difference could perhaps be explained by the fact 7834

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box, and found that mechanical misalignments as great as 1 mm can be compensated for by adjusting the offset voltages of the deflector electrodes. We measured the resolving power as a function of the electronic misalignment. In this experiment, the inlet electrode and the deflector voltages were V0 = 4 kV and Vd = 4 kV. Figure 4c.1 (and 4c.2 in closer detail) shows how the resolving power falls when misalignment voltages are increased (transversal misalignments are plotted with circles, while longitudinal misalignments are plotted using squares). As shown in Figure 4c.1, transversal misalignments of only 5 V (over 5 kV) reduce the resolving power of the TM-IMS (a double peak would be observed in the spectrum), while longitudinal misalignments as high as 100 V are required to affect it. Fortunately, current state of the art high voltage systems easily provide the necessary accuracy. We also varied the wave amplitude of only one of the deflector voltages, and found that the system can handle differences as high as 500Vpp (Volts peak to peak). The results of this experiment are shown in Figure 4c.1 with diamond marks. We conclude that the configuration is quite robust, high precision machining is not needed, and the required accuracy is far below current standards. The only requirement is to be careful to adjust the offset voltages so as to avoid transversal misalignment. Alignment is done manually; the operator can evaluate if the TM-IMS is properly aligned by visual inspection of the spectrum; basically, a double peak would indicate that the system is not properly aligned. Wave Shape. As demonstrated mathematically in the patent1 for the case involving uniform electric fields, the TMIMS could operate with any wave shape as long as the deflection in one direction and the following counter-deflection are symmetrical to ensure that ions are brought back to the axis; put in more mathematical terms, the wave function has to be periodic ( f(t + T) = f(t)) and antisymmetric (f(−t) = −f(t)). Throughout this study we are using sinusoidal waves. In this experiment we tested a triangular wave to check experimentally that other common wave shapes also produce mobility filtration. As expected, the triangular wave also produced a well-defined peak. Its signal is slightly higher and its resolving power is slightly lower. We speculate that the triangular wave produces lower deflections, which could explain the higher signals and the lower resolving powers, because its effective voltage is lower than that of the sinusoidal wave. This result is still very preliminary, but it brings out two very important points: (i) that different wave shapes can be successfully utilized, and (ii) that the wave shape affects the performance, and that therefore there is much room for optimization. Outlet Flow Rate. The results of this study are shown in Figure 4d. We characterized the resolving power as a function of the flow rate sampled through the TM-IMS outlet which, in first approximation, equates with the ionic flow outputted from the TM-IMS. According to the theoretical model, for flows higher than the critical (3 lpm), the resolving power should drop following an inversely proportional evolution, while lower flows should not have an important impact on the resolving power. We would naturally expect a smooth transition between the two described behaviors for flows near 3 lpm, but experimental results show that, although the resolving power decreases with increasing flows, the theoretical and the experimental models match only qualitatively. Resolving power seems rather to decrease linearly with the flow; it starts to be reduced for flows as low as 0.5 lpm, while the theoretical model underestimates the resolving power at higher flows. For

Figure 4. Measured resolving power of the first peak as a function of the ratio Vd/V0 for a fixed V0. Figure 4b shows the measured Resolving power of the first peak as a function of the voltage V0 for a fixed ratio Vd/V0. Figure 4c shows the measured and normalized loss of resolving power as a function of the different misalignments; the effects of transversal voltage, longitudinal voltage, and different wave amplitude voltages are represented respectively with circles, squares, and diamonds. Insert 4c.2 shows the effect of the transversal misalignment in closer detail. Figure 4d shows the measured and normalized loss of resolving power as a function of the outlet flow rate exiting through the TM-IMS outlet slit.

that the real configuration is not uniform. The axial and deflector electric fields are partially shielded, and ions might travel more slowly and be subjected to higher diffusion effects than those hypothesized in our very basic model. The results of this study are shown in Figure 4b. Electronic Alignment. The high voltage system that we used in this first prototype is based on expensive laboratory multipurpose electronic equipment. The final commercial system should use specific and cheaper elements. One objective of this study is to establish the design criteria for this electronic equipment. Ideally, the TM-IMS should be mechanically and electronically symmetrical. The inlet and the outlet slits should be aligned, while deflector electrodes should be centered on the symmetry plane, and their offset voltages should be exactly half of the inlet electrode voltage to ensure that trajectories coalesce at the outlet slit and not elsewhere. But, for obvious reasons, it is impossible to create such ideal symmetry. With reference to figure 2, vertical and horizontal directions are named transversal and longitudinal, respectively. When the TM-IMS is transversally misaligned, trajectories coalesce near the outlet slit, but not exactly at it, and the spectrum shows a double peak pattern: ions cannot reach the outlet slit at the resonance frequency, but slightly spread-out trajectories (slightly higher and slightly lower frequencies) produce an intense pulsed output responsible for the double peak. By increasing one deflector offset voltage and decreasing the other, the height of coalescence can by transversally deflected and the quality of the spectrum can be recovered. Similarly, longitudinal misalignments reduce the resolving power but can be corrected by modifying the two deflector offset voltages. We deliberately destroyed the mechanical symmetry by introducing gauges between the electrodes and the insulator 7835

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example, assuming that diffusion and flow rate effects are commensurable at 3lpm, resolving power at 6lpm should be approximately 25, but it is actually near 40. This result suggests that finer models (perhaps based on numerical computations) are required to better understand the effect of the flow rate.

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CONCLUSIONS The main conclusion of the present study is that the TM-IMS works. The configuration is very robust, as indicated by the fact that, although this first prototype was designed “by eye” and is not optimized, its performance is nonetheless comparable to current planar DMAs that require a very careful design to avoid turbulence. However, our understanding of the behavior of the system is still limited. To give an example; the experiments show that resolving power grows linearly with the deflector voltage, as predicted by the theory, but then it suddenly starts falling, and we do not know the exact reasons for this. The proposed theoretical models match experimental results, but only qualitatively. Interestingly, electric fields are much easier to model than fluid flows that require the solving of the nonlinear Navier−Stokes equations in the entire domain. A numerical tool for the optimization of the geometry, that could use computationally inexpensive methods specifically optimized for resolving of the electric fields, could help in the design of better geometries and in the interpretation of the results. The experiments with sinusoidal and triangular wave shapes show that there is room here also for optimization. The results of this study are very promising, but the TM-IMS still produces an undesired nonzero signal of nonselected pulsed ions. In the theoretical studies we concluded that the most promising configuration would be the Two Stage TM-IMS, although, for simplicity, we decided to start with the single TM-IMS. The results of this study suggest that a Two Stage TM-IMS could well produce an operative system providing true mobility filtration approaching the ideal 100% transmission.

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ASSOCIATED CONTENT

S Supporting Information *

Appendix I. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Postal address: P. Tec. Boecillo, Parcela 205 Edificio CARTIF, Boecillo, Valladolid, Spain 47151. Tel.: +34 983 130 154 Fax: +34 983 130 411. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are most grateful to Juan and Gonzalo F. de la Mora for their valuable insights and comments. We are also grateful to our colleagues at SEADM, and especially to Daoiz Zamora, for his help with the development of SW. This work was developed under the project VEAME funded by the “Plan AVANZA” (Spanish Government).

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REFERENCES

(1) Vidal-de-Miguel, G. Method and Apparatus to Produce Steady Beams of Mobility Selected Ions Via Time Dependent Electric Fields. U.S. Patent Application No. 20100243883, 2010. 7836

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