Int. J. Ion Mobil. Spec. (2012) 15:31–39 DOI 10.1007/s12127-011-0084-7

ORIGINAL RESEARCH

Selective ion suppression as a pre-separation method in ion mobility spectrometry using a pulsed electron gun Philipp Cochems & Frank Gunzer & Jens Langejuergen & Andre Heptner & Stefan Zimmermann

Received: 9 September 2011 / Revised: 19 October 2011 / Accepted: 21 October 2011 / Published online: 5 November 2011 # Springer-Verlag 2011

Abstract Ion mobility spectrometry is a well-known method for fast trace gas detection. Detection limits in the very low ppb- and even ppt-range, fast response times down to a second and good separation power combined with a reasonable instrumental effort make ion mobility spectrometry more and more attractive. Aiming for higher separation power we investigate the ion specific lifetime of different ion species in a field free reaction region of a drift tube ion mobility spectrometer equipped with a pulsed non-radioactive electron gun. When turning off the electron gun ionization stops and the total ion concentration in the reaction region starts to decrease, while different ion species have different decay times. By varying the time delay between the end of the ionization and the injection pulse transferring all remaining ions of one polarity from the reaction region into the drift region the individual decay times can be measured. Our experimental data show that the lifetime of ion species in a field free reaction region mainly depends on ion-ion-recombination and charge transfer reactions leading to significant lifetime differences. Therefore, short-lived ions can be effectively suppressed

Presented in parts at the ISIMS conference, July, 2011 P. Cochems : J. Langejuergen : A. Heptner : S. Zimmermann (*) Institute of Electrical Engineering and Measurement Technology, Dept. of Sensors and Measurement Technology, Leibniz University Hannover, Appelstr. 9A, Hannover, Germany e-mail: [email protected] F. Gunzer Physics Department, Entrance El Tagamoa El Khames, German University in Cairo, New Cairo City, Cairo, Egypt

in the reaction region by introducing a sufficient time delay between the end of the ionization and the injection pulse. This allows detecting even smallest concentrations of long-lived ions in a complex shortlived background. From our experimental data it can be also concluded that wall losses and the ion transport within the sample gas stream out of the reaction region just play a minor role in the ion loss. Keywords Ion mobility spectrometry . Ion suppression . Ion-ion recombination . Non-radioactive electron source . Pulsed electron gun

Introduction Ion mobility spectrometry (IMS) is a well-known technique for fast trace gas detection with detection limits in the very low ppb- and even ppt-range, fast response times down to a second and very good separation power considering its reasonable instrumental effort. Typical applications are the detection of chemical warfare agents, toxic industrial compounds, explosives and drugs of abuse. Due to its analytical performance combined with an affordable price IMS becomes more and more popular in other applications. In [1] Eiceman et al. and in [2] Borsdorf et al. give a comprehensive description of IMS fundamentals, different instruments and applications. Recent developments in IMS are summarized by Borsdorf et al. in [3]. Here we use a drift tube IMS, where analytes are ionized in the reaction region by chemical gas phase reactions at atmospheric pressure using a non-radioactive pulsed electron gun, see Fig. 1. A detailed description of the electron gun is given in [4]. Fundamentals can be found in [5–7]. In continuous mode the electron gun emits free electrons with an average

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Fig. 1 Principle of selective ion suppression in a field free reaction region: After 10 ms of ionization the electron gun is turned off (a). The total ion concentration starts to decrease (b). Depending on both the ion lifetime and the time delay between the end of the ionization and the injection just long-lived ions are still present in the reaction region (c). Short-lived ions are suppressed, while long-lived ions can be separated in the drift region on the basis of their ion mobility. Ion shutter principle: The electron gun potential is pulled 350 V above the

shutter grid potential for 350 μs. The shutter grid is just a plane grid here having constant electrical potential (no Bradbury-Nielsen shutter). The potential difference between the electron gun and the shutter grid leads to a high electrical field of 1750 V/cm in the reaction region that injects all positive ions present in the reaction region through the shutter grid into the drift region (d). In the negative mode all potentials are vice versa

kinetic energy of 10 keV, which is comparable to radioactive beta-sources, such as 63Ni or 3H emitting free electrons with an average energy of 17 keV and 5.7 keV respectively [8]. Here we use the electron gun in pulsed mode. After 10 ms of ionization the electron gun is turned off and the total ion concentration in the reaction region starts to decrease. In [9] we showed that different ion species have different lifetimes in the reaction region. This allows effective suppression of short-lived ions in the reaction region by introducing a sufficient time delay between the end of the ionization and the injection pulse transferring all remaining ions of one polarity from the reaction region into the drift region. In [10] and [11] we investigated the lifetime of positive H3O+(H2O)n and negative O2-(H2O)n reactant ions and positive dimethyl methylphosphonate (DMMP) product ions at different IMS parameters. Here we discuss more fundamental experiments revealing the mechanism of ion

loss in a field free reaction region. It is important to note, that we do not use a Bradbury-Nielsen shutter. The electric potential of the ionization source is pulled up for 350 μs instead leading to a high electrical field in the reaction region that injects all ions of one polarity into the drift region, see Fig. 1. At all other times no electrical field is present in the reaction region. Driven by the electrical field in the drift region different ion species move with different drift velocities towards the detector, where the ion current is measured. The drift velocity depends on the ion mobility, so that different ion species can be separated in the drift region on the basis of their ion mobility. Controlling both the ionization time and the delay time allows investigating the ion formation kinetics [12] and ion lifetimes. With the right timing it is possible to suppress either slow-forming or short-lived ions in the reaction region. Here we discuss the suppression of shortlived ions.

Int. J. Ion Mobil. Spec. (2012) 15:31–39 Table 2 Operating parameters of the electron gun

Experimental For all experiments a drift tube IMS is used with an inner diameter of 15.2 mm, a 2.5 mm long reaction region and a 70.5 mm long drift region. The operating parameters are summarized in Table 1. A pulsed electron gun from Optimare Analytics, Germany is used for ionization. The operating parameters are given in Table 2. It is important to note, that both the IMS and the electron gun used here have slightly different geometries compared to the IMS and the electron gun used in [4] and [9–11]. Furthermore, the IMS operating parameters differ. Therefore, the experimental results cannot be easily compared. All chemicals were purchased from Sigma Aldrich, Germany, see Table 3. The electric potentials and mass flows are controlled and recorded by an external control unit connected to a Laptop. Furthermore, humidity is monitored. The sample gas is generated by mixing dry clean air with analyte containing carrier gas coming from a permeation oven. The permeation rate is calculated from the weight loss per time of the permeation tube containing the liquid analyte. For all analytes the permeation oven temperature is constant at 35 °C. The carrier gas is dry clean air constantly flowing with 600 ml/min trough the permeation oven. Up to 550 ml/ min of the analyte containing carrier gas can be mixed with up to 2000 ml/min of dry clean air to finally generate the sample gas. The analyte concentration in the sample gas can be calculated from the permeation rate, the carrier gas flow rate through the permeation oven and the mixing ratio between the carrier gas and the dry clean air. A reference system to measure the analyte concentration was not available. However, for our conclusions the exact absolute analyte concentration is not important, since we compare relative analyte concentrations. The ionization time is set to 10 ms (electron gun on) for all experiments. Furthermore, the integral of the ion current measured at the detector without any time delay is kept constant. Thus, the total

Table 1 Operating parameters and of the IMS Parameter

Value

Temperature

293 K

Drift tube pressure Drift gas

1018 mbar clean compressed air, 500 ml/min, dew point of −55 °C clean compressed air containing analytes, 5 ml/min, dew point of −55 °C 285 V/cm 1750 V/cm 350 μs 16 Hz

Sample gas

Drift field Injection field Injection pulse Injection frequency

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Parameter

Value

Average electron energy Electron current in the reaction region

10 keV ≈ 25 pA

Electron pulse

10 ms

Delay time Emitter window

0…15 ms 1×1 mm2

initial ion concentration in the reaction region is constant independent of the ion species present. The time delay between the end of the ionization and the injection pulse transferring all remaining ions of one polarity from the reaction region into the drift region can be varied from 0 to 15 ms. Figures 1 and 2 show the principle of selective ion suppression.

Results and discussion It is assumed that after 10 ms of ionization a constant ion distribution is reached in the reaction region containing even slow-forming ion species, such as dimers. Even longer ionization times result in similar ion mobility spectra. When stopping the ionization the total ion concentration in the reaction region starts to decrease. By varying the time delay between the end of the ionization and the ion injection both the decay of the total ion concentration and the decay of individual ion species can be measured. For example, Fig. 3 shows positive ion mobility spectra of dry clean air at different delay times. Just the positive reactant ion peak (RIP+) is present, so that the positive reactant ion concentration equals the total positive ion concentration in the reaction region not considering any primary ions. As can be seen in Fig. 4, the positive reactant ion peak decreases within T10 =12 ms down to 10% of its initial concentration. In the following we discuss possible mechanisms that could lead to the observed ion loss. Assuming a homogenously distributed ion concentration and having no potential difference between the ionization source and the shutter grid a field free reaction region results. Therefore, directional ion movement is just possible within the sample Table 3 Chemicals Name

Ordering #

DMMP 1-Octanol Acetone 1,1,2-Trichlorethane

D169102 95446 650501 46262

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Fig. 2 Timing diagram of the electron gun (on: electron emission; off: no electron emission) and the ion shutter (closed: no electrical field in the reaction region; open: high electrical field in the reaction region)

gas stream or driven by diffusion towards the walls of the reaction region. With a sample gas flow rate of 5 sccm/min and a reaction region volume of 0.45 cm3 it takes at least 5 s to transport all ions out of the reaction region within the sample gas stream. Comparing this with the measured decay time of T10 =12 ms shows that the ion transport within the sample gas stream cannot be of major relevance to the ion loss. The only other directional ion transport mechanism in the reaction region is diffusion. Figure 5 shows the simulated ion loss in the reaction region just considering diffusion and wall losses. COMSOL 4.1 is used for this simulation. Wall losses are modeled by a zero-concentration boundary condition. The initial condition is assumed to be a homogenously distributed cylindrical ion cloud inside the reaction region, see insert in Fig. 5. Assuming an electron penetration depth of about 2 mm at atmospheric pressure [14] and an emitter window size of 1×1 mm2 the initial ion

Fig. 3 Positive ion mobility spectra of dry clean air at different delay times. Just the positive reactant ion peak is clearly present

Int. J. Ion Mobil. Spec. (2012) 15:31–39

Fig. 4 Decay of the positive reactant ion concentration (solid circles) and the negative reactant ion concentration (empty squares). Assuming that the ion loss is mainly caused by ion-ion recombination the measured decay curves should follow Eq. 2. As can be seen, the calculated decay (solid line) fits the measured decay

cloud is about 2 mm in height and 5 mm in diameter. The diffusion coefficient D is derived from the measured ion mobility of the positive reactant ions, see Table 4, via the Nernst-Einstein Eq. 1 to D=0.057 cm2s−1: D ¼ kB

TK q

ð1Þ

where kB is Boltzmann’s constant in CVK−1, T is the temperature in the drift tube in K, K is the mobility in cm2V−1 s−1 and q is the ionic charge in C [13]. The simulated decay time constant is T10 =280 ms. Thus, wall losses cannot cause the measured decay of the positive reactant ion concentration either leading to the conclusion, that ion-ion recombination mainly defines the ion loss in the reaction region.

Fig. 5 Simulated ion loss of the positive reactant ions in the reaction region just considering diffusion and wall losses

Int. J. Ion Mobil. Spec. (2012) 15:31–39

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Table 4 Ion mobilities and reduced ion mobilities of the measured positive and negative reactant and product ion peaks Peak

Ion mobility in cm2s−1 V−1

Reduced ion mobility in cm2s−1 V−1

RIP+

2.26

2.12

RIPAcetone monomer

2.40 2.07

2.25 1.94

DMMP monomer

2.03

1.90

DMMP dimer 1-Octanol monomer

1.61 1.53

1.51 1.43

1-Ocatnol dimer

1.19

1.11

1,1,2-Trichlorethane

2.57

2.40

Figure 6 shows negative ion mobility spectra of dry clean air at different delay times. The only peak present is the negative reactant ion peak (RIP-), so that the positive reactant ions can just recombine with the negative reactant ions. Therefore, both decay curves should match. As can be seen in Fig. 4, the positive reactant ion concentrations decay fits the negative reactant ion concentration decay. This again shows that wall losses cannot be of major relevance to the ion loss in the reaction region since diffusion controlled wall losses would cause different decay times for different ion species with different diffusion coefficients and ion mobilities respectively. As discussed later, even for positive and negative ion species with significant ion mobility differences the decay time remains nearly constant as long as just one ion species of each polarity is present in the reaction region. Finally, the form of the simulated decay curve does not match the measured ion loss, which again indicates that ion-ion recombination dominates the decay time rather than diffusion controlled wall losses. A detailed description of

Fig. 6 Negative ion mobility spectra of dry clean air at different delay times. Just the negative reactant ion peak is clearly present

ion-ion recombination can be found in [15–17]. Assuming charge conservation the positive reactant ion concentration cp is similar to the negative reactant ion concentration cn and the ion loss caused by ion-ion recombination can be described as follows: R¼

dc c0 ¼ kc2 ) cðtÞ ¼ 1 þ kc0 t dt

ð2Þ

where R is the recombination rate in mol cm−3 s−1, c=cn = cp is the positive or negative reactant ion concentration in mol cm−3, c0 is the initial positive or negative reactant ion concentration in mol cm−3, k is the recombination coefficient in mol−1cm 3s −1, and t is the time in s. The recombination coefficient k depends on the ion mobilities of the recombining ion species [15]:
ð3Þ k  Kp þ Kn where Kp is the ion mobility of the positive ions in cm2s−1 V−1 and Kn is the ion mobility of the negative ions in cm2s−1 V−1. As shown in Fig. 4, the measured decay curves can be fitted with Eq. 2 indicating that ion-ion recombination dominates the ion loss in a field free reaction region. When analytes are present in the sample gas, the conditions in the reaction region totally change. For example, the formation of positive and negative analyte ions results in an equivalent loss of positive and negative reactant ions. Since the recombination rate decreases with a decreasing ion concentration, see Eq. 2, the decay time constant of the reactant ions should increase when analyte ions are present in the reaction region. However, in addition to ion-ion recombination charge transfer reactions from reactant ions to neutrals need to be considered. While the total ion concentration in the reaction region continuously decreases after stopping the ionization due to ion-ion recombination, charge transfer reactions do not affect the total ion concentration but can cause a significant change in the lifetime of individual ion species. For example, a concentration of just 5 ppb Acetone in dry clean air leads to a significant lifetime reduction of the positive reactant ions from T10 =12 ms to T10 =2.4 ms while the Acetone ions have a decay time constant of T10 =15 ms, see Figs. 7 and 8. This can just be explained by proton transfer reaction from positive reactant ions to Acetone neutrals after stopping the ionization. Since proton and electron transfer reactions strongly depend on the proton and electron affinities of the reaction partners high proton and electron affine substances should have longer lifetimes. For example, Fig. 9 shows ion mobility spectra of 5 ppb DMMP in dry clean air at different delay times. Due to proton transfer reactions the DMMP dimer ion concentration initially increases before

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Fig. 7 Positive ion mobility spectra of dry clean air containing 5 ppb of Acetone at different delay times

Fig. 9 Positive ion mobility spectra of dry clean air containing 5 ppb of DMMP at different delay times

ion-ion-recombination gets predominant causing the ion concentration to decrease. This leads to an exceptional long decay time constant. At a time delay of 15 ms just 50% of the initial DMMP dimer ion concentration is lost, see Fig 10. With an increasing DMMP concentration both the reactant ion concentration and the DMMP monomer ion concentration decrease while the DMMP dimer ion concentration increases. In our experimental setup at a DMMP concentration of 90 ppb just DMMP dimer ions are present in the reaction region after 10 ms of ionization. Thus, the negative reactant ions can only recombine with positive DMMP dimer ions. Figure 11 shows the decay curve of the negative reactant ion concentration which surprisingly fits the decay curves of the negative and positive reactant ion concentrations measured without any analytes present. Thus, negative reactant ions seem to recombine with positive reactant ions as fast as with positive DMMP dimer

ions. As described in Eq. 3, the recombination coefficient depends on the ion mobility of the recombining ion species, so that different ion mobilities should lead to different decay curves. The ion mobilities of all investigated ion species can be found in Table 4. Thus, replacing all positive reactant ions by positive DMMP dimer ions should lead to a decrease of the recombination coefficient by 14%, see Eq. 4. However, this could not be observed.    KDMMP KRIP KRIP 1 kDMMP =kRIPþ ¼ 1þ þ ð4Þ KRIPþ KRIPþ KRIPþ

Fig. 8 Decay of the positive reactant ion concentration (solid circles) and the positive Acetone monomer ion concentration (empty triangles)

At a DMMP concentration of 5 ppb positive reactant ions as well as positive DMMP monomer and dimer ions are present in the reaction region, see Fig. 9, so that the negative reactant ions can now recombine with different positive ion species. Again, the decay curve of the total positive ion concentration matches the decay of the positive

Fig. 10 Decay of the positive reactant ion concentration (solid circles) and the positive DMMP monomer and dimer ion concentrations (empty triangles and empty circles)

Int. J. Ion Mobil. Spec. (2012) 15:31–39

reactant ion concentration measured without any analytes present in the reaction region. Therefore, the recombination coefficient of the negative reactant ions seems to be independent of the chemical structure of their positive recombination partners. Expressed in a more general way, in our reaction region the decay time of the total ion concentration seems to be independent of the recombining ion species. However, ion losses based on any directed transport in electrical fields e.g. between oppositely charged ions or caused by an inhomogeneous ion distribution in the reaction region should depend on the ion mobility. As shown in Fig. 11, the decay of the total positive ion concentration is nearly independent of the positive ion species present in the reaction region and therefore, independent of the ion mobility. Further investigations are required to give a sound explanation of the observed behavior. We assume that charging effects could affect the decay. However, this still allows selective ion suppression since different ion species have significant lifetime differences when different analytes and ion species of one polarity are present in the reaction region at the same time. From our experimental data we conclude that the individual lifetime of different ion species is not a result of individual ion-ion recombination coefficients but mainly depends on charge transfer reactions, which in turn depend on the electron and proton affinity. Furthermore, the ion stability needs to be considered. To substantiate this theory a mixture of 25 ppb DMMP (proton affinity of 902 kJ/mol) and 65 ppb 1Octanol (proton affinity of 846 kJ/mol) in dry clean air is further investigated. Due to the much higher proton affinity

Fig. 11 Decay of the negative and positive reactant ion concentration (empty circles and solid circles) without DMMP present in the reaction region, the negative reactant ion concentration (solid triangle) at 90 ppb DMMP and the total positive ion concentration (empty triangles) at 5 ppb DMMP

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Fig. 12 Positive ion mobility spectra of dry clean air containing a mixture of 25 ppb DMMP and 65 ppb 1-Octanol at different delay times

DMMP product ions should have a significant longer lifetime than 1-Octanol product ions even though 1Octanol product ions have lower ion mobilities. As can be seen in Figs. 12 and 13, the 1-Octanol monomer and dimer ion peaks decrease significantly faster than the DMMP dimer peak, which has an extensively long decay time constant of T50 >>15 ms. Since the total positive ion concentration decays as soon as the electron emission from the e-gun stops the initial increase of the DMMP dimer ion concentration can just be explained by proton transfer reactions from other positive ions in the reaction region to DMMP neutrals forming DMMP monomer ions, which in turn form DMMP dimer ions with other DMMP neutrals. We believe that the fast decay of the 1-Octanol monomer ion concentration is also

Fig. 13 Decay of the positive reactant ion concentration (solid circles), the positive 1-Octanol monomer ion concentration (solid triangle) and the positive DMMP monomer and dimer ion concentrations (empty triangle and empty circle)

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caused by proton transfer reactions to DMMP neutrals. Figures 12 and 13 show again that suppression of shortlived ions in the reaction region is independent of the ion mobility. Thus, ion suppression in the reaction region can be used as an orthogonal pre-separation method as long as unwanted ions are short-lived. In the experiments so far negative reactant ions recombined with different positive ion species. At a concentration of 1 ppm of 1,1,2-Trichlorethane in the sample gas negative product ions are formed, see Fig. 14. As can be seen in Fig. 15, the total negative ion concentration decay is similar to the decay of the negative reactant ions measured without any analytes present in the reaction region. Generally speaking, in our experimental set up and for constant initial ion concentrations the decay of the total ion concentration is nearly independent of the positive and negative ion species present in the reaction region. However, as explained above this still allows selective ion suppression in the reaction region.

Conclusions In this paper we discuss several aspects of the ion loss in the reaction region of a drift tube ion mobility spectrometer equipped with a non-radioactive pulsed electron gun for pulsed ionization. By varying the delay time between the end of the ionization and the injection of all remaining positive or negative ions into the drift region the ion loss of the total ion concentration and individual ion species can be measured. Here the concentration decay of the reactant ions and the product ions of Acetone, Dimethyl methylphosphonate, 1-Octanol and 1,1,2-Trichlorethane as well as possible mechanisms of the ion loss are discussed.

Int. J. Ion Mobil. Spec. (2012) 15:31–39

Fig. 15 Decay of the total negative ion concentration (empty triangles) and the negative reactant ion concentration measured without any analytes present in the reaction region (empty squares)

From our experimental data we draw the following conclusions: &

&

& &

& Fig. 14 Negative ion mobility spectra of dry clean air containing 1 ppm of 1,1,2-Trichlorethane at different delay times. The product ions of 1,1,2-Trichlorethane are probably Cl−-Cluster

The simulated decay of the positive reactant ions just considering diffusion controlled wall losses gives a decay time constant of T10 =280 ms, which does not match the measured decay time constant of T10 =12 ms. Thus, wall losses just play a minor role in the ion loss. Due to the low sample gas flow rate of 5 sccm/min the ion transport in the sample gas stream cannot be of major relevance to the ion loss either. It takes at least 5 s to transport all ions out of the reaction region within the sample gas stream. The ion loss in a field free reaction region is mainly caused by ion-ion recombination. In our experimental set up and for similar initial ion concentrations the decay of the total ion concentration remains nearly constant independent of the ion species present in the reaction region. Furthermore, different ion species with different ion mobilities show similar decay times as long as the ion species investigated is the only ion species of its polarity in the reaction region. Thus, an ion specific recombination coefficient could not be observed. Furthermore, the ion-ion recombination seems to be nearly independent of the ion mobility although the recombination coefficient should depend on the ion mobility. Further investigations are required to give a sound explanation of this behavior. We assume that charging effects in the reaction region also affect the ion loss. However, different ion species have significant lifetime differences when other analytes and product ions are present in the reaction region. This can be only explained by charge transfer reactions, which can even cause an increase of the concentration of certain ion

Int. J. Ion Mobil. Spec. (2012) 15:31–39

&

species after stopping the ionization. Since the ion lifetime obviously depends on charge transfer reactions electron and proton affine substances should have longer lifetimes. Further investigations are required to substantiate this theory. Due to significant lifetime differences, selective ion suppression in the reaction region is possible even though the decay time of the total ion concentration seems to be independent of the ion species present in the reaction region. This allows suppression of unwanted short-lived ions in the reaction region. Since ion suppression is independent of the ion mobility it can be used as an orthogonal pre-separation method.

It is important to note, that by varying the ionization time—not discussed here but just as interesting—another pre-separation method becomes available. Short ionization times would lead to a suppression of slow-forming ions in the reaction region.

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