Application of a Nonradioactive Pulsed Electron Source for Ion

Mar 30, 2010 - Marvin Ihlenborg , Jürgen Grotemeyer , Frank Gunzer. Sensors and ... Wolfgang Baether , Stefan Zimmermann , Frank Gunzer. Sensors and ...
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Anal. Chem. 2010, 82, 3756–3763

Application of a Nonradioactive Pulsed Electron Source for Ion Mobility Spectrometry Frank Gunzer,*,† Stefan Zimmermann,‡ and Wolfgang Baether§ Physics Department, German University in Cairo, Entrance El Tagamoa El Khames, New Cairo City 11835, Cairo, Egypt, Sensors and Measurement Technology, Institute of Electrical Engineering and Measurement Technology, Appelstrasse 9a, Leibniz University Hannover, 30167 Hannover, Germany, and Research Unit, Draegerwerk AG & Co. KGaA, D-23542 Luebeck, Germany Ion mobility spectrometry (IMS) is a well-known method for detecting hazardous compounds in air. Typical applications are the detection of chemical warfare agents, highly toxic industrial compounds, explosives, and drugs of abuse. Detection limits in the low part per billion range, fast response times, and simple instrumentation make this technique more and more popular. Common ion mobility spectrometers work by employing a radioactive source to provide electrons with high energy to ionize analytes in a series of chemical reactions. General security as well as regulatory concerns related to radioactivity result in the need for a different ionization source which on the other hand produces ions in a similar manner as a radioactive source since the ion chemistry is wellknown. Here we show the application of a novel nonradioactive source that produces spectra similar to those obtained with radioactive tritium sources. Using this source in a pulsed mode offers the additional advantage of selecting certain analytes by their recombination time and thus significantly increasing the selectivity. The successful isolation of a target signal in the presence of contaminants using a pulsed electron beam or more precisely the difference in recombination times will be demonstrated for the case of dimethyl-methylphosphonate (DMMP) showing the potential of this source to reduce the possibility for false-positive detection of corresponding chemical warfare agents (CWA) by IMS. Ion mobility spectrometry (IMS) emerged as an analytical technique in the early 1970s. Detection limits in the part per billion range and fast response times of just a few seconds make this technique more and more popular. Typical applications are the detection of chemical warfare agents, toxic industrial compounds, explosives, and drugs of abuse.1-4 IMS is well-established today for real-time monitoring of ambient air. It compares favorably to * To whom correspondence should be addressed. Phone: +2 02 7590 668. Fax: +2 02 7590 668. E-mail: [email protected]. † German University in Cairo. ‡ Leibniz University Hannover. § Draegerwerk AG & Co. KGaA. (1) Williamson, C. S. Nutr. Bull. 2008, 33, 4–7. (2) Eiceman, G. A. Trends Anal. Chem. 2002, 21, 259–275. (3) Eiceman, G. A.; Karpas, Z. Ion Mobility Spectrometry; CRC Press: Boca Raton, FL, 2005. (4) McDaniel, E. W.; Mason, A. E. The Mobility and Diffusion of Ions in Gases; John Wiley & Sons: New York, 1973.

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other analytical methods with respect to size, weight, handling, and instrumentation. In general, the separation of species in an ion mobility spectrometer works via the drift time principle. After ionization, the sample is led by electric fields into the drift region where the ions are accelerated by a low electric field (typically a few 100 V/cm). These ions subsequently collide with the air molecules and depending on their mobility (which in turn depends on numerous parameters, e.g., structure and size) reach the detector after different drift times. The techniques used to ionize the sample are as diverse as the fields of application of IMS: electrospray,5-9 typically for liquid samples, optical (high-intensity light sources10,11 and lasers,12,13 X-ray14), or corona discharge,15-18 but the most common one especially in commercial devices is a radioactive source (mainly 63Ni but also 241Am and 3H). The different sources ionize with different energies: 63Ni emits electrons with a maximum energy of 67 keV and an average energy of 17 keV, 241Am emits short-range R particles at over 5.4 MeV, 3H emits electrons at less energy than 63Ni with a (5) Yamagaki, T.; Sato, A. Anal. Sci. 2009, 2, 985–988. (6) Wittmer, D.; Chen, Y. H.; Luckenbill, B. K.; Hill, H. H., Jr. Anal. Chem. 1994, 66, 2348–2355. (7) Chen, Y. H.; Siems, W. F.; Hill, H. H. Anal. Chim. Acta 1996, 334, 75–84. (8) Wu, C.; Siems, W. F.; Hill, H. H., Jr. Anal. Chem. 2000, 72, 396–403. (9) Harris, G. A.; Nyadong, L.; Fernandez, F. M. Analyst 2008, 133, 1297– 1301. (10) See, e.g., patents Spangler, G. E.; Roehl, J. E.; Patel, G. B.; Dorman, A. Photoionization ion mobility spectrometer. U.S. Patent 5,388,931, August 16, 1994. Cohen, M.; Carroll, D. I.; Wernlund, R. F.; Kilpatrick, W. D. Apparatus and Methods for Separating, Concentrating, Detecting, and Measuring Trace Gases. U.S. Patent 3,699,333, October 17, 1972. Yang, W.; Hsi, P. C. Selective photo-ionization detector using ion mobility spectrometry. U.S. Patent 6,509,562, January 21, 2003. Do ¨ring, H.; Arnold, G.; Adler, J.; Ro ¨bel, T.; Riemenschneider, J. DE Patent 19609582, 1996. (11) Sielemann, S.; Baumbach, J. I.; Schmidt, H. Int. J. Ion Mobility Spectrom. 2002, 5, 143–148. (12) Matsaev, V.; Gumerov, M.; Krasnobaev, L.; Pershenkov, V.; Belyakov, V.; Christyakov, A.; Boudovitch, V. Int. J. Ion Mobility Spectrom. 2002, 5, 112– 114. (13) Oberhu ¨ ttinger, C.; Langmeier, A.; Oberpriller, H.; Kessler, M.; Goebel, J.; Mu ¨ ller, G. Int. J. Ion Mobility Spectrom. 2009, 12, 23–32. (14) See, e.g., patents Do¨ring, H. U.S. Patent 2002185593, 2002. Do ¨ring, H. Ionization chamber with electron source. U.S. Patent 6,429,426, August 6, 2002. (15) See, e.g., patents Taylor, S. J.; Turner, R. B.; Arnold, P. D.; Patent WO9311554, 1993. Taylor, S. J.; Turner, R. B.; Arnold, P. D. Corona discharge ionization source. U.S. Patent 5,684,300, November 14, 1997. Xu, J.; Ramsay, J. M.; Whitten, W. B. U.S. Patent 2004164238, 2004. (16) Xu, J.; Whitten, W. B.; Lewis, T.A,; Ramsey, J. M. Int. J. Ion Mobility Spectrom. 2009, 4, 3–6. (17) Hill, C.; Thomas, P. Int. J. Ion Mobility Spectrom. 2002, 5, 155–160. (18) Schmidt, H.; Baumbach, J. I.; Sielemann, S.; Wember, M.; Klockow, D. Int. J. Ion Mobility Spectrom. 2001, 4, 39–42. 10.1021/ac100166m  2010 American Chemical Society Published on Web 03/30/2010

maximum value of 18.6 keV and an average value of 5.7 keV.19 The reason for the use of such sources is simplicity. When a radioactive source for ionization is employed, no external power supply is needed; also maintenance is not required. Despite these advantages of radioactive sources, their application is discouraged in commercial devices due to technical, organizational, and related financial complications. These sources require certain procedures regarding permits, licenses, and disposal, for example, which lead to extra costs. Therefore, a special set up of the drift tube is required to ensure that no radioactive byproduct can leave the tube or instrument. Thus, research was conducted in order to find similarly efficient ionization sources that can work without radioactive substances. However, the ionization mechanism using β radiation is quite well understood so that any replacement should also ionize via highly energetic electrons providing similar if not equivalent reactant ion chemistry known from radioactive β emitters. The concept of the new electron source used here is based on the early work (1894) of P. Lenard who showed that electrons, generated in an electrical discharge tube, could pass through a thin aluminum foil. The material which has been employed recently is not aluminum but silicon nitride with a thickness of typically 300 nm. Such foils are routinely manufactured, and they can be used with much lower electron kinetic energies. Instead of 50 keV as in the case of aluminum (thickness 2650 nm), only 10 keV is needed. Thus the silicon nitride membrane allows the nonradioactive production of electron beams in a discharge tube with energies close to those of a 3H source, especially when compared with the average electron kinetic energy.20 Additionally, the electron source offers the possibility to control parameters such as electron beam intensity over time and thus to produce pulsed electron beams with a defined temporal profile. In contrast to radioactive sources which constantly produce free electrons, this nonradioactive source allows for the introduction of delay times between the ion formation process and the ion extraction in which no free electrons are being produced. This in turn means that after their production, ions can recombine before the extraction process starts since within the reaction chamber both positive and negative ions are produced. From the literature it can be generally concluded that different analyte ions can have different recombination times under atmospheric pressure.21,22 We found that the ion recombination time is also characteristic for the analyte (such as the ion mobility). An additional selection parameter is thus introduced which can be used to separate the ion species before their extraction. Separation means in this context signal intensity reduction, as the recombination process removes ions so that the corresponding intensity decreases over time. The article is organized as follows: After a brief description of the nonradioactive electron source and its capability of reproducing RIP spectra, which have been previously obtained with a tritium source, in the continuous mode is demonstrated. Furthermore, the influence of humidity on the RIP intensity decay time (19) Guharay, S. K.; Dwivedi, P.; Hill, H. H., Jr. IEEE Trans. Plasma Sci. 2008, 36, 1458–1470. (20) Gunzer, F.; Ulrich, A.; Baether, W. Int. J. Ion Mobility Spectrom. 2010, DOI: 10.1007/s12127-009-0034-9. (21) Ferguson, E. F. Annu. Rev. Phys. Chem. 1975, 26, 17–38. (22) Polley, C. W., Jr.; Illies, A. J.; Meisels, G. G. Anal. Chem. 1980, 52, 1797– 1803.

in the pulsed mode is shown as humidity dependence is a major aspect for any detector to be used under ambient conditions. The application of the pulsed mode is then described for two substances with different recombination times, acetone and DMMP, which clearly shows the advantage of controlling the temporal profile of free electron production. The article concludes by showing how this approach can be used to extract a DMMP signal out of the signal background obtained with a contaminated IMS. EXPERIMENTAL SECTION General Setup. Figure 1a shows the basic setup. The central component is the drift tube. We use a drift tube as a central component and an amplifier from a Draeger Ion Mobility Spectrometer GSM with a 3 mm long reaction region, a 5.5 cm long drift region, and a 0.5 mm long collector region. In the reaction region, the reactant and product ions are produced by chemical reactions. The electric potential of the source grid is given by the high voltage supply 1 (called HV1 in Figure 1), and the source wall potential is given by the high voltage supply called HV2 (low voltage). Initially, a small electric field gradient of 3 V/cm is present in the reaction region in order to prevent drift losses into the drift region. The collector wall is at zero potential. At a frequency of 30 Hz, the source wall potential is increased so that the field strength in the reaction region reaches about 1000 V/cm for typically 100 µs. The voltage for this pulse is provided by HV2 (high voltage), and the HV Pulse Generator controls the temporal width of the pulse. The pulse is used in order to bring the ions after the ionization process out of the reaction region into the drift region. In this region a permanent field of ∼200 V/cm forces the ions toward the collector region. On their way through the drift region, the ions collide with the air molecules (the drift tube is operated at atmospheric pressure) of an air flow of 300 mL/ min that flows in the opposite direction of the ions’ motion. The flow rate is controlled by the mass flow controller MFC. In the collector region, the ions experience a field of 1000 V/cm that moves them toward the detector, which is formed by a simple Faraday cup. In this experiment, the voltages HV1, HV2 (low), and HV2 (high) are provided by high voltage supplies of type dc power supply HCP35-3500 (FUG Company, Germany). The delay generator that generates the trigger pulses for the HV Pulse Generator is a Pulse Generator model 9512 (Quantum), and the HV Pulse Generator is a PVX-4140 (DEI). The oscilloscope used to monitor the trigger pulses, the reference pulse from the Pulse Generator, and the output of the detector unit is a Wavejet 314 (LeCroy, Germany). Nonradioactive Electron Source. The electron source (a detailed description can be found in ref 20) used instead of the standard radioactive 3H source requires a special voltage supply which is a custom-made system by Optimare Wilhelmshaven, Germany. Per software (also provided by Optimare) it is possible to set all the required voltages for static as well as pulsed operation. The electron source works at high vacuum (better than 10-6 mbar). To maintain the vacuum we use a turbomolecular pump system of type HiCube from Pfeiffer Vacuum, Germany. The electrons are emitted thermally by a filament which is heated by supplying a heating voltage UH. Electron extraction is achieved by three grids (G1, G2, and Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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Figure 1. (a) General set up; (b) illustration of the voltages supplied to the drift tube (left) and the timing of the voltage pulse for the electron gun and the reaction region (right). The electron gun produces free electrons and thus triggers the formation of analyte ions (top). Then a delay ∆t is introduced in which the ions recombine (middle). Finally the extraction pulse is supplied which accelerates the remaining ions into the drift region where they are separated due to their different mobilities (bottom).

G3). With certain potentials at grid G1, electrons can enter the tube as a constant current or pulsed. With a volatge supplied to grid G2, electrons are accelerated to the focus system G3 consisting of two parallel metal plates and a corresponding focus voltage. The emission and focusing groups are on a potential UK. For application in IMS, the potential UK can be set to values between 8 and 12 kV. The kinetic electron energy is then high enough to pass through the silicon nitride foil and ionize molecules in the reaction room. The size of the silicon 3758

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nitride window is about 0.5 mm2. The regular radioactive source in the Draeger IMS is a 3H source (circular shaped metal plate with 10 mm diameter) having 50 MBq of activity. It has been replaced by a source punch with a diameter of about 10 mm that emits the electron beam produced by the described nonradioactive source. A summary of the voltages applied can be found in Table 1. The silicon nitride window limits the maximum kinetic energy (and thus UK) as well as the amount of electrons (which can be

Table 1. Summary of the Experimental Parameters for the IMS and the Electron Source device IMS

parameter extraction pulse (source’s potential) reaction room grid drift gas flow

electron source

high voltage UK grid voltage G1 (Vg1) grid voltage G2 focus voltage G3 heater voltage UH pressure

value low 1999.95 V high 2243.0 V high time 111 µs 2001 V air: 495 mL/min; DMMP: 60 mL/min analyte: 3 mL/min 8 kV low: -150.0 V high: 8.0 V high time: 25 µs 150 V 300 V 7.0 V 8.6 × 10-7 mbar

controlled via the heater voltage UH) that can be produced. At high currents larger than ∼7 µA, the window can be damaged and in the worst case the vacuum sealing is broken. Another factor that limits the maximum kinetic energy of the electrons is the width of the reaction region. If the kinetic energy is too high, the electrons pass through the region and reach the drift region where then ions are produced, which can form additional peaks in the spectrum, but usually these ions increase the background noise. Pulsed Operation. The operation principle for pulsed operation is as follows: The power supply sets the voltage of G1 from -150 V to a higher voltage Vg1 that allows the electrons to reach grid G2, from where they are accelerated to the focusing unit G3 and finally out of the electron tube through the silicon nitride window. At -150 V, the electrons cannot reach G2 and the electron beam is stopped. In continuous operation, G1 is kept at Vg1, while in pulsed operation the voltage is set with a frequency of 30 Hz to Vg1 and maintained at that level for a certain time (which can be chosen between 0 and 100 µs) before it is switched back to -150 V. The result is an electron pulse with a certain temporal width. The power supply creating that pulse triggers also the delay generator, which then triggers after a certain delay time the high voltage generator, which in turn produces the high voltage pulse for the extraction of the ions out of the reaction region of the IMS into the drift region. One operation cycle consists consequently of an electron beam which produces ions for a certain time interval (in the range of tens of microseconds) within the reaction region. After a certain delay (typically in the millisecond range), a high voltage pulse (duration typically hundreds of microseconds) extracts the ions into the drift region, from where they reach the detector and produce the ion signal (drift time in the drift region typically a few milliseconds). The pulsed ion production is the main difference compared to the standard operation using a radioactive source. Instead of extracting ion pulses out of the reaction region where the electron source continuously produces new ions, the ion production can be additionally controlled and by choosing proper delays a further reaction step can be introduced during which the ions can recombine, before the remaining ions are extracted by the extraction pulse. Thus, with dependence on the delay time in relation to the recombination time, the signal intensity reduces and can even reach zero. The timing of the voltages is illustrated in Figure

1b. This setup allows for the determination of recombination times of analyte ions under different ambient parameters, e.g., humidity. From the perspective of using the device as a detector, additional selectivity has been introduced, since different ions species with different recombination times can be filtered out by varying the delay time, i.e., ion species with shorter recombination times can be suppressed. RESULTS AND DISCUSSION Figure 2 shows reduced mobility spectra for the RIP obtained with the standard radioactive 3H source and the novel nonradioactive source in continuous mode. Both peaks have the same mobility and the same width, which shows that the nonradioactive source produces similar spectra for the RIP. The quality of the signals is also equivalent so that there is no significant difference between them. Figure 3 goes a step further and proves, by showing corresponding drift time spectra obtained from a device which is contaminated with unknown substances, that not only RIP but different substances’ spectra are correspondingly similar. The upper spectrum has been recorded employing the standard tritium source, while the middle spectrum has been recorded using the nonradioactive electron source in continuous mode. In the case of different ionization mechanisms, additional peaks typically appear in the drift time spectrum. For identical peaks, identical ionization mechanisms are usually required.23 In Figure 3, both spectra (top and middle spectrum) show the same peaks. The RIP at K0 ) 2.01 cm2 V-1 s-1 has as expected a quite low intensity compared to the main contaminant at 1.37 cm2 V-1 s-1. The signal-to-noise ratio is worse for the nonradioactive source, but all peaks are clearly distinguishable. Especially the RIP is in this case stronger compared to the central contaminant’s peak. It should be noted that for higher operation voltages (extraction pulse lower voltage 2799 V, higher voltage 3200 V, high time is 750 µs, reaction room grid voltage 2800 V), the intensity relations of the peaks when using the radioactive source become closer to the intensity relations of the nonradioactive source used with the normal voltages. This is shown in the bottom spectrum of Figure 3. We conclude that the nonradioactive source produces ions in the same manner as the tritium source. The signal-to-noise ratio could be improved by using higher values for UH and UK, but the ones employed for the spectrum in Figure 3 were chosen in order to be well within a range that is best for the source’s stability. The focus of this article is the pulsed operation of the source. Thus the following graphs show the intensity of different peaks depending on the delay time introduced between end of the electron beam and the beginning of extraction. Figure 4 shows the intensity decay (i.e., decay of the maximum peak intensity) for the RIP shown in Figure 2 for increasing delay times. Approximately 2.2 ms after the electron beam stops, the intensity drops to ∼6% of its initial value (i.e., ions are extracted directly when the electron beam is stopped). The intensity decrease is a consequence of the recombination of the ions in the reaction region of the IMS. Space charge or Coulombic effects in the drift region are too small in the setup used here to result in such an intensity decay24,25 due to the dimensions involved and would only (23) Borsdorf, H.; Rudolph, M. Int. J. Mass Spectrom. 2001, 208, 67–72. (24) Mariano, A. V.; Su, W.; Guharay, S. K. Anal. Chem. 2009, 81, 3385–3391.

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Figure 2. RIP peak recorded using the radioactive 3H source and the nonradioactive electron source. The intensities shown are normalized so that the maximum intensity in the spectrum using the nonradioactive source is 100.

Figure 3. Comparison of the mobility spectra of a radioactive source (upper spectrum) and the nonradioactive, continuous electron source using standard voltages (middle spectrum); the lower spectrum shows the mobility spectrum of the radioactive source using higher voltages. The intensities shown are normalized so that the most intense peak in the lower spectrum has an intensity of 1.

become significant in miniature IMS devices.26,27 Furthermore, these effects are too small in the reaction region due to the concurrent presence of positively and negatively charged particles. Another effect that could cause the signal decay is if the ions drift far enough to reach the source grid, where they would lose their charge. Simple drift time calculations3 show that the time necessary to reach this grid is beyond 10 ms and thus not visible in this graph. It should be noted that even in the case of DMMP (25) Tolmachev, A. V.; Clowers, B. H.; Belov, M. E.; Smith, R. D. Anal. Chem. 2009, 81, 4778–4787.

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where delay times longer than 10 ms were investigated, no change of the decay curve’s shape (which should be the consequence of any additional signal loss mechanism) could be observed at any time. Thus, we conclude that the recombination is the dominant mechanism leading to the signal decay here. The strength of the decay in turn seems to depend on the substance or more precisely its electron/proton affinity so that it can be used to select certain (26) Zimmermann, S.; Barth, S.; Baether, W.; Ringer, J. Anal. Chem. 2008, 80, 6671–6676. (27) Barth, S.; Baether, W.; Zimmermann, S. IEEE Sens. J. 2009, 9, 377–382.

Figure 4. Intensity decay of DMMP, acetone, and RIP over increasing delay time between ion production and ion extraction. The intensities have been normalized so that the maximum intensity of each curve is 1.

Figure 5. Humidity dependence of the RIP signal over delay time; the upper spectrum shows absolute, and the lower spectrum shows normalized intensities.

molecules. Before this is demonstrated, Figure 5 shows the dependence of the decay of the RIP intensity on the relative humidity of the sample gas in the IMS. Humidity is an important factor for all devices using ambient air during operation, which makes in general special filtering necessary to keep it constant. Here it can be seen that the decay time of the RIP is independent of the humidity as for all humidity levels used in the experiment it is constant (lower spectrum in Figure 5, where the normalized intensities of the upper spectrum of Figure 5 are displayed over the delay time); the decay times are the same as for the case of

dry air shown in Figure 4. Furthermore not only the decay time is constant, also the intensity is relatively independent in this setup from the humidity, as can be seen in the upper spectrum of Figure 4. Only for 1% relative humidity, the intensity of the RIP is ∼25% higher, but for all other humidity levels the intensity is the same. The high potential of the possibility to introduce the delay time between ion production and ion extraction can be shown when using substances with different recombination times. Thus we have repeated the measurements (dry air) with acetone and the chemical warfare agent simulant (for G series Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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nerve agents28) DMMP. The corresponding graphs are also shown in Figure 4. Initially, DMMP has a ∼5 times higher signal strength than acetone (not visible in the graph which shows normalized intensities). It furthermore has a very large recombination time and corresponding intensity decay time compared to acetone. As can be seen, the acetone signal nearly vanishes after a delay of ∼2600 µs, while the DMMP signal still has ∼55% of its initial intensity. The initial intensity reduction is typical for DMMP in our experiments and currently under further investigation. From the graphs, the signal decay times can be calculated, which are 412 ± 37 µs for acetone and 3166 ± 38 µs for DMMP. Thus, it becomes clear that the delay time can be used as a parameter to suppress certain unwanted signals if they have a lower recombination time; a further discrimination parameter is introduced into ion mobility spectrometry so that substances can be selected not only by their drift time but also by their recombination time in the previously described manner. To further demonstrate the advantage of this possibility, i.e., to introduce arbitrary delay times, we have used a contaminated drift tube with DMMP. The unknown contaminating substances formed a broad background from which two peaks of correspondingly unknown substances and the peak of DMMP (at 1.40 cm-2 V-1 s-1 reduced mobility) could be clearly determined, but the peak height was only ∼15% larger than the background. The DMMP concentration in the sample gas was chosen so that the DMMP peak’s intensity was also only ∼15% larger than the background (see again Table 1). This is shown in the bottom graph of Figure 6, where the asymmetric distribution of the background signal is also visible. Because of the different recombination times and especially due to the relatively long recombination time of DMMP, the background and other signals with lower recombination times can be filtered out. Figure 6 shows in the remaining graphs how with increasing delay time between ion production and ion extraction, the background as well as the RIP and the unknown substance’s peak decrease in intensity. The background’s signal intensity reduction due to recombination is strongest, so that after a delay of 100 and 300 µs all peaks are better distinguishable from the background. As already shown in Figure 4, DMMP has in this delay range first a slightly decreased intensity (100 µs) which then increases (300 µs) until it reaches its maximum value at a ∼500 µs delay. The consequence is that in this experiment after a relatively short delay of 300 µs, the DMMP peak intensity is already ∼5 times higher than the background. The peak’s true shape becomes dominant so that the asymmetry is reduced. At a delay time of 1000 µs, all the other substances have vanished from the spectrum and only the DMMP peak remains visible with nearly the same intensity (which is now ∼15 times higher than the background) as in the case of the nondelayed extraction pulse (see again Figure 6). As can also be seen in the same figure, the peak positions are not affected by the delay, as all peak centers remain at the same drift time. The peak widths (fwhm, for the 300 µs and 1000 µs delays) are also constant. Thus, the ambiguity of signals regarding DMMP (i.e., is it a DMMP signal or the signal from any contaminant) could be cleared by introducing a delay time of 1000 µs in between ion (28) Steiner, W. E.; Klopsch, S. J.; English, W. A.; Clowers, B. H.; Hill, H. H. Anal. Chem. 2005, 77, 4792–4799.

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Figure 6. Demonstration how the delay time can be used to isolate the DMMP peak at K0 ) 1.40 in the presence of contaminating substances and background noise. After 300 µs, the background is strongly reduced while the peak intensity is nearly unchanged. The intensities shown are normalized so that the maximum intensity in the top spectrum is 1.

formation and ion extraction without losing any of the DMMP signal’s intensity or resolution. CONCLUSIONS AND OUTLOOK In this article, we have shown the application of a pulsed nonradioactive electron source. A comparison with drift time spectra of a tritium source shows that this novel electron source can produce similar drift time spectra. As the electron producing process and thus the ionization of the analyte molecules can be controlled, different delay times between ion production and ion extraction can be introduced. These in turn lead to different signal intensities due to recombination processes in the reaction region which have time constants characteristic for the analyte molecules. Thus signals with shorter recombination times can be suppressed. This was demonstrated comparing the signal decay of acetone and the CWA simulant DMMP. Furthermore, it could be shown how the decay time can be used to suppress all signals with lower recombination time in a contaminated IMS tube except the DMMP signal. Thus a further selection parameter besides drift time is introduced into ion mobility spectrometry which can be used e.g. in the previously described fashion to reduce the chance of falsepositive detection results for DMMP and thus, because DMMP is a CWA-simulant for the nerve gas GB, to potentially reduce the false-positive detection rate of IMS for corresponding CWA.

Investigations of the influence of humidity on the RIP intensity decay time show a behavior of the decay time independent from the humidity level. Further investigations have to show the influence on other substances’ recombination times in order to verify and generalize this independence. The possible enhancement of the selectivity of IMS regarding CWA was here shown for DMMP. This substance has a relatively large proton affinity and thus in general a stronger IMS signal. Future research should investigate if the observed larger recombination time is a general consequence of larger proton affinities, and if thus the here shown results can be extended to other CWA

simulants (e.g., DIMP) and the corresponding CWA. The results presented already show the great potential pulsed electron beams have to increase the selectivity of IMS devices not only for the detection of CWA but for the application of IMS in general by using the recombination time as an additional experimental parameter.

Received for review January 20, 2010. Accepted March 22, 2010. AC100166M

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