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An ion-focusing aspiration condenser as an ion mobility spectrometer
Sensors and Actuators B 125 (2007) 428–434
An ion-focusing aspiration condenser as an ion mobility spectrometer Stefan Zimmermann ∗ , Nora Abel, Wolfgang Baether, Sebastian Barth Draegerwerk AG, Research Unit, Moislinger Allee 53-55, D-23542 Luebeck, Germany Received 7 August 2006; received in revised form 15 February 2007; accepted 22 February 2007 Available online 2 March 2007
Abstract Ion mobility spectrometry is a well-known method for detecting hazardous compounds in air. Detection limits in the low ppb range and fast response times make this technique more and more popular. Typical applications are the detection of chemical warfare agents, toxic industrial compounds, explosives and drugs. This paper describes the concept and preliminary results of a simple planar ion-focusing aspiration condenser as an ion mobility spectrometer (IMS). Compared to other aspiration condenser IMS with identical ion carrier gas and drift gas, our design has separate gas flows. This allows fluidic focusing of the ion carrier gas by means of geometric constrains. Thus, ions are focused before separation, which results in improved separation power. Furthermore, humidity in the ion carrier gas does not effect peak position in the ion mobility spectrum when using dry drift gas. In addition, the system is easy to fabricate at low-cost, has small dimensions and low power consumption. First measurements of 1-Octanol in air demonstrate the potential of this novel concept. © 2007 Elsevier B.V. All rights reserved. Keywords: Ion mobility spectrometer; IMS; Aspiration condenser; MEMS
ual ion swarms containing just one ion species. An ion mobility spectrum results by plotting the ion current over drift time.
1. Introduction The principle of ion mobility spectrometry (IMS) is based on the characteristic drift velocity of gas phase ions in an electric field at atmospheric pressure. The ion formation process is described in detail in Ref. [1]. For low electric fields the drift velocity vd is proportional to the electric field strength E as in Eq. (1): vd = KE
(1)
The proportionality coefficient K is called ion mobility and depends on the pressure, temperature, ion mass and collision cross section [1,2]. Under constant conditions the ion mobility K is a characteristic measure for a certain ion species. In time-of-flight IMS, herein after referred to as drift tube IMS, an ion swarm of different ion species is injected into a homogeneous drift field, which forces the ions towards a detector. The time needed for the ions to travel the distance d between the ion shutter and detector is called drift time td . Thus, different ion mobilities K lead to different drift times td as in Eq. (2). The initial ion swarm eventually separates in a number of individ∗
Corresponding author. Tel.: +49 451 882 4546; fax: +49 451 882 3921. E-mail address: [email protected] (S. Zimmermann).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.02.038
td =
d d = vd KE
(2)
Most drift tube IMS have high resolution of R > 20. The definition of the resolution R for drift tube IMS is given in Eq. (3), where td is the drift time and Wt,1/2 the temporal peak width measured at half height [3]: R=
td Wt,1/2
(3)
However, drift tube IMS are expensive precision instruments and miniaturization is difficult due to the complex design. Peak broadening caused by field inhomogeneity, space charge effects and diffusion becomes more significant at small dimensions, which limits the resolution of miniaturized drift tube IMS [4–6]. A different miniaturization approach is based on field asymmetric ion mobility spectrometry (FAIMS), also referred to as differential mobility spectrometry (DMS), where a strong highfrequency electric field is used for ion separation. The separation principle is based on the effect of the electric field strength on the ion mobility. A detailed description of FAIMS is given in Ref. [7]. Miller et al. [8] developed a micro-machined planar FAIMS using standard MEMS fabrication technologies. This
S. Zimmermann et al. / Sensors and Actuators B 125 (2007) 428–434
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Fig. 1. Schematic diagram of the aspiration condenser with two ion species. Ions travel within a gas stream through a transverse electric field, which deflects the ions towards a number of detector electrodes. Ions with different ion mobilities are collected at different detector electrodes. Pattern recognition based on the ion currents In is employed to identify the ion species.
device is easy to fabricate and has good separation power but suffers from high power consumption due to the required strong high-frequency electric field, which makes the development of a hand-held FAIMS difficult. The objective of our research is the development of a simple IMS design that is easy to fabricate at low-cost, has small dimensions, low power consumption and sufficient separation power. Our concept is based on the aspiration condenser developed by Puumalainen [9] and Paakkanen et al. [10], where ions travel within a gas stream through a transverse electric field, which forces the ions towards a number of detector electrodes, see Fig. 1. Different ion species with different ion mobilities are collected at different detector electrodes. Pattern recognition is employed for ion identification. Major drawbacks of this concept are the low resolving power and the immense training effort, which is required for identifying gas mixtures. The low resolving power results from poor spatial ion separation mainly caused by space charge effects and diffusion. Both effects can be reduced by increasing the flow rate so that ion concentration and drift time decrease. However, the flow rate is limited since laminar flow conditions are required. An improved aspiration condenser was developed by Sacristan et al. [11,12]. This swept-field aspiration condenser uses
a variable electric drift field to move all ion species across a single detector electrode, see Fig. 2. A drift voltage scan with n discrete steps is used instead of n individual detector electrodes. The resulting I(V) curve is transformed into an ion mobility spectrum by applying the discrete inverse Tammet transform [13]. This allows ion identification by locating the peak positions in the ion mobility spectrum—a tremendous advantage over pattern recognition when analyzing unknown gas mixtures. However, reconstruction of the ion mobility spectrum is difficult in practice, especially for low signal-to-noise ratios. The required regularization of the discrete direct Tammet transform limits the resolution of the swept-field aspiration condenser. Our approach to gain resolving power is a swept-field aspiration condenser like design with ion focusing. This allows direct measurement of ion mobility spectra and has higher separation power. 2. Ion-focusing aspiration condenser The concept of the ion-focusing aspiration condenser is shown in Fig. 3. It is based on two different gas flows that form parallel gas streams under laminar flow conditions. The ions travel within the upper ion carrier gas stream, which is focused
Fig. 2. Schematic diagram of the swept-field aspiration condenser with two ion species. Ions travel within a gas stream through a variable transverse electric field, which deflects the ions towards the bottom electrode setup. Ions with different ion mobilities are collected at the detector electrode at different drift voltages. The resulting I(V) curve can be transformed back into an ion mobility spectrum by applying the discrete inverse Tammet transform [9].
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Fig. 3. Schematic diagram of the direct ion-focusing aspiration condenser with two ion species. The ions travel within the upper focused ion carrier gas stream. The parallel drift gas stream fills most of the separation region so that a thin layer of ion carrier gas is formed at the counter electrode. While traveling through the transverse electric field ions are forced out of the ion carrier gas flow through the drift gas flow towards the bottom electrode setup. An ion mobility spectrum is directly measured by scanning the drift voltage.
by means of geometric constrains. The parallel drift gas stream fills most of the separation region so that a thin layer of ion carrier gas is formed at the counter electrode. While traveling through the transverse electric field ions are forced out of the ion carrier gas through the drift gas towards the bottom electrode setup. Ion species with different ion mobilities are eventually separated into individual ion beams. An ion mobility spectrum can be directly measured by varying the drift voltage so that the ion beams are successively moved over the detector electrode. Separation power is improved compared to previously mentioned aspiration condenser designs since all ions travel through the entire drift field, which results in a more effective ion separation. Without focusing only ions close to the counter electrode are fully separated. Furthermore, reconstruction of the ion mobility spectrum is not necessary. Still, space charge effects and diffusion cause poor spatial ion separation. Both effects can be reduced by increasing the flow rates in order to minimize ion concentration and drift time. As mentioned, the flow rates are limited since laminar flow conditions are required. A major advantage of separate drift and ion carrier gas is that ambient humidity in
the ion carrier gas does not affect the peak position as shown below. One major drawback of the direct ion-focusing aspiration condenser is the required narrow detector electrode of just a few microns in width to maintain resolving power. For easier fabrication an alternative electrode design with an integrating detector electrode is used, see Fig. 4. This first order differential ion-focusing aspiration condenser requires ion mobility spectrum reconstruction. The ion mobility spectrum results from differentiating the I(V) curve. Due to high signal-to-noise ratio reconstruction is possible without massive signal degradation. 3. Experimental All measurements were carried out with a prototype of the planar first order differential ion-focusing aspiration condenser, see Fig. 5 for device dimensions. Purified dry air with a relative humidity below 0.1% at 22 ◦ C was used as sample gas, ion carrier gas and drift gas. Gas phase analytes were mixed into the sample gas stream well before entering the ionization region.
Fig. 4. Schematic diagram of the first order differential ion-focusing aspiration condenser with two ion species. The ions travel within the upper focused ion carrier gas stream. The parallel drift gas stream fills most of the separation region so that a thin layer of ion carrier gas is formed at the counter electrode. While traveling through the transverse electric field ions are forced out of the ion carrier gas flow through the drift gas flow towards the bottom electrode setup. An ion mobility spectrum is derived from scanning the drift voltage and differentiating the measured I(V) curve.
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Fig. 5. Dimensions of the planar first order differential ion-focusing aspiration condenser. The separation region is 20 mm in depth. The electrodes are sputtered Au thin film electrodes.
After passing the ionization region the sample gas is referred to as ion carrier gas. Beta emission from a tritium source with an activity of 500 MBq and a circular active area of 78.5 mm2 was used for ionization. The dimensions of the ionization chamber were 10 mm × 10 mm × 1 mm. The entire sample gas passed through the ionization chamber. All measurements were carried out at 700 ml/min of sample gas and ion carrier gas, respectively, 3.4 l/min of drift gas and a temperature of 22 ◦ C. The I(V) curves were recorded by stepping the drift voltage from −24 to +24 V in 400 steps. The positive and negative ion mobility spectra were derived by differentiating the I(V) curves. Ion mobility spectra of different sample gases containing just purified air and purified air with 55 and 110 ppb of 1-Octanol were measured. 1-Octanol was purchased from Merck. 4. Results and discussion Figs. 6 and 7 show the measured integrated and reconstructed original ion mobility spectrum of purified dry air containing no analytes. The positive and negative reactant ion peaks (RIP) are identified at drift voltages of UP,RIP+ = 5.94 V and UP,RIP− = −5.64 V. The drift time is constant for all ion species and can be calculated to td = 0.73 ms as in Eq. (4), assuming laminar flow conditions, no transverse ion transport caused by the gas stream and no ion transport in the direction of flow caused
Fig. 7. Ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and drift gas. The positive and negative reactant ion peaks (RIP) are identified at UP,RIP+ = 5.94 V and UP,RIP− = −5.64 V with mobilities of KRIP+ = 2.32 cm2 /V s and KRIP− = 2.19 cm2 /V s. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
by the electric field. A is the cross sectional area of the separation region, xE the width of the detector electrode, vx the mean gas flow velocity in the separation region and V the total gas flow rate. The mobilities KRIP+ = 2.19 cm2 /V s and KRIP− = 2.30 cm2 /V s can be calculated as in Eq. (5), where h is the height of the separation region, hs the ion carrier gas inlet height and UP the peak position. td =
xE xE A h − 0.5hs h(h − 0.5hs ) = = = vx V KE KUP
(4)
K=
h(h − 0.5hs )V AxE UP
(5)
Assuming the initial ion concentration Gaussian in shape and diffusion as the major effect on peak broadening the spatial peak width W of the ion beam can be approximated through Eq. (6). W0 is the initial peak width and Wd the contribution of diffusion. Peak width is measured at 6.25% of the peak height, which equals two times the peak width at half height and about 4.7 times the standard deviation [14]. A rough estimation of W0 is the height of the ion carrier gas inlet. Wd can be calculated through Eq. (7), where σ D is the standard deviation of the diffusion process as described in Eq. (8). The diffusion coefficient D is calculated through the Nernst–Einstein Eq. (9), where kB is Boltzmann’s constant, T the absolute temperature and q the charge of the ion. W 2 = W02 + Wd2 = h2s + 64 ln(2) Wd = 4 2 ln(2)σD σD = 2Dtd D=
Fig. 6. Integrated ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and drift gas. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
kB TK q
kB T (h − 0.5hs ) q UP
(6) (7) (8) (9)
Mapping the spatial peak width of the ion beam onto the xaxis with the beam center at xE gives a rough estimation of the
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spatial peak width WE when moving the ion beam on the detector electrode. 2 xE WE = W 1 + (10) h − 0.5hs Spectral peak width WU can be calculated through Eqs. (11) and (12). Eq. (11) describes the voltage required to move ions with mobility K from the center of the carrier gas inlet to a certain x-position. U = vx
h(h − 0.5hs ) xE = UP Kx x
WU = 2WU,1/2 = U(xE − 0.5WE ) − U(xE + 0.5WE ) WE = UP xE 2 xE − 0.25WE2
(11)
(12)
Spectral peak width at half height WU,1/2 of the positive and negative reactant ion peak can be calculated to 1.55 and 1.51 V. The measured spectral peak width is 2.32 V for the positive and 2.19 V for the negative reactant ion peak. The mismatch between the calculated and measured peak width is mainly due to a more complex ion transport in the gas flow than assumed. Simulations show ion transport in y-direction and even small vortices that cause peak broadening. Furthermore, field inhomogeneity might cause ion transport in the direction of flow and the required smoothing of the raw data before differentiating results in additional peak broadening. Assuming that the peak width behaves as in Eq. (12), fitting the measured peak widths with function (13) gives a good estimation of the peak width at half height. The fitting parameters are A = 2.218 V1/2 , B = 7.925 V and C ≈ 0 V. √ √ U P UP + C U P UP WU,1/2 = A ≈A (13) UP + B UP + B One major drawback of previous aspiration condenser designs is the strong effect of ambient humidity especially on the peak position. In the ion-focusing aspiration condenser the drift gas can be filtered so that the humidity in the separation region remains constant. Due to the low ion carrier gas flow rate as compared to the drift gas flow rate ambient humidity in the ion carrier gas does not significantly change the humidity in the separation region. Fig. 8 shows a significant shift of the peak position for dry ion carrier gas but humid drift gas with relative humidity of 45% at 22 ◦ C. The total ion current remains constant. However, the peak height reduces due to its increasing width, see Eq. (10). On the other hand, there is no peak shift noticeable for dry drift gas and humid ion carrier gas with relative humidity of 45% at 22 ◦ C. Thus, both the positive and the negative reactant ion peak position are independent from ion carrier gas humidity when keeping the drift gas dry. The total ion current slightly increases with increasing humidity in the ion carrier gas, which can be explained by changing recombination and wall reaction rates. For a hand-held device a drift gas loop with filters can be used to keep the drift gas dry and clean during operation, see Fig. 9. The ion-focusing aspiration condenser can be also operated with identical gas streams for the ion carrier
Fig. 8. Ion mobility spectrum (solid line) with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and purified air with relative humidity of 45% at 22 ◦ C as drift gas compared to the ion mobility spectrum (dashed line) with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and drift gas. A significant peak shift is visible. The positive and negative reactant ion peaks are at UP,RIP+ = 7.42 V and UP,RIP− = −6.95 V for humid drift gas and dry sample gas. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
Fig. 9. Schematic flow diagram of a hand-held ion-focusing aspiration condenser IMS with separate sample gas and drift gas.
gas and drift gas, as shown in Fig. 10, but humidity in the drift gas would cause peak shift as mentioned above. Furthermore, analytes in the drift gas would react with reactant ions while traveling through the drift gas. This would result in a decreased separation power. However, separation power could still be better compared to other aspiration condenser IMS without ion focusing. Further investigations are required here. The effect of humidity on the in mobility spectrum will be investigated in detail in the future. Figs. 11 and 12 show the ion mobility spectra of 1-Octanol for different concentrations. Only the positive ion mobility spectrum is presented since no negative product ions are formed. Table 1 lists the ion species, peak positions and intensities of all identified peaks.
Fig. 10. Schematic flow diagram of a hand-held ion-focusing aspiration condenser IMS with identical sample gas and drift gas. Humidity will affect both peak position and height. However, separation power could sill be better compared to other aspiration condenser IMS.
S. Zimmermann et al. / Sensors and Actuators B 125 (2007) 428–434
Fig. 11. Ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as drift gas and purified air with relative humidity below 0.1% at 22 ◦ C containing 55 ppb of 1-Octanol as sample gas. Fitting the spectrum with Gaussian peaks reveals four individual peaks, which are identified as the reactant ion peak, the proton-bound Octanol monomer, dimer and trimer. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
Fig. 12. Ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as drift gas and purified air with relative humidity below 0.1% at 22 ◦ C containing 110 ppb of 1-Octanol as sample gas. Fitting the spectrum with Gaussian peaks reveals four individual peaks, which are identified as the reactant ion peak, the proton-bound Octanol monomer, dimer and trimer. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
As expected, the positive reactant ion peak intensity decreases with an increasing 1-Octanol concentration. At low 1-Octanol concentrations the proton-bound Octanol monomer is mainly formed. Due to slower recombination and wall reaction rates of
433
Fig. 13. Difference between the original and the fitted ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as drift gas and purified air with relative humidity below 0.1% at 22 ◦ C containing 55 ppb of 1Octanol (circles) and 110 ppb of 1-Octanol (dots) as sample gases. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
the product ion cluster the total ion current increases with an increasing 1-Octanol concentration. This also affects the negative reactant ion peak since its recombination rate decreases with increasing 1-Octanol concentration. Thus, the total negative ion current increases. For higher 1-Octanol concentrations the proton-bound Octanol dimer and trimer are formed. The overlapping peaks can be separated by fitting the spectrum with Gaussian peaks. The peak width depends on the peak position as described in Eq. (13). Fig. 13 shows the difference between the measured and fitted ion mobility spectra. The small peak at UP = 7.06 V could indicate an additional low concentrated ion species present in the ion carrier gas. The origin of this impurity is not identified yet. The resolution RA for the ion-focusing aspiration condenser can be written as in Eq. (14), which corresponds to the definition of resolution for drift tube IMS as described in Eq. (3). By applying Eqs. (13) and (5) the resolution RA can be written as in Eq. (15). Thus, the resolution of the ion-focusing aspiration condenser is not a constant as compared to drift tube IMS. A resolution of RA = 2.6 results for the reactant ion peak position at UP = 5.94 V. The resolution increases for higher voltages UP and lower ion mobilities K, respectively. RA =
UP WU,1/2
(14)
RA ≈
UP + B K + B √ √ = A UP A K
(15)
5. Conclusions Table 1 Peak position and intensity of the reactant ion peak and proton-bound Octanol monomer, dimer and trimer peaks Position UP [V]
Reactant ion Monomer Dimer Trimer
5.94 8.75 11.05 13.30
Intensity [pA/V] 55 ppb 1-Octanol
110 ppb 1-Octanol
10.08 9.37 5.63 0.91
4.06 5.33 8.18 5.62
The concept and preliminary results of a novel ion-focusing aspiration condenser as an ion mobility spectrometer are presented. The simple design is easy to fabricate at low-cost. Small dimensions and low power consumption allow the development of a hand-held system. Due to separate ion carrier gas and drift gas flows the ion carrier gas can be focused by means of geometric constrains. Compared to other aspiration condenser IMS ion focusing results in better separation power since all ions travel through the entire deflection field. Another advantage of sep-
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arate drift gas and ion carrier gas is that ambient humidity in the ion carrier gas does not affect the peak position of the reactant ion peak when operating with dry drift gas. Other effects of humidity, such as sensitivity loss, were not investigated. References [1] G.A. Eiceman, Z. Karpas, Ion Mobility Spectrometry, second ed., CRS Press, Taylor & Francis Group, 2005. [2] H.E. Revercomb, E.A. Mason, Theory of plasma chromatography/gaseous electrophoresis—a review, Anal. Chem. 47 (7) (1975) 970–983. [3] W.F. Siems, C. Wu, E.E. Tarver, H. Hill Jr., Measuring the resolving power of ion mobility spectrometers, Anal. Chem. 66 (23) (1994) 4195– 4201. [4] G.E. Spangler, K.N. Vora, J.P. Carrico, Miniature ion mobility spectrometer cell, J. Phys. E: Sci. Instrum. 19 (3) (1986) 191–198. [5] J.I. Baumbach, D. Berger, J.W. Leonhardt, D. Klockow, Ion mobility sensor in environmental analytical chemistry—concept and first results, Int. J. Environ. Anal. Chem. 52 (1–4) (1993) 189–193. [6] J. Xu, W.B. Whitten, J.M. Ramsey, Space charge effects on resolution in a miniature ion mobility spectrometer, Anal. Chem. 72 (23) (2000) 5787–5791. [7] I.A. Buryakov, E.V. Krylov, E.G. Nazarov, U.K. Rasulev, A new method of separation of multi-atomic ions by mobility at atmospheric pressure using a high-frequency amplitude-asymmetric strong electric filed, Int. J. Mass Spectrom. Ion Process. 128 (3) (1993) 143–148. [8] R.A. Miller, G.A. Eiceman, E.G. Nazarov, A.T. King, A novel micromachined high field asymmetric waveform ion mobility spectrometer, Sens. Actuators B 67 (3) (2000) 300–306. [9] P. Puumalainen, Method for detection of foreign matter contents in gases, Patent No. US 5,047,723, 1991. [10] H. Paakkanen, E. K¨arp¨anoja, T. K¨att¨o, T. Karhap¨aa¨ , A. Oinonen, H. Salmi, Method and equipment for definition of foreign matter contents in gases, Patent Application No. WO 94/16320, 1994. [11] E. Sacristan, Ion mobility method and device for gas analysis, Patent No. US 5,455,417, 1995. [12] E. Sacristan, A.A. Solis, A swept-field aspiration condenser as an ion mobility spectrometer, IEEE Trans. Instrum. Meas. 47 (3) (1998) 769–775.
[13] H.F. Tammet, The aspiration method for the determination of atmosphericion spectra, Scientific Notes of Tartu State University, No. 195, 1967, Trans. IPST, 1970. [14] S. Rokushika, H. Hatano, M.A. Baim, H. Hill Jr., Resolution measurement for ion mobility spectrometry, Anal. Chem. 57 (9) (1985) 1902–1907.
Biographies Stefan Zimmermann received his diploma in electrical engineering in 1996 and his PhD in electrical engineering in 2001 from the Technical University Hamburg-Harburg (TUHH), Germany. He worked at the Department of Microsystems Technology at the TUHH from 1996 to 2001. His research focused on MEMS design and fabrication. In 2001, Dr. Zimmermann joined the Berkeley Sensor and Actuator Center at the University of California, Berkeley, USA, as a post-doctoral research engineer with support of a Feodor–Lynen Fellowship of the Alexander von Humboldt Foundation. His research focused on BioMEMS and the development of a disposable continuous glucose monitor. In 2004, Dr. Zimmermann joined the Research Unit of the Draegerwerk AG, Germany. He is currently working on MEMS for medical and safety applications. Nora Abel studied physics at the Technical University Berlin, Germany, with concentration on laser physics and medical technology. In 2005, she joined the Research Unit of the Draegerwerk AG, first as an intern for infrared optical multi-gas analysis. In 2006 she started working on her diploma thesis in data analysis of ion mobility spectra. Wolfgang Baether received his diploma in organic chemistry in 1980. His doctoral thesis was in the field of organic mass spectrometry, 1984, University of Bielefeld, Germany. In 1984 he joined the Draegerwerk AG, Luebeck, Germany. His work focused on detector tube development. In 1989, he became head of the tube development and application technology department. In 1998, he joined the Draeger Research Group as a technology scout for gas measuring technologies based on ionization in the gas phase with a focus on ion mobility spectrometry. Sebastian Barth received his diploma in information technology in 2006 from the Otto-von-Guericke University Magdeburg, Germany. His studies concentrated on digital signal processing, electrical engineering and speech processing. Sebastian Barth is currently working at the Research Unit of the Draegerwerk AG, Germany, towards his PhD degree. His research focuses on the simulation and numerical analysis of gas phase sensors.
An ion-focusing aspiration condenser as an ion mobility spectrometer Stefan Zimmermann ∗ , Nora Abel, Wolfgang Baether, Sebastian Barth Draegerwerk AG, Research Unit, Moislinger Allee 53-55, D-23542 Luebeck, Germany Received 7 August 2006; received in revised form 15 February 2007; accepted 22 February 2007 Available online 2 March 2007
Abstract Ion mobility spectrometry is a well-known method for detecting hazardous compounds in air. Detection limits in the low ppb range and fast response times make this technique more and more popular. Typical applications are the detection of chemical warfare agents, toxic industrial compounds, explosives and drugs. This paper describes the concept and preliminary results of a simple planar ion-focusing aspiration condenser as an ion mobility spectrometer (IMS). Compared to other aspiration condenser IMS with identical ion carrier gas and drift gas, our design has separate gas flows. This allows fluidic focusing of the ion carrier gas by means of geometric constrains. Thus, ions are focused before separation, which results in improved separation power. Furthermore, humidity in the ion carrier gas does not effect peak position in the ion mobility spectrum when using dry drift gas. In addition, the system is easy to fabricate at low-cost, has small dimensions and low power consumption. First measurements of 1-Octanol in air demonstrate the potential of this novel concept. © 2007 Elsevier B.V. All rights reserved. Keywords: Ion mobility spectrometer; IMS; Aspiration condenser; MEMS
ual ion swarms containing just one ion species. An ion mobility spectrum results by plotting the ion current over drift time.
1. Introduction The principle of ion mobility spectrometry (IMS) is based on the characteristic drift velocity of gas phase ions in an electric field at atmospheric pressure. The ion formation process is described in detail in Ref. [1]. For low electric fields the drift velocity vd is proportional to the electric field strength E as in Eq. (1): vd = KE
(1)
The proportionality coefficient K is called ion mobility and depends on the pressure, temperature, ion mass and collision cross section [1,2]. Under constant conditions the ion mobility K is a characteristic measure for a certain ion species. In time-of-flight IMS, herein after referred to as drift tube IMS, an ion swarm of different ion species is injected into a homogeneous drift field, which forces the ions towards a detector. The time needed for the ions to travel the distance d between the ion shutter and detector is called drift time td . Thus, different ion mobilities K lead to different drift times td as in Eq. (2). The initial ion swarm eventually separates in a number of individ∗
Corresponding author. Tel.: +49 451 882 4546; fax: +49 451 882 3921. E-mail address: [email protected] (S. Zimmermann).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.02.038
td =
d d = vd KE
(2)
Most drift tube IMS have high resolution of R > 20. The definition of the resolution R for drift tube IMS is given in Eq. (3), where td is the drift time and Wt,1/2 the temporal peak width measured at half height [3]: R=
td Wt,1/2
(3)
However, drift tube IMS are expensive precision instruments and miniaturization is difficult due to the complex design. Peak broadening caused by field inhomogeneity, space charge effects and diffusion becomes more significant at small dimensions, which limits the resolution of miniaturized drift tube IMS [4–6]. A different miniaturization approach is based on field asymmetric ion mobility spectrometry (FAIMS), also referred to as differential mobility spectrometry (DMS), where a strong highfrequency electric field is used for ion separation. The separation principle is based on the effect of the electric field strength on the ion mobility. A detailed description of FAIMS is given in Ref. [7]. Miller et al. [8] developed a micro-machined planar FAIMS using standard MEMS fabrication technologies. This
S. Zimmermann et al. / Sensors and Actuators B 125 (2007) 428–434
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Fig. 1. Schematic diagram of the aspiration condenser with two ion species. Ions travel within a gas stream through a transverse electric field, which deflects the ions towards a number of detector electrodes. Ions with different ion mobilities are collected at different detector electrodes. Pattern recognition based on the ion currents In is employed to identify the ion species.
device is easy to fabricate and has good separation power but suffers from high power consumption due to the required strong high-frequency electric field, which makes the development of a hand-held FAIMS difficult. The objective of our research is the development of a simple IMS design that is easy to fabricate at low-cost, has small dimensions, low power consumption and sufficient separation power. Our concept is based on the aspiration condenser developed by Puumalainen [9] and Paakkanen et al. [10], where ions travel within a gas stream through a transverse electric field, which forces the ions towards a number of detector electrodes, see Fig. 1. Different ion species with different ion mobilities are collected at different detector electrodes. Pattern recognition is employed for ion identification. Major drawbacks of this concept are the low resolving power and the immense training effort, which is required for identifying gas mixtures. The low resolving power results from poor spatial ion separation mainly caused by space charge effects and diffusion. Both effects can be reduced by increasing the flow rate so that ion concentration and drift time decrease. However, the flow rate is limited since laminar flow conditions are required. An improved aspiration condenser was developed by Sacristan et al. [11,12]. This swept-field aspiration condenser uses
a variable electric drift field to move all ion species across a single detector electrode, see Fig. 2. A drift voltage scan with n discrete steps is used instead of n individual detector electrodes. The resulting I(V) curve is transformed into an ion mobility spectrum by applying the discrete inverse Tammet transform [13]. This allows ion identification by locating the peak positions in the ion mobility spectrum—a tremendous advantage over pattern recognition when analyzing unknown gas mixtures. However, reconstruction of the ion mobility spectrum is difficult in practice, especially for low signal-to-noise ratios. The required regularization of the discrete direct Tammet transform limits the resolution of the swept-field aspiration condenser. Our approach to gain resolving power is a swept-field aspiration condenser like design with ion focusing. This allows direct measurement of ion mobility spectra and has higher separation power. 2. Ion-focusing aspiration condenser The concept of the ion-focusing aspiration condenser is shown in Fig. 3. It is based on two different gas flows that form parallel gas streams under laminar flow conditions. The ions travel within the upper ion carrier gas stream, which is focused
Fig. 2. Schematic diagram of the swept-field aspiration condenser with two ion species. Ions travel within a gas stream through a variable transverse electric field, which deflects the ions towards the bottom electrode setup. Ions with different ion mobilities are collected at the detector electrode at different drift voltages. The resulting I(V) curve can be transformed back into an ion mobility spectrum by applying the discrete inverse Tammet transform [9].
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Fig. 3. Schematic diagram of the direct ion-focusing aspiration condenser with two ion species. The ions travel within the upper focused ion carrier gas stream. The parallel drift gas stream fills most of the separation region so that a thin layer of ion carrier gas is formed at the counter electrode. While traveling through the transverse electric field ions are forced out of the ion carrier gas flow through the drift gas flow towards the bottom electrode setup. An ion mobility spectrum is directly measured by scanning the drift voltage.
by means of geometric constrains. The parallel drift gas stream fills most of the separation region so that a thin layer of ion carrier gas is formed at the counter electrode. While traveling through the transverse electric field ions are forced out of the ion carrier gas through the drift gas towards the bottom electrode setup. Ion species with different ion mobilities are eventually separated into individual ion beams. An ion mobility spectrum can be directly measured by varying the drift voltage so that the ion beams are successively moved over the detector electrode. Separation power is improved compared to previously mentioned aspiration condenser designs since all ions travel through the entire drift field, which results in a more effective ion separation. Without focusing only ions close to the counter electrode are fully separated. Furthermore, reconstruction of the ion mobility spectrum is not necessary. Still, space charge effects and diffusion cause poor spatial ion separation. Both effects can be reduced by increasing the flow rates in order to minimize ion concentration and drift time. As mentioned, the flow rates are limited since laminar flow conditions are required. A major advantage of separate drift and ion carrier gas is that ambient humidity in
the ion carrier gas does not affect the peak position as shown below. One major drawback of the direct ion-focusing aspiration condenser is the required narrow detector electrode of just a few microns in width to maintain resolving power. For easier fabrication an alternative electrode design with an integrating detector electrode is used, see Fig. 4. This first order differential ion-focusing aspiration condenser requires ion mobility spectrum reconstruction. The ion mobility spectrum results from differentiating the I(V) curve. Due to high signal-to-noise ratio reconstruction is possible without massive signal degradation. 3. Experimental All measurements were carried out with a prototype of the planar first order differential ion-focusing aspiration condenser, see Fig. 5 for device dimensions. Purified dry air with a relative humidity below 0.1% at 22 ◦ C was used as sample gas, ion carrier gas and drift gas. Gas phase analytes were mixed into the sample gas stream well before entering the ionization region.
Fig. 4. Schematic diagram of the first order differential ion-focusing aspiration condenser with two ion species. The ions travel within the upper focused ion carrier gas stream. The parallel drift gas stream fills most of the separation region so that a thin layer of ion carrier gas is formed at the counter electrode. While traveling through the transverse electric field ions are forced out of the ion carrier gas flow through the drift gas flow towards the bottom electrode setup. An ion mobility spectrum is derived from scanning the drift voltage and differentiating the measured I(V) curve.
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Fig. 5. Dimensions of the planar first order differential ion-focusing aspiration condenser. The separation region is 20 mm in depth. The electrodes are sputtered Au thin film electrodes.
After passing the ionization region the sample gas is referred to as ion carrier gas. Beta emission from a tritium source with an activity of 500 MBq and a circular active area of 78.5 mm2 was used for ionization. The dimensions of the ionization chamber were 10 mm × 10 mm × 1 mm. The entire sample gas passed through the ionization chamber. All measurements were carried out at 700 ml/min of sample gas and ion carrier gas, respectively, 3.4 l/min of drift gas and a temperature of 22 ◦ C. The I(V) curves were recorded by stepping the drift voltage from −24 to +24 V in 400 steps. The positive and negative ion mobility spectra were derived by differentiating the I(V) curves. Ion mobility spectra of different sample gases containing just purified air and purified air with 55 and 110 ppb of 1-Octanol were measured. 1-Octanol was purchased from Merck. 4. Results and discussion Figs. 6 and 7 show the measured integrated and reconstructed original ion mobility spectrum of purified dry air containing no analytes. The positive and negative reactant ion peaks (RIP) are identified at drift voltages of UP,RIP+ = 5.94 V and UP,RIP− = −5.64 V. The drift time is constant for all ion species and can be calculated to td = 0.73 ms as in Eq. (4), assuming laminar flow conditions, no transverse ion transport caused by the gas stream and no ion transport in the direction of flow caused
Fig. 7. Ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and drift gas. The positive and negative reactant ion peaks (RIP) are identified at UP,RIP+ = 5.94 V and UP,RIP− = −5.64 V with mobilities of KRIP+ = 2.32 cm2 /V s and KRIP− = 2.19 cm2 /V s. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
by the electric field. A is the cross sectional area of the separation region, xE the width of the detector electrode, vx the mean gas flow velocity in the separation region and V the total gas flow rate. The mobilities KRIP+ = 2.19 cm2 /V s and KRIP− = 2.30 cm2 /V s can be calculated as in Eq. (5), where h is the height of the separation region, hs the ion carrier gas inlet height and UP the peak position. td =
xE xE A h − 0.5hs h(h − 0.5hs ) = = = vx V KE KUP
(4)
K=
h(h − 0.5hs )V AxE UP
(5)
Assuming the initial ion concentration Gaussian in shape and diffusion as the major effect on peak broadening the spatial peak width W of the ion beam can be approximated through Eq. (6). W0 is the initial peak width and Wd the contribution of diffusion. Peak width is measured at 6.25% of the peak height, which equals two times the peak width at half height and about 4.7 times the standard deviation [14]. A rough estimation of W0 is the height of the ion carrier gas inlet. Wd can be calculated through Eq. (7), where σ D is the standard deviation of the diffusion process as described in Eq. (8). The diffusion coefficient D is calculated through the Nernst–Einstein Eq. (9), where kB is Boltzmann’s constant, T the absolute temperature and q the charge of the ion. W 2 = W02 + Wd2 = h2s + 64 ln(2) Wd = 4 2 ln(2)σD σD = 2Dtd D=
Fig. 6. Integrated ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and drift gas. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
kB TK q
kB T (h − 0.5hs ) q UP
(6) (7) (8) (9)
Mapping the spatial peak width of the ion beam onto the xaxis with the beam center at xE gives a rough estimation of the
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spatial peak width WE when moving the ion beam on the detector electrode. 2 xE WE = W 1 + (10) h − 0.5hs Spectral peak width WU can be calculated through Eqs. (11) and (12). Eq. (11) describes the voltage required to move ions with mobility K from the center of the carrier gas inlet to a certain x-position. U = vx
h(h − 0.5hs ) xE = UP Kx x
WU = 2WU,1/2 = U(xE − 0.5WE ) − U(xE + 0.5WE ) WE = UP xE 2 xE − 0.25WE2
(11)
(12)
Spectral peak width at half height WU,1/2 of the positive and negative reactant ion peak can be calculated to 1.55 and 1.51 V. The measured spectral peak width is 2.32 V for the positive and 2.19 V for the negative reactant ion peak. The mismatch between the calculated and measured peak width is mainly due to a more complex ion transport in the gas flow than assumed. Simulations show ion transport in y-direction and even small vortices that cause peak broadening. Furthermore, field inhomogeneity might cause ion transport in the direction of flow and the required smoothing of the raw data before differentiating results in additional peak broadening. Assuming that the peak width behaves as in Eq. (12), fitting the measured peak widths with function (13) gives a good estimation of the peak width at half height. The fitting parameters are A = 2.218 V1/2 , B = 7.925 V and C ≈ 0 V. √ √ U P UP + C U P UP WU,1/2 = A ≈A (13) UP + B UP + B One major drawback of previous aspiration condenser designs is the strong effect of ambient humidity especially on the peak position. In the ion-focusing aspiration condenser the drift gas can be filtered so that the humidity in the separation region remains constant. Due to the low ion carrier gas flow rate as compared to the drift gas flow rate ambient humidity in the ion carrier gas does not significantly change the humidity in the separation region. Fig. 8 shows a significant shift of the peak position for dry ion carrier gas but humid drift gas with relative humidity of 45% at 22 ◦ C. The total ion current remains constant. However, the peak height reduces due to its increasing width, see Eq. (10). On the other hand, there is no peak shift noticeable for dry drift gas and humid ion carrier gas with relative humidity of 45% at 22 ◦ C. Thus, both the positive and the negative reactant ion peak position are independent from ion carrier gas humidity when keeping the drift gas dry. The total ion current slightly increases with increasing humidity in the ion carrier gas, which can be explained by changing recombination and wall reaction rates. For a hand-held device a drift gas loop with filters can be used to keep the drift gas dry and clean during operation, see Fig. 9. The ion-focusing aspiration condenser can be also operated with identical gas streams for the ion carrier
Fig. 8. Ion mobility spectrum (solid line) with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and purified air with relative humidity of 45% at 22 ◦ C as drift gas compared to the ion mobility spectrum (dashed line) with purified air with relative humidity below 0.1% at 22 ◦ C as sample gas and drift gas. A significant peak shift is visible. The positive and negative reactant ion peaks are at UP,RIP+ = 7.42 V and UP,RIP− = −6.95 V for humid drift gas and dry sample gas. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
Fig. 9. Schematic flow diagram of a hand-held ion-focusing aspiration condenser IMS with separate sample gas and drift gas.
gas and drift gas, as shown in Fig. 10, but humidity in the drift gas would cause peak shift as mentioned above. Furthermore, analytes in the drift gas would react with reactant ions while traveling through the drift gas. This would result in a decreased separation power. However, separation power could still be better compared to other aspiration condenser IMS without ion focusing. Further investigations are required here. The effect of humidity on the in mobility spectrum will be investigated in detail in the future. Figs. 11 and 12 show the ion mobility spectra of 1-Octanol for different concentrations. Only the positive ion mobility spectrum is presented since no negative product ions are formed. Table 1 lists the ion species, peak positions and intensities of all identified peaks.
Fig. 10. Schematic flow diagram of a hand-held ion-focusing aspiration condenser IMS with identical sample gas and drift gas. Humidity will affect both peak position and height. However, separation power could sill be better compared to other aspiration condenser IMS.
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Fig. 11. Ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as drift gas and purified air with relative humidity below 0.1% at 22 ◦ C containing 55 ppb of 1-Octanol as sample gas. Fitting the spectrum with Gaussian peaks reveals four individual peaks, which are identified as the reactant ion peak, the proton-bound Octanol monomer, dimer and trimer. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
Fig. 12. Ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as drift gas and purified air with relative humidity below 0.1% at 22 ◦ C containing 110 ppb of 1-Octanol as sample gas. Fitting the spectrum with Gaussian peaks reveals four individual peaks, which are identified as the reactant ion peak, the proton-bound Octanol monomer, dimer and trimer. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
As expected, the positive reactant ion peak intensity decreases with an increasing 1-Octanol concentration. At low 1-Octanol concentrations the proton-bound Octanol monomer is mainly formed. Due to slower recombination and wall reaction rates of
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Fig. 13. Difference between the original and the fitted ion mobility spectrum with purified air with relative humidity below 0.1% at 22 ◦ C as drift gas and purified air with relative humidity below 0.1% at 22 ◦ C containing 55 ppb of 1Octanol (circles) and 110 ppb of 1-Octanol (dots) as sample gases. Note: After passing the ionization chamber the sample gas is referred to as ion carrier gas.
the product ion cluster the total ion current increases with an increasing 1-Octanol concentration. This also affects the negative reactant ion peak since its recombination rate decreases with increasing 1-Octanol concentration. Thus, the total negative ion current increases. For higher 1-Octanol concentrations the proton-bound Octanol dimer and trimer are formed. The overlapping peaks can be separated by fitting the spectrum with Gaussian peaks. The peak width depends on the peak position as described in Eq. (13). Fig. 13 shows the difference between the measured and fitted ion mobility spectra. The small peak at UP = 7.06 V could indicate an additional low concentrated ion species present in the ion carrier gas. The origin of this impurity is not identified yet. The resolution RA for the ion-focusing aspiration condenser can be written as in Eq. (14), which corresponds to the definition of resolution for drift tube IMS as described in Eq. (3). By applying Eqs. (13) and (5) the resolution RA can be written as in Eq. (15). Thus, the resolution of the ion-focusing aspiration condenser is not a constant as compared to drift tube IMS. A resolution of RA = 2.6 results for the reactant ion peak position at UP = 5.94 V. The resolution increases for higher voltages UP and lower ion mobilities K, respectively. RA =
UP WU,1/2
(14)
RA ≈
UP + B K + B √ √ = A UP A K
(15)
5. Conclusions Table 1 Peak position and intensity of the reactant ion peak and proton-bound Octanol monomer, dimer and trimer peaks Position UP [V]
Reactant ion Monomer Dimer Trimer
5.94 8.75 11.05 13.30
Intensity [pA/V] 55 ppb 1-Octanol
110 ppb 1-Octanol
10.08 9.37 5.63 0.91
4.06 5.33 8.18 5.62
The concept and preliminary results of a novel ion-focusing aspiration condenser as an ion mobility spectrometer are presented. The simple design is easy to fabricate at low-cost. Small dimensions and low power consumption allow the development of a hand-held system. Due to separate ion carrier gas and drift gas flows the ion carrier gas can be focused by means of geometric constrains. Compared to other aspiration condenser IMS ion focusing results in better separation power since all ions travel through the entire deflection field. Another advantage of sep-
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arate drift gas and ion carrier gas is that ambient humidity in the ion carrier gas does not affect the peak position of the reactant ion peak when operating with dry drift gas. Other effects of humidity, such as sensitivity loss, were not investigated. References [1] G.A. Eiceman, Z. Karpas, Ion Mobility Spectrometry, second ed., CRS Press, Taylor & Francis Group, 2005. [2] H.E. Revercomb, E.A. Mason, Theory of plasma chromatography/gaseous electrophoresis—a review, Anal. Chem. 47 (7) (1975) 970–983. [3] W.F. Siems, C. Wu, E.E. Tarver, H. Hill Jr., Measuring the resolving power of ion mobility spectrometers, Anal. Chem. 66 (23) (1994) 4195– 4201. [4] G.E. Spangler, K.N. Vora, J.P. Carrico, Miniature ion mobility spectrometer cell, J. Phys. E: Sci. Instrum. 19 (3) (1986) 191–198. [5] J.I. Baumbach, D. Berger, J.W. Leonhardt, D. Klockow, Ion mobility sensor in environmental analytical chemistry—concept and first results, Int. J. Environ. Anal. Chem. 52 (1–4) (1993) 189–193. [6] J. Xu, W.B. Whitten, J.M. Ramsey, Space charge effects on resolution in a miniature ion mobility spectrometer, Anal. Chem. 72 (23) (2000) 5787–5791. [7] I.A. Buryakov, E.V. Krylov, E.G. Nazarov, U.K. Rasulev, A new method of separation of multi-atomic ions by mobility at atmospheric pressure using a high-frequency amplitude-asymmetric strong electric filed, Int. J. Mass Spectrom. Ion Process. 128 (3) (1993) 143–148. [8] R.A. Miller, G.A. Eiceman, E.G. Nazarov, A.T. King, A novel micromachined high field asymmetric waveform ion mobility spectrometer, Sens. Actuators B 67 (3) (2000) 300–306. [9] P. Puumalainen, Method for detection of foreign matter contents in gases, Patent No. US 5,047,723, 1991. [10] H. Paakkanen, E. K¨arp¨anoja, T. K¨att¨o, T. Karhap¨aa¨ , A. Oinonen, H. Salmi, Method and equipment for definition of foreign matter contents in gases, Patent Application No. WO 94/16320, 1994. [11] E. Sacristan, Ion mobility method and device for gas analysis, Patent No. US 5,455,417, 1995. [12] E. Sacristan, A.A. Solis, A swept-field aspiration condenser as an ion mobility spectrometer, IEEE Trans. Instrum. Meas. 47 (3) (1998) 769–775.
[13] H.F. Tammet, The aspiration method for the determination of atmosphericion spectra, Scientific Notes of Tartu State University, No. 195, 1967, Trans. IPST, 1970. [14] S. Rokushika, H. Hatano, M.A. Baim, H. Hill Jr., Resolution measurement for ion mobility spectrometry, Anal. Chem. 57 (9) (1985) 1902–1907.
Biographies Stefan Zimmermann received his diploma in electrical engineering in 1996 and his PhD in electrical engineering in 2001 from the Technical University Hamburg-Harburg (TUHH), Germany. He worked at the Department of Microsystems Technology at the TUHH from 1996 to 2001. His research focused on MEMS design and fabrication. In 2001, Dr. Zimmermann joined the Berkeley Sensor and Actuator Center at the University of California, Berkeley, USA, as a post-doctoral research engineer with support of a Feodor–Lynen Fellowship of the Alexander von Humboldt Foundation. His research focused on BioMEMS and the development of a disposable continuous glucose monitor. In 2004, Dr. Zimmermann joined the Research Unit of the Draegerwerk AG, Germany. He is currently working on MEMS for medical and safety applications. Nora Abel studied physics at the Technical University Berlin, Germany, with concentration on laser physics and medical technology. In 2005, she joined the Research Unit of the Draegerwerk AG, first as an intern for infrared optical multi-gas analysis. In 2006 she started working on her diploma thesis in data analysis of ion mobility spectra. Wolfgang Baether received his diploma in organic chemistry in 1980. His doctoral thesis was in the field of organic mass spectrometry, 1984, University of Bielefeld, Germany. In 1984 he joined the Draegerwerk AG, Luebeck, Germany. His work focused on detector tube development. In 1989, he became head of the tube development and application technology department. In 1998, he joined the Draeger Research Group as a technology scout for gas measuring technologies based on ionization in the gas phase with a focus on ion mobility spectrometry. Sebastian Barth received his diploma in information technology in 2006 from the Otto-von-Guericke University Magdeburg, Germany. His studies concentrated on digital signal processing, electrical engineering and speech processing. Sebastian Barth is currently working at the Research Unit of the Draegerwerk AG, Germany, towards his PhD degree. His research focuses on the simulation and numerical analysis of gas phase sensors.