Field Switching Combined with BradburyâNielsen Gate for Ion

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Field switching combined with BradburyNeilsen gate for ion mobility spectrometry Chuang Chen, Mahmoud Tabrizchi, Weiguo Wang, and Haiyang Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b01737 • Publication Date (Web): 05 Jul 2015 Downloaded from http://pubs.acs.org on July 9, 2015

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Analytical Chemistry

Field switching combined with Bradbury-Nielsen gate for ion mobility spectrometry Chuang Chen1, Mahmoud Tabrizchi2,*, Weiguo Wang1, and Haiyang Li1,* 1

Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, People’s Republic of China. 2

*

Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111.

Corresponding authors. Haiyang Li, Tel.: +86-411-84379510, e-mail: [email protected];

Mahmoud Tabrizchi, e-mail: [email protected].

ACS Paragon Plus Environment

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Abstract

Bradbury-Nielsen gate (BNG) is commonly used in ion mobility spectrometers. It, however, transmits only a small fraction of the ions into the drift region, typically 1%. In contrast, all ions in the ionization chamber could be efficiently compressed into the drift region by field switching gate (FSG). We report in this paper on the simultaneous use of BNG and field switching (FS) to enhance the ion utilization of the BNG. In this technique, the FS collects the ions existing in the region between the FS electrode and the BNG and drives them quickly going through the BNG in the period of gate opening. The BNG acts as the retarding field in the reported FSG to stop ions from diffusing into the drift region in the period of gate closing. Using this technique, an increase of at least ten-fold in the ion peak height without any loss of resolution is achieved for acetone comparing with the BNG only approach at a gate pulse width of 150 µs, and an even larger improvement factor of 21 is achieved for heavier DMMP dimer ions. This technique can be adapted to the current BNG-based ion mobility instruments to significantly enhance their sensitivity without any modification of the drift tube hardware.


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Introduction Drift tube ion mobility spectrometry (IMS) is widely used in portable detection instruments, especially for the detection and identification of narcotics, explosives, chemical warfare agents, and other hazardous substances1, 2. The system usually consists of two parts, an ionization region and an ion drift region separated by an ion shutter or shutter grid that controls the injection of ions into the drift tube. Ions are generated in the ionization region, and an ion pulse is admitted to the drift region for a short period of time during which the ion shutter is open. The pulsed ion group then travels down the drift region at a velocity proportional to the electric field and the ion mobility3-5. Ion shutter is a key element in the design of every drift tube ion mobility spectrometer. The signal intensity in IMS depends not only on the number of ions that are generated in the ionization region but also on the percentage of the ions penetrating through the shutter grid to traverse the drift region. Low ion transmission of a shutter grid usually results in a poor signalto-noise ratio6, 7. The most commonly used ion shutter type in ion mobility spectrometers is so-called BradburyNielsen gate (BNG). So far, several methods have been used to make BNG such as etching8, weaving9-11, machine-weaving12, 13, micromachining14, hand-woven and print-circuit-board (PCB) methods6, 15. Usually, a BNG consists of two sets of parallel wires placed in single plane perpendicular to the moving direction of the drift ions16. The two sets of wires are biased to an electrical potential, creating an orthogonal field, to deflect or block ion passage into the drift region. One of the drawbacks of using BNG is that it only allows ions to pass through into the drift region for a relatively short opening period, typically 0.2 ms comparing with the long operational


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period of 20 ms, which means that almost 99% of the ions produced are annihilated on the shutter grid wires. Moreover, depletion effect is an intrinsic characteristic of the structure and operation principle for BNG6, 7, 17. The perpendicular electric field produced when it is closed not only exists within the shutter grid plane, but also disperses beyond the shutter grid plane. That dispersion distorts the electric fields within the vicinity of the shutter grid plane and therefore reduces the effective opening time to shorter than the pulse duration applied on the BNG and leads to mobility discrimination and further losses of ions. A less commonly used ion shutter for IMS is the field switching gate (FSG) reported earlier by Jenkins18 and Leonhardt et al.19 as well as most recently by Kirk et al.20, 21. In this type of design, a simple metallic net grid is used to separate the field switching region from the drift region. The field switching region is a narrow field-free region where an ion source like Tritium is contained to build up ion population. The drift field inside the field switching region is periodically switched on and off to inject the ions into the drift region. The underlying operational principles of both FSG and BNG are presented in Figure S-1. A comprehensive comparison between FSG and BNG is given by Kirk et al.20. By using FSG, the surviving ions in the field switching region that are accumulated over a relatively long closing period are compressed into a short ion pulse, typically 5µs20, 21. This increases ion density and instantaneously collects ions more efficiently, nearly all the ions in the field switching region. The FSG, however, exhibits limited flexibility concerning the ion source. It is difficult to work with ion sources of long ionization depth like 63Ni or UV sources or ion sources requiring an electric field for ionization such as corona discharge or electrospray, while the BNG is universally applicable. Based on the theoretical considerations and field simulations, Kirk et al.20


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concluded that the BNG, even its operating principle results in field non-homogeneities, is of more universally applicable and typically less complex for long injection widths. In this work, to enhance the utilization of the ions in the ionization region for BNG, we propose combining field switching (FS) with BNG to take advantage of the high efficient ion collection capability of FSG. Aided by SIMION simulation, we demonstrate that the FS enhances the signal to noise ratio of the conventional BNG-based IMS instruments by at least a factor of 10 with no sacrifice in the resolving power. Operation principle of the combined FS and BNG A simple model was proposed to describe the ionic loss mechanism in the vicinity of BNG shutter grid6, where BNG was assumed to function as a chopper cutting the ion beam into successive ion slices by opening and closing repeatedly. When BNG closes, the dispersion of the perpendicular electric field beyond the shutter grid plane increases its actual cutting width6. That width is larger than the diameter of the grid wires. No ion is available at the immediate vicinity of the grid wires when BNG opens. Ions are available at a distance of half the actual cutting width from the shutter grid plane. To pass through the BNG, those ions have to travel that distance first. Obviously, increasing the electric field in the ionization region enhances ion migration towards the shutter grid. The concept of combining FS with BNG is schematically shown in Figure 1. During the closing period, the BNG is closed to stop the ions in the ionization region from penetrating into the drift region and the electrode Efs is kept at a normal voltage forming a homogenous electric field to drive ions in the ionization region moving towards the BNG. During the opening period, the BNG is opened and a high voltage pulse is simultaneously applied to the Efs to build a strong electric field between the Efs and BNG for quickly sweeping the ions in this region into the drift


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region. In this concept, the ion source is moved out of the field switching region. The operations of the ion source and the combined FS and BNG are independent. The length of the field switching region can be freely defined and fixed according to need if a high voltage switch satisfying the experiment requirements is available. The ion source is also free to choose, no worrying about their ionization depth or required ionization electric field. SIMION Simulation In a previous study, aided by SIMION simulation, a three-zone theory was proposed to characterize the electric field features within the vicinity of the BNG adjacent to the drift region22. During the closing period of BNG, as the electric potential for one set of the gate wires is set constantly and the other set is applied with an extra gating voltage difference (GVD), there are electric potential ridges and valleys formed in the vicinity of the high and low voltage grid wires, respectively. The electric field in the vicinity of the low voltage grid wires is divided into three zones: the depletion, dispersion, and compression zones. It was found that part of the ions passed the BNG is annihilated in the depletion zone, causing peak heights to lower. Obviously, increasing the GVD enlarges the depletion zone and thus depletes more ions, resulting in a much more reduced peak area. This is consistent with the results reported by Eiceman et al.17 and Tadjimukhamedov et al.23. Here, the SDS model of SIMION simulation was used to explore how the ions behave within the vicinity of the BNG adjacent to the ionization region. The repulsion between ions was not considered and the geometry of the IMS drift tube was simplified as described below to clearly show the electric field and ion distribution features. Two 1 mm-thick circular electrodes were positioned in parallel to each other at y = 0 mm and y = 8.1 mm and to which were applied with potentials of 300 and 570 volts, respectively. Thus a homogeneous electric field of E0 = 386


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V/cm was established between them. A BN gate15, with a wire diameter of 0.1 mm and an adjacent wire-center distance of 1 mm, was placed in the middle of the two plates separating the space into the ionization and drift regions. The potential for one set of the shutter grid wires was fixed at 435 volts, and the other set was applied with an extra GVD of 135 volts to create an electric field 3.5 times that of the drift field6, 7, 24 when the gate is closed. Figures 2a and 2c display the iso-potential diagrams for GVDs equal to 0 volts (BNG open) and 135 volts (BNG closed), respectively. Clearly, when the BNG closed, potential ridges and valleys formed at the vicinity of the BNG, and the drift region obtained a field stronger than the ionization region. This is due to the extra applied GVD of 135 volts. To further understand how those potential ridges and valleys affect ion behavior, a uniform distribution of 10000 protonated water ions in the ionization region was created at timing start point t = 0 µs when the BNG was about to change from open to close (Figure 2b). Then, the BNG closed for the next 50 µs while the ions were migrating. As shown in Figure 2d, at timing point t = 50 µs, the trailing edge of the ion packet distanced about 1/4 mm from its origin. The leading edge, however, stayed behind the BNG while the ions in the vicinity of the high voltage grid wires were depleted, forming a zigzag-like pattern. The inset in Figure 2d provides a better display of the depletion occurring within 1 mm from the grid wires. In fact, the ions were diffracted into the potential valleys around the low voltage grid wires by the potential ridges around the high voltage grid wires. Thus, in the next opening period, the majority portion of the ions available to pass through the shutter grid existed at a short distance from the shutter grid plane. As Figure 2f displays, at timing point t = 80 µs, the depletion gap behind the BNG shutter grid was just filled up and only few ions of the ion packet penetrated the shutter grid into the drift region after the BNG reopened for next 30 µs. If a strong field was


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applied behind the shutter grid at timing point t = 50 µs when the BNG was about to reopen, it would help the ions to fill the gap quickly and penetrate through the gate before it closed again. In addition, the ions in the ionization regions would move faster as increasing electric field behind the shutter grids. Consequently, those ions farther away from the shutter grid would also be collected and compressed into the drift region to give rise to enhanced signal intensity. To validate the above speculation, at timing point t = 50 µs, the GVD was removed and the voltage on the top electrode was simultaneously switched from 570 volts to 2070 volts for 30 µs when watching the movement of the ions behind the shutter grid. As Figure 2h displays, at timing point t = 80 µs, all the ions in the ionization region were efficiently swept into the drift region and compressed into a narrow ion slice. This is mainly due to the highly enhanced electric field (4671 V/cm) in the ionization region and the so-caused distinctive electric field difference between the ionization and the drift regions. That distinctive electric field difference forms a spatial compression condition25 for the ions going through the BNG, as shown in Figure 2g. Experimental The ion mobility spectrometer was constructed at Dalian Institute of Chemical Physics. As schematically displayed in Figure 3, the IMS cell consisted of a drift region of 7.2 cm in length and an ionization region of 1.6 cm in length. The drift region was confined by 9 discrete stainless steel guard rings (each 1 mm thick) evenly separated by Teflon rings (each 7 mm thick). The ionization source was a cylindrical 10 mCi 63Ni source with. A BN gate with an adjacent wirecenter space of 1 mm and a wire diameter of 0.1 mm was placed between the ionization region with an inner diameter of 14 mm and the drift region with an inner diameter of 25 mm. The IMS temperature was kept at 100 degrees Celsius.


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A high voltage power supply (HV1) with adjustable output from 0 to +5000 volts and a serial of 1MΩ resistors were used to form a homogenous electric field 338 V/cm across the IMS drift tube. Two high voltage pulse generators (4130 DEI, USA) and two isolated high voltage power supplies were used to create two high voltage pulses (Figure 3, bottom). One high voltage pulse (blue) with amplitude of 3000 volts was applied to the annular electrode EFS for collecting the ions existing in the region between the EFS and the BNG. The second high voltage pulse (red) with amplitude of 80 volts was applied to the BNG for chopping a narrow slice of ions into the drift region. A digital delay/pulse generator was used for time correlation between the two pulses, as shown in Figure 3. The ion current signal collected by the faraday cup was firstly converted to voltage signal via a homebuilt preamplifier of 109 V/A, and then fed to an Oscilloscope (Tektronix, 2024C) for recording. 16 times average was used to obtain each single spectrum. Ambient air purified by activated carbon, silica gel, and fresh molecular sieves trap was used as both drift and carrier gases at flow rates of 200 and 10 mL·min-1, respectively. The humidity of the air was kept below 1 ppmv. Acetone of analytical purity was carried into the IMS using a permeation vial (1.5 mL, Agilent), and the concentration was adjusted high enough (> 20 ppmv) to react with most of the reactant ions and form a single protonated acetone peak for convenient investigation. Dimethyl-methylphosphonate (DMMP) samples of different concentrations in air produced the same way were introduced into the IMS to test the performance of the proposed method. Results and Discussion a) BNG operation only


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For this experiment, the shutter grid was repeatedly opened and closed to chop the ion beam into the drift region. The pulse amplitude on the BNG was set to 80 volts, which is sufficient for eliminating the background. The electrode EFS was held at its normal voltage (2970 volts). As shown in Figure 4, a distinct ion peak originated from acetone with 0.15 nA height could be seen in the spectrum for a shutter pulse width of 150 µs. It has a characteristic drift time of 7.7 ms and a FWHM (full width at half maximum) of 260 µs. Accordingly, a resolving power3 based on the quotient of the drift time divided by the FWHM for the acetone peak is calculated to be 30. b) FS with BNG always open For this experiment, the BNG was left totally open by removing the voltage difference between the two sets of grid wires. A flat signal of about 1.5 nA arising from the continuous ionization of the 63Ni source was observed. However, after applying a high voltage pulse jumping from 2970 volts to 5970 volts to the electrode EFS, a perturbation is observed in the flat signal, as displayed in Figure 5a. The signal sharply increases and then quickly decreases to near zero before it finally reaches its initial background value. The peak is the result of applying the high voltage pulse, which collects the ions just existing between the electrode EFS and the BNG and injects them on top of the existing background ion current. This injection removes almost all the ions from the region between the electrode EFS and the BNG. The application of the high voltage pulse meanwhile lifts the potential of the electrode EFS to a level much higher than that of the ion source resulting in a reversed electric field between the ion source and the electrode Efs. No ion reaches the electrode EFS as long as the pulse exists. As a result, an ion current decrease region is observed immediately after the peak and wider than the applied high voltage pulse width. As the ions in the region between the shutter grid and the electrode EFS are limited, further increase in the pulse


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width does not help in collecting more ions but only results in the widening of the ion current decrease region. c) FS with BNG always closed For this experiment, the BNG was always closed by setting the GVD at 80 volts in order to avoid any background. The electrode EFS was then subjected to the high voltage pulse jumping from 2970 volts to 5970 volts. As shown in Figure 5b, distinct ion peaks at drift time of 7.58 ms originated from acetone as FS pulse width varied from 50 to 400 µs even the BNG is always closed. Comparison between Figure 5a and 5b reveals that the flat signal and the ion current decrease region disappeared while the peak remained. In fact, as the transmission of ions through the BNG depends on the ratio of the perpendicular electric field to the longitudinal one6, the BNG becomes transparent for ions at high longitudinal fields, that is, during the high voltage pulse duration. However, it still blocks the ions outside the time window of the high voltage pulse. The FWHM of the ion peak in Figure 5b reduces as the FS pulse width reduces and reaches its minimum value of 210 µs at pulse width of 100 to 150 µs, showing a resolving power of 36. Further reduce in FS pulse width results in asymmetric peak shape and wider FWHM, like 260 µs at FS pulse width of 50 µs. That might be caused by the electric field distortion associated with a closed BNG, which more likely matters at relatively short pulse width. This working mode is actually a pure FSG mode, where the retarding filed in the reported FSG design18-21 is substituted by an always closed BNG. d) FS and BNG operating simultaneously Figure 5b shows that the FS with a closed BNG can generate ion mobility spectra as well as the BNG only operation, e.g. an acetone ion peak of 1.25 nA was observed at FS pulse width of 150 µs which is 8 times that obtained when only BNG was used. The operations of the two


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events are independent. They can operate simultaneously if the BNG starts earlier than the FS. The reason for starting the BNG earlier is that the FS creates an ion current decrease region (Figure 5a) so that there is no ion to be chopped by the BNG in the period immediately after the FS. To observe the two events simultaneously, we started the BNG 2 ms ahead of the FS (de1ay = 2 ms). As shown in Figure 6 (blue), two distinct ion peaks originated from the same acetone ions. In fact, they were created by the two independent gating methods with a slight time lapse between the two events. The two peaks have characteristic drift times of 7.7 and 9.58 ms, respectively. The drift time difference is mostly due to the time lapse between the two pulses. If the delay is finely adjusted, the two peaks overlap to give a single stronger peak as observed in Figure 6 (red). The peak height is over 1.75 nA, which is 11.5 times that obtained when only the BNG was used (Figure 4). In addition, the FWHM of the peak is 260 µs, showing a resolving power of 30, the same as that of the peak in Figure 4. e) Enhanced Sensitivity In order to see the advantage of combining FS with BNG, the response sensitivities of the instrument to a test compound of DMMP under the two conditions of conventional BNG and combined FS and BNG were investigated. Figure 7 shows the ion mobility spectra of 5 ppbv DMMP in air recorded under the two conditions. Under the conventional BNG condition, the 5 ppbv DMMP shows a distinct ion peak at drift time of 10.1 ms (Figure 7, blue), corresponding to a reduced mobility of 1.41cm2/V·s, which is assigned to be DMMP dimer ions26. The peak has a height of 3.5 pA, nearly 3 times the background noise level (1.1 pA). Thus, the limit of detection3 (LOD) of DMMP is defined as 5 ppbv for this case. Under the combined FS and BNG condition, the peak of the 5 ppbv DMMP shows the same drift time 10.1 ms but with a highly


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enhanced peak height of 80.3 pA (Figure 7, red), 22 times the peak height obtained under conventional BNG condition. The practical LOD of DMMP under the combined FS and BNG condition is about 200 pptv, 25 times lower than before. The much lowered LOD of DMMP obtained with the new gating method comes from two aspects. First, the highly enhanced electric field in the region between the electrode EFS and the BNG increases the drift velocity of the DMMP ions existing in that region and thus more ions can pass through the BNG into the drift region during the gate opening period. Second, the relative transmission efficiency of the DMMP ions through the BNG is also increased as the electric field between the electrode EFS and the BNG is highly enhanced, which is verified by Figure S-2. Conclusion The BNG only allows a small portion of ions to pass through into the drift tube. Applying a strong pulsed field behind the BNG right at the time of gate opening compensates part of the ion loss. The high field pulse collects ions in the vicinity of and even far from the shutter grid and injects them into the drift tube. In addition, it also increases the transmission efficiency of ions, especially ions with low reduced mobility, through the BNG during the opening time. The current method can be easily applied to the existing drift tube ion mobility spectrometers already using BNG without any change in the drift tube hardware, however, a fast HV-switch is needed to implement this technique. In this design, the ion source is placed out of the FS region to make sure its flexibility of working with all kinds of ion sources. It also can work as a pure FSG if the BNG is always closed to avoid background ion current. However, the use of the BNG also introduces field


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distortion effect which is associated with the BNG. Its effect on the peak shape would be carefully explored in our following work. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21405158, 21077101, and 21177124) and the Chinese Academy of Sciences Visiting Fellowship for Researchers from Developing Countries (Grant No. 2013FFGA0011). We would like to express our heartfelt thanks to Dr. Rong Zhang for committing time on helping with the revision of our manuscript. Supporting Information Available Additional information as noted in the text. This information is available free of charge via the Internet at http://pubs.acs.org/. Reference (1)

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Figure Captions: Figure 1. Illustrations for the operational principle of field switching combined with Bradbury-Nielsen gate, where EFS refers to the electrode on which field switching voltage is applied and BNG refers to Bradbury-Nielsen gate. Figure 2. SIMION simulation of the electric field (left column: a, c, and e) and the ion distribution profiles (right column: b, d, and f) around the BNG wires at timing start point the BNG was about to change from right open to closed (t = 0 µs and GVD = 0 V), the BNG closed for the next 50 µs (t = 50 µs and GVD = 135 V), and the BNG reopened for the next 30 µs (t = 80 µs, GVD = 0 V and Ei = 386 V/cm), respectively. For comparison, at timing point t = 50 µs, the GVD was abruptly removed and the voltage on the top electrode was simultaneously switched from 570 volts to 2070 volts for 30 µs (t = 80 µs, GVD = 0 V and Ei = 4671 V/cm), g and h are the corresponding electric filed and ion distribution profiles. GVD stands for the gating voltage difference between the two sets of BNG wires and Ei stands for the electric field strength in the ionization region. Figure 3. Schematic of the coupled field switching and BNG ion mobility spectrometer (top) and the high voltage pulses applied to the electrode EFS and the BNG (bottom). Figure 4. Acetone signal obtained by the IMS under BNG only mode. Figure 5. Acetone signals obtained by the IMS after imposing a high voltage pulse of 3000 volts to the electrode EFS: a) BNG is always open and b) BNG is always closed. Figure 6. Acetone signals obtained by the IMS operating with the combined FS and BNG: 2 ms of delay between the two events (blue) and 120 µs of delay (red). The pulse widths of the FS and the BNG were both 150 µs. Figure 7. Signals of 5 ppbv DMMP obtained by the IMS at the conventional BNG condition (blue) and the combined FS and BNG condition (red). The pulse widths of the FS and the BNG were both 150 µs.


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Figures: Figure 1

Figure 2


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Figure 3

Figure 4


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Figure 5

Figure 6


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Figure 7

For TOC only.


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Field Switching Combined with BradburyâNielsen Gate for Ion

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Field Switching Combined with BradburyâNielsen Gate for Ion