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A novel micromachined high-field asymmetric waveform-ion mobility spectrometer
Sensors and Actuators B 67 Ž2000. 300–306 www.elsevier.nlrlocatersensorb
A novel micromachined high-field asymmetric waveform-ion mobility spectrometer R.A. Miller a,) , G.A. Eiceman b, E.G. Nazarov b, A.T. King a a b
The Charles Stark Draper Laboratory, MS-37, 555 Technology Square, Cambridge, MA 02139, USA Department of Chemistry and Biochemistry, New Mexico State UniÕersity, Las Cruces, NM 88003, USA Received 9 November 1999; received in revised form 19 April 2000; accepted 16 May 2000
Abstract The fabrication and characterization of a novel micromachined high-field asymmetric waveform-ion mobility spectrometer ŽFA-IMS. is described. The spectrometer has a 3 = 1 = 0.2 cm3 rectangular drift tube and a planar electrode configuration. The planar configuration permits simple construction using microfabrication technology where electrodes and insulating regions are made with deposited metal films on glass substrates. The spectrometer is characterized using organic vapors Žincluding acetone, benzene, and toluene. at ambient pressure and with air as the drift gas. Ions are created in air at ambient pressure using photo-ionization with a 10.6 eV photo discharge lamp Ž l s 116.5 nm.. The micromachined FA-IMS exhibited behavior consistent with conventional FA-IMS designs where compensation voltage was effective in discriminating between ion species in high-field radio-frequency ŽRF. regimes. Excellent resolution of benzene and acetone ions in mixtures illustrates an advantage of the FA-IMS over low-field ion mobility spectrometry. Detection of toluene at concentrations as low as 100 ppb has been demonstrated. Improvements in detection limits, by as much as 100 = , are anticipated with improved ionization source designs. The ability to transport both positive and negative ions simultaneously through the FA-IMS drift tube is demonstrated here for the first time. Ion intensity is found to be proportional to sample concentration, although clusters of sample ions and neutrals at high concentrations illustrate the need for a drift region which is kept free of sample neutrals. Micromachining promises cost, size, and power reductions enabling both laboratory and field instruments. q 2000 Elsevier Science S.A. All rights reserved.
1. Introduction Ion mobility spectrometry ŽIMS. is an instrumental method based upon the separation of gas phase ions through size-to-charge rather than mass-to-charge ratios w1x. Since both sample ionization and sample characterization occur at ambient pressure in air or nitrogen, IMS instruments can be made field-portable. Examples of successful on-site applications of IMS include the hand-held chemical agent monitor ŽCAM. which is used as a personal detector for chemical warfare agents and the Ionscan and Orion which are used at airports to screen handbags for explosives w1x. The hand-held CAM weighs about 6 kg and a subsequent military initiative, the mini-IMS, is roughly 500 g w2x.
)
Corresponding author. Tel.: q1-617-258-2815. E-mail address: [email protected] ŽR.A. Miller..
The possibility of creating mobility spectrometers with dimensions under a few centimeters, while maintaining a suitable level of analytical performance, has been considered a next step in reducing the size and cost of IMS analyzers w3,4x. This would be possible if the drift tube design could be simplified over traditional configurations, if current performance levels could be maintained, and if fabrication methods were made amenable to mass production. Recent attempts at miniaturizing drift tubes for IMS have generally led to spectrometers with reduced performance in terms of sensitivity or resolution w5x. Others have sought to improve drift tubes through novel methods of ion separation by ion transport across flow gradients w6x or through high electric fields with radio-frequency ŽRF. asymmetric waveforms applied between adjacent conducting surfaces w7–12x. This last approach, the so-called high field asymmetric waveform-ion mobility spectrometer ŽFA-IMS., offers a greatly simplified drift tube design over conventional mobility spectrometers. The FA-IMS drift
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 5 3 5 - 9
R.A. Miller et al.r Sensors and Actuators B 67 (2000) 300–306
tube does not require ion-shutters, voltage dividers, and aperture grids typical of conventional ion mobility spectrometers. In the FA-IMS, ions are carried between parallel plate electrodes by a flow of gas while an asymmetric high voltage RF waveform causes the ions to follow trajectories perpendicular to the flow. Under these high-field conditions, mobilities become field dependent, in contrast to the low-field regimes used in conventional IMS drift tubes where the mobilities are essentially constant with field w13x. A second electric field produced by a weak DC voltage is superimposed on the high voltage RF waveform. The weak DC field typically ranges from y1000 to q1000 Vrcm, while the peak amplitude of the RF field is around 20,000 Vrcm. Both fields are applied transverse to the carrier gas flow. For a given ion species and RF field, a certain compensating DC field will allow these ions to traverse the drift region and register at a detector without striking the parallel plate electrodes. Previous investigations into the role of the high-fields on ion behavior w7–10x illustrate that this approach is a feasible method for ion characterization, however, the method is not well described and the link between molecular structure and compensation voltage is still unclear w12,13x. In spite of these uncertainties, the prospects of improved detection limits without the need for a shutter have motivated the development of conventionally machined FA-IMS. However, the manufacture of conventional FA-IMS drift tubes is complex, labor intensive, and expensive. Micromachining is a fabrication technology that utilizes approaches and fabrication techniques Že.g., batch fabrication and photolithography. which are similar to those used in the integrated circuit industry. Adopting this fabrication technology should allow the realization of low cost, high precision FA-IMS spectrometers. In addition, new device configurations not practical with conventional machining such as spectrometer arrays can be realized. In this article, the construction of a FA-IMS fabricated by micromachining techniques is described and performance results are presented.
2. Principle of operation In FA-IMS, ions are transported longitudinally through a drift tube by a carrier gas stream as shown in Figs. 1 and 2. An asymmetric RF electric field EŽ t . Žoriented at 908 to the gas flow. is produced by applying a voltage between two, parallel plate, ion filter electrodes. The RF electric field Ž Emax ) 10,000 Vrcm. causes the ions to oscillate in a direction transverse to the carrier gas flow during ion transport through the length of the drift tube. Since the amplitude of the RF field is asymmetric, an ion velocity
301
Fig. 1. Schematic of the FA-IMS system showing the arrangement of electrical controls, gas flows, and drift tube.
towards the top electrode as described in Eq. Ž1. will differ from its velocity towards the bottom ion filter electrode: Õ Hs K Ž E . E Ž t .
Ž 1.
here Õ H is the ion velocity perpendicular to the gas flow direction, K Ž E . is the field dependent mobility, and EŽ t . is the electric field strength. This difference in ion velocities is due not only to the differences in electric field strengths Emin - Emax but also to the dependence of ion mobility on electric field strength at fields greater than 10,000 Vrcm w14,15x. Therefore, for each period of applied RF field, the ion is drawn towards one ion filter electrode by a net amount D h and total displacement of the ion in space includes the number of RF periods as nD h. The average value of "D h for an ion species is determined by the duty cycle of the RF field and the field dependence of the mobility K Ž E .. The resultant motion of the ion along the drift tube under the influence of field and flow yields a trajectory that is specific to each ion species, and only ions with a total transverse displacement less than the drift channel height can reach the detector. All other ions collide with the filter electrodes are neutralized and are removed from the drift region by the gas flow. While all ions interact with the applied RF field and are drawn toward the drift tube walls, select ions can be kept in the flowing gas by applying particular low DC voltages to the ion filter electrodes. The voltages generate fields and subsequently forces on the ions that oppose their net RF-induced transverse motion. Thus, an ion can be passed between the ion filter electrodes and transported to the detector and this DC Žor compensation. voltage is a characteristic of the ion species. Sweeping the compensation voltage over a range of voltages produces a high-field equivalent of a mobility spectrum of the different ion species present in the gas stream. The unique aspect of this approach to ion characterization is that part of the ion interaction with the RF field occurs under high-field conditions where ion mobilities are dependent upon the electric field w13x. In traditional lowfield IMS, ion mobilities are independent of electric field and are reasonably well described through the Mason–
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R.A. Miller et al.r Sensors and Actuators B 67 (2000) 300–306
Fig. 2. Diagram of the anticipated motion or trajectory of ions in a FA-IMS with the DC compensation voltage superimposed on the RF field.
Schamp equation. The Mason–Schamp equation links mobilities to reduced mass and cross-section areas for ion– molecule collisions w14,15x. In contrast, the relationship between molecular structure and the coefficient of mobility in the high electric field for multi-atomic organic ions is not well-described and the relationship between conventional terms of reduced mass, collisional cross-section, and the high-field influence is unknown. Nevertheless, the limited publications on large, conventionally machined FA-IMS analyzers suggest that different ions species appear at characteristic compensation voltages. Additionally, since the FA-IMS operates in a regime that differs from the low-field, conventional IMS, some ion species that cannot be resolved in the conventional IMS appear well resolved in the FA-IMS w7–12,16,17x. In short, the RF field interacts with ions in ways that allow ion separations based upon ion molecular interactions, which are not present in mobility spectrometers operating in low-field regimes.
wanted metal residing on top of the photoresist while leaving behind the metal electrodes on the Pyrex. Once metal electrodes have been formed, the Pyrex wafers are diced into rectangular pieces. For spectrometers intended to be used with photo-ionization sources, holes are drilled into the top Pyrex pieces to provide a means for the photons to enter the ionization region. The silicon wafer thickness defines the gap between the top and bottom electrodes of the ion filter. In the present method, the silicon wafer is diced into strips that are 25 mm long, 2 mm wide, and 0.5 mm thick. The silicon strips are aligned with the metal electrodes and bonded to the Pyrex as shown in Fig. 3. The anodic bonding process uses a combination of high electric field Ž; 1000 V., heat Ž3508C. and applied pressure to permanently attach the Pyrex pieces to the silicon strips. Metal electrodes also make contact to these silicon strips allowing control of their potential. A Teflon test fixture provides a means for interfacing a vapor generator to the spectrometer for experimental measurements. The RF asymmetric field required to filter the ions is produced by a soft-switched semi-resonant circuit which incorporates a flyback transformer to generate the high voltage pulses, similar circuits have been described by Krylov w18x. The circuit, shown in Fig. 4, provides a peak-to-peak RF voltage of 1400 V at a frequency of 2 MHz with a duty cycle of approximately 30%. This RF voltage is superimposed onto the compensation voltage. In these studies, sample ionization was accomplished using photo-ionization discharge lamps. A miniature 10.6 eV Ž l s 116.5 nm. photo-discharge lamp was positioned above the hole in the top Pyrex wafer of the spectrometer to permit the photons to enter the drift tube. The bottom ion filter electrode was grounded while the high voltage asymmetric field and compensation voltages were applied to the upper filter plate. An amplifier was used to detect the charge collected at the detector plate. A polynomial waveform synthesizer model 2920 was used to sweep the compensation voltage and a Tektronics model TDS340A
3. Experimental 3.1. Instrumentation A schematic of the micromachined drift tube is shown in Fig. 3 and consists of an ion filter, a detector electrode, and a deflector electrode. The spectrometer is fabricated from two Pyrex wafers and one heavily boron doped silicon wafer. Metal electrodes are formed on the top and bottom Pyrex wafers defining the ion filter, deflector, and detector plates. The metal electrodes are fashioned on the Pyrex wafers by first coating the wafers with Shipley 1822 photoresist and patterning the resist using a photolithographic process. The patterned photoresist is then coated ˚ gold on 400 A˚ titanium. by sputter with metal Ž1200 A deposition. A lift-off process is used to remove the un-
Fig. 3. Photograph of a micromachined field asymmetric ion mobility spectrometer.
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4. Results and discussion 4.1. Influence of applied RF fields on ion behaÕior in drift tube
Fig. 4. Schematic of circuit used to generate the high voltage, asymmetric, RF electric field.
storage oscilloscope was used to record the signal from the amplifier and produce a spectrum for the sample gas. 3.2. Sample generation and gas supply Clean air was provided using a pure air generator Žmodel 737, Addco, Miami, FL. with gas scrubbing through beds of activated charcoal and 13X molecular sieve. Purified air was split into three independently controlled parts with 1–4 lrmin used as carrier gas flow, 10–300 mlrmin used to operate a sample generator based upon diffusion, and 0–300 mlrmin used as a dilution gas flow with the sample flow. In experiments with mixtures of chemicals, the dilution gas line was also used for preparing and introducing an additional vapor sample. Sample vapors were prepared in the vapor generator using glass diffusion tubes or Teflon permeation cells. All three parts of the flow were combined and entered the drift tube shown in Fig. 3. The vapor generator is a hermetically sealed, glass container Ž200 ml. located in a thermostated aluminum block at 35–808C via a Minco CT 137 controller. All supply lines after the vapor generator were kept at about 508C to minimize wall-adsorption. This flow system allowed a constant flow to be delivered to the drift tube while permitting changes in vapor concentration by adjusting the ratio of sample to dilutent flows. The diffusion or permeation tubes were weighed over several days to gravimetrically determine concentrations of the sample flow.
The mobility spectrum for acetone obtained using the micromachined high-field asymmetric ion mobility spectrometer is shown in Fig. 5, where the DC voltage is swept from y15 to q15 V, in the absence Ždashed line. and presence Žsolid line. of an applied RF field. In the spectrum without the RF field applied, ions arrived at the detector through bulk flow in the drift gas and there was no need for a compensation voltage to control ion motion. Consequently, the ion peak appears at a compensation voltage at or very near 0 V DC. The slight shift from zero compensation voltage is believed due to the manner in which the vapor generator was interfaced to the FA-IMS. This caused a slight asymmetry in the gas flow, which tended to deflect the ions preferentially toward one of the ion filter electrodes. Here, a slightly positive compensation voltage was required to overcome this deflection. The baseline for this spectrum is constant and low throughout the remainder of the sweep including the extremes beyond "5 V DC Žnot shown.. The intensity of this peak was about 700 mV with a near Gaussian peak shape and peak width at half height of 0.5 V DC. Application of a high-field RF waveform affected ion behavior and the ion was detected at a compensation voltage of y3.5 V DC. At values lower than y4.5 V DC, the baseline was flat and no ions were detectable. At values ranging from y2.8 V to about q1.5 V DC, the baseline was elevated slightly suggesting the presence of ions. This baseline perturbation was dependent on, and proportional to, the concentration of acetone in the gas flow and can be explained by ion–molecule cluster reactions that likely occur in the drift tube between the ion source and the detector. Such reac-
3.3. Chemicals Chemicals used in this work were acetone, benzene, and toluene. All three chemicals were purchased from Aldrich Chemical ŽSt. Louis, MO. and used without further purification.
Fig. 5. FA-IMS spectra for acetone in air with and without the RF field applied. The product ion was created by photo-ionization and ions created in the same source and gas were identified using mass spectrometry as MHq.
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tions, as shown in Eqs. Ž2. and Ž3., would lead to association and dissociation reactions throughout this region since the ions and neutral species would move together in the drift gas flow. Acetone clusters are well known and dimers and trimers may be anticipated to form at ambient temperature.
lM M lM
Mq q M ion
Mq 2 q dimer
neutral
neutral
q 2
Ž 2.
dimer q 3
Ž 3.
trimer
Although peak intensities with and without the RF field applied differed by nearly 30%, the areas under the total spectrum including the baseline perturbations were found to be the same. Thus, the transmission coefficient for ions in the drift region under the influence of the RF and DC fields is close to 100%, implying ion losses to surfaces were minor or not detectable. Minor Ž20%. broadening of the peak shape was observed with the application of the RF field to the drift tube. The reason for the peak broadening is not known yet, though may be attributed to increased ion paths from the oscillations. The flow rate of the carrier gas affects both peak intensity and peak area. Without an RF field, 2.5 lrmin of gas flow provides the largest peak heights and areas. At higher flows, the intensities decrease. This decrease was at first attributed to turbulent flow caused by the hole in the Pyrex wall used for ionization. However, finite-element analysis did not confirm this hypothesis and showed laminar flow at flow rates in excess of 3 lrmin. Further investigations are being performed to determine the cause of the decrease in intensity at higher flows. The maximum amplitude of the RF waveform also affects the appearance of spectral profiles and 1000 V Žor ; 20,000 Vrcm. showed optimum yield for ions during passage down the drift tube. The ability to predict the value of the compensation voltage for different chemicals is limited at present by inadequate understanding of the effect of field strength on mobility. This is true for all further discussions below and constitutes a main theme for future research in FA-IMS analyzers. 4.2. PositiÕe and negatiÕe ions in micromachined high FA-IMS Mobility spectra for benzene in Fig. 6 Žtop two traces., showed field dependent behavior similar to acetone. The product ion for benzene Žtentatively identified as Mq Ø from separate PID-MS studies. appeared as expected near zero DC compensation voltage without applied RF fields. A compensation voltage of y10.5 V DC was needed, with the RF voltage applied, in order to register product ions at the detector. As with acetone, a broad shift of the baseline between the product ion and zero compensation voltage was observed and here extended from y8 to q2 V DC. The intensity of this shift was proportional to benzene concentration and the broad spectral band is consistent
Fig. 6. FA-IMS spectra for benzene vapor in air without and with RF field applied. The top spectrum is without RF field and a with positive deflector plate bias. The middle spectrum shows spectra with positive deflector plate bias and with the RF field applied. The bottom spectrum shows response with the RF field and a negative deflector plate bias.
with the formation of ion–molecule clusters per Eqs. Ž2. and Ž3.. This interpretation, though tentative, is supported by evidence from high-pressure mass spectrometry where cluster ions of benzene have been measured. The presence of negative molecular-based ions, along with positive ions in the drift gas during transport toward the detector was initially considered implausible owing to the presence of oxygen with a high electron affinity in the drift gas. However, reversal of the deflector electrode polarity without any other changes to the drift tube or electronics allowed negative ions to be detected ŽFig. 6 bottom trace.. The intensities of the negative ions, however, were substantially lower in comparison to the positive ions Žroughly 10% of the positive ion intensity.. Still, this signal demonstrates that negative ions were passed through the drift region and two peaks appeared at compensation voltages of q3 and q0.5 V DC. Negative ions were not observed with other vapors used in this study and should best be explored with another ion source such as a beta-emitter. The location and peak shape is beyond current models and the identity of this ion was not determined. The proportional response with vapor concentration of benzene is not informative and further conclusions must await mass spectral analysis. Nonetheless, the ability to detect simultaneously positive and negative ions is an unexpected possible benefit of the micromachined FA-IMS design that has not been previously recognized. 4.3. Response to sample Õapor concentrations The response of the analyzer to increases in concentrations of sample vapors in air is shown in Figs. 7 and 8. Photo-ionization detectors in gas chromatography are known to provide working ranges of 10 3 or better, so proportional response was anticipated and was seen in the
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the range of concentrations down to 0.1 ppm. Apparent limits of detection ŽLOD. based upon signal to noise calculations are estimated at 60–70 ppb but may be lower since the response curve becomes shallow below 0.1 ppm. At present, causes for the deviation from linearity in Fig. 8 for very low concentrations are unknown. 4.4. Resolution of multiple ion species in mixtures
Fig. 7. Mobility spectra for FA-IMS for toluene with increasing concentration from bottom to top Ža. without RF field and Žb. with the RF field.
high-field mobility spectra. Spectra for toluene without and with the RF field are shown in Fig. 7a and b, respectively with vapor concentrations ranging from 0.1 to 0.7 ppm. As seen in both sets of spectra, the product ion intensity was proportional Žabove the detection limit. to vapor concentrations. The peak heights and areas in Fig. 7b are plotted in Fig. 8 as a function of concentration and appear linear over
Fig. 8. Peak areas and intensities at various concentrations of toluene vapor.
The ability to distinguish between multiple ion species in a drift tube is a measure of the resolution or analytical selectivity of the device. The practical analytical selectivity or resolution in IMS is dictated not only by the number of discrete peaks that can fit within the total spectrum band width but depends on the ion generation process as well. This fact has permitted the IMS to be successfully applied and dominate the chemical agent and explosives detection arenas. Nonetheless, a measure of instrument performance is how well ion species can be separated. Fig. 9 shows typical spectra from a traditional low-field time-of-flight drift tube for the small organic molecules, benzene, toluene, and acetone w19x. The product ion peaks are all close to the reactant ion peak and have nearly the same drift times Žthe source of charge used for these spectra is a radioactive ion source.. For example, drift times from these spectra are 4.44 ms for the reactant ion peak, 4.86 ms for acetone, 5.04 ms for toluene and 4.84 ms for benzene. In a mixture, these chemicals would be difficult to resolve using a low-field drift tube. In contrast, good separation of product ions in the FA-IMS drift tube is evident for the pairs of chemicals shown in Fig. 10. In the top spectrum, benzene and acetone are well separated at compensation voltages of y10.5 and y3.5 V DC, respectively. In a mixture of toluene and acetone two distinct peaks are observed Žbottom trace. at y7.0 and y3.5 V DC. These peaks occur at compensation voltages identical to those measured for toluene and acetone individually. In addition, a peak at a compensation voltage of y9.5 V is also present. The presence of this unexpected peak at y9.5 V DC is be-
Fig. 9. Mobility spectra from conventional time-of-flight IMS drift tube for acetone, toluene, and benzene. Source: Ref. w20x. The product ions all appear between 4.8–5.0 ms.
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Acknowledgements The authors would like to thank Dr. William P. Kelleher for helpful discussions, Brenda A. Hugh for her assistance with the fabrication, and Mr. John T. Kauppinen Jr. for his assistance with spectrometer characterization. Support from the FBI through contract no. J-FBI-98-111 to GAE and EGN, is gratefully acknowledged.
References Fig. 10. FA-IMS spectra for chemical pairs of benzene–acetone and toluene–acetone. The peak for acetone is at y3.5 V.
lieved due to interaction between the acetone and toluene as the ions and vapors move through the drift region. In addition, the baseline perturbations increase for both mixtures with increased vapor levels and the peaks for benzene and toluene occur at slightly more negative compensation voltages. Both of these effects are presently ascribed to the formation of ion–molecule clusters in the drift gas flow. The presence of sample neutrals with sample ions in the flowing gas stream must be rectified in future drift tube designs. This has been recognized by others and corrected by isolating the ion formation region from ion identification region, and performing the identification in clean air w11,12x. Similar modifications will be incorporated into future designs of the micromachined FA-IMS. This will lead to reduced clustering and to the realization of robust, miniature, low cost FA-IMS.
5. Conclusions A micromachined FA-IMS has been microfabricated and successfully demonstrated with the detection of acetone, toluene, and benzene. The parallel electrode design of this analyzer allows mass-production methods to be used in the construction of the analyzer. The ion separations were distinct and well resolved for the chemical pairs acetone–benzene and acetone–toluene which are not easily separated using low-field mobility spectrometry. A detection threshold of about 60 ppb was demonstrated for toluene with an UV photo-discharge lamp. The response of the spectra to concentration variations suggests that the ion characterization should be performed in a carrier gas free of sample neutrals. Findings suggest that sample neutral ion clusters in the drift region lead to baseline perturbations. Through planned modifications to the drift tube design, the drift gas will be kept free of sample neutrals in future spectrometers.
w1x G.A. Eiceman, Z. Karpas, Ion Mobility Spectrometry, CRC Press, Boca Raton, FL, 1993. w2x C.S. Harden, Chemical warfare agent detection by ion mobility spectrometry, PITTCON ’99 Book of Abstracts, March 7–12, Orlando, Florida, 1999. w3x J.I. Baumbach, D. Berger, J.W. Leonhardt, D. Klockow, Ion mobility sensor in environmental analytical chemistry — concept and first result, Int. J. Environ. Anal. Chem. 52 Ž1993. 189. w4x R. Bradshaw, Engineering approaches and compromises in the design of small IMS spectrometers, 3rd International Workshop on Ion Mobility Spectrometry, Galveston, TX, Oct. 16–19, 1994. w5x J. Xu, B. Whitten, M. Ramsey, Miniature ion mobility spectrometry, 8th International Workshop on Ion Mobility Spectrometry, Buxton, UK, Aug. 8–12, 1999. w6x E. Sacristan, A.A. Solis, IEEE Trans. Instrum. Meas. 47 Ž1998. 769–775. w7x I.A. Buryakov, E.V. Krylov, A.L. Makas’, E.G. Nazarov, V.V. Pervukhin, U.Kh. Rasulev, Sov. Tech. Phys. Lett. 17 Ž6. Ž1991. June. w8x I.A. Buryakov, E.V. Krylov, E.G. Nazarov, U.Kh. Rasulev, Int. J. Mass Spectrom. Ion Processes 128 Ž1993. 143–148. w9x B. Carnahan, S. Day, V. Kouznetsov, A. Tarasov, Development and applications of a transverse field compensation ion mobility spectrometer, 3rd International Workshop on Ion Mobility Spectrometry, Cambridge, UK, August 6–9, 1995. w10x B. Carnahan, S. Day, V. Kouznetsov, M. Matyjaszczyk, A. Tarassov, Field ion spectrometry — a new analytical technology for trace gas analysis, Proceedings of the 1996 International Conference on Advances in Instrumentation and Control, ISA ’96, 1996. w11x R.W. Purves, R. Guevremont, S. Day, C.W. Pipich, M.S. Matyjaszczyk, Rev. Sci. Instrum. 69 Ž1998. 4094–4105. w12x R. Guevremont, R.W. Purves, Rev. Sci. Instrum. 70 Ž1999. 1370. w13x W. McDaniel, A. Mason, The Mobility and Diffusion of Ions in Gasses, Wiley, 1993, pp. 273–326. w14x E.A. Mason, H.W. Schamp Jr., Ann. Phys. ŽN. Y.. 4 Ž1958. 233. w15x E.A. Mason, E.W. Mc Daniel, Transport Properties of Ions in Gases, Wiley, New York, 1988. w16x D.E. Riegner, C.S. Harden, B. Shoff, G. Ewing, Evaluation of ion dynamics in field ion spectrometry, Proceedings of 1997 ERDEC Conference, MD, 1997, Nov. w17x R.G. Ewing, B. Shoff, Detection of chemical warfare materials with a high-field asymmetric waveform ion mobility spectrometer, Abstracts of 8th International Conference on Ion Mobility Spectrometry, Buxton, UK, ISIMS ’99, 1999. w18x E.V. Krylov, Instrum. Exp. Tech. 40 Ž5. Ž1997. 628–631. w19x S.E. Bell, E.J. Nazarov, Y.F. Wang, G.A. Eiceman, Anal. Chim. Acta 394 Ž1999. 121–133.
A novel micromachined high-field asymmetric waveform-ion mobility spectrometer R.A. Miller a,) , G.A. Eiceman b, E.G. Nazarov b, A.T. King a a b
The Charles Stark Draper Laboratory, MS-37, 555 Technology Square, Cambridge, MA 02139, USA Department of Chemistry and Biochemistry, New Mexico State UniÕersity, Las Cruces, NM 88003, USA Received 9 November 1999; received in revised form 19 April 2000; accepted 16 May 2000
Abstract The fabrication and characterization of a novel micromachined high-field asymmetric waveform-ion mobility spectrometer ŽFA-IMS. is described. The spectrometer has a 3 = 1 = 0.2 cm3 rectangular drift tube and a planar electrode configuration. The planar configuration permits simple construction using microfabrication technology where electrodes and insulating regions are made with deposited metal films on glass substrates. The spectrometer is characterized using organic vapors Žincluding acetone, benzene, and toluene. at ambient pressure and with air as the drift gas. Ions are created in air at ambient pressure using photo-ionization with a 10.6 eV photo discharge lamp Ž l s 116.5 nm.. The micromachined FA-IMS exhibited behavior consistent with conventional FA-IMS designs where compensation voltage was effective in discriminating between ion species in high-field radio-frequency ŽRF. regimes. Excellent resolution of benzene and acetone ions in mixtures illustrates an advantage of the FA-IMS over low-field ion mobility spectrometry. Detection of toluene at concentrations as low as 100 ppb has been demonstrated. Improvements in detection limits, by as much as 100 = , are anticipated with improved ionization source designs. The ability to transport both positive and negative ions simultaneously through the FA-IMS drift tube is demonstrated here for the first time. Ion intensity is found to be proportional to sample concentration, although clusters of sample ions and neutrals at high concentrations illustrate the need for a drift region which is kept free of sample neutrals. Micromachining promises cost, size, and power reductions enabling both laboratory and field instruments. q 2000 Elsevier Science S.A. All rights reserved.
1. Introduction Ion mobility spectrometry ŽIMS. is an instrumental method based upon the separation of gas phase ions through size-to-charge rather than mass-to-charge ratios w1x. Since both sample ionization and sample characterization occur at ambient pressure in air or nitrogen, IMS instruments can be made field-portable. Examples of successful on-site applications of IMS include the hand-held chemical agent monitor ŽCAM. which is used as a personal detector for chemical warfare agents and the Ionscan and Orion which are used at airports to screen handbags for explosives w1x. The hand-held CAM weighs about 6 kg and a subsequent military initiative, the mini-IMS, is roughly 500 g w2x.
)
Corresponding author. Tel.: q1-617-258-2815. E-mail address: [email protected] ŽR.A. Miller..
The possibility of creating mobility spectrometers with dimensions under a few centimeters, while maintaining a suitable level of analytical performance, has been considered a next step in reducing the size and cost of IMS analyzers w3,4x. This would be possible if the drift tube design could be simplified over traditional configurations, if current performance levels could be maintained, and if fabrication methods were made amenable to mass production. Recent attempts at miniaturizing drift tubes for IMS have generally led to spectrometers with reduced performance in terms of sensitivity or resolution w5x. Others have sought to improve drift tubes through novel methods of ion separation by ion transport across flow gradients w6x or through high electric fields with radio-frequency ŽRF. asymmetric waveforms applied between adjacent conducting surfaces w7–12x. This last approach, the so-called high field asymmetric waveform-ion mobility spectrometer ŽFA-IMS., offers a greatly simplified drift tube design over conventional mobility spectrometers. The FA-IMS drift
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 5 3 5 - 9
R.A. Miller et al.r Sensors and Actuators B 67 (2000) 300–306
tube does not require ion-shutters, voltage dividers, and aperture grids typical of conventional ion mobility spectrometers. In the FA-IMS, ions are carried between parallel plate electrodes by a flow of gas while an asymmetric high voltage RF waveform causes the ions to follow trajectories perpendicular to the flow. Under these high-field conditions, mobilities become field dependent, in contrast to the low-field regimes used in conventional IMS drift tubes where the mobilities are essentially constant with field w13x. A second electric field produced by a weak DC voltage is superimposed on the high voltage RF waveform. The weak DC field typically ranges from y1000 to q1000 Vrcm, while the peak amplitude of the RF field is around 20,000 Vrcm. Both fields are applied transverse to the carrier gas flow. For a given ion species and RF field, a certain compensating DC field will allow these ions to traverse the drift region and register at a detector without striking the parallel plate electrodes. Previous investigations into the role of the high-fields on ion behavior w7–10x illustrate that this approach is a feasible method for ion characterization, however, the method is not well described and the link between molecular structure and compensation voltage is still unclear w12,13x. In spite of these uncertainties, the prospects of improved detection limits without the need for a shutter have motivated the development of conventionally machined FA-IMS. However, the manufacture of conventional FA-IMS drift tubes is complex, labor intensive, and expensive. Micromachining is a fabrication technology that utilizes approaches and fabrication techniques Že.g., batch fabrication and photolithography. which are similar to those used in the integrated circuit industry. Adopting this fabrication technology should allow the realization of low cost, high precision FA-IMS spectrometers. In addition, new device configurations not practical with conventional machining such as spectrometer arrays can be realized. In this article, the construction of a FA-IMS fabricated by micromachining techniques is described and performance results are presented.
2. Principle of operation In FA-IMS, ions are transported longitudinally through a drift tube by a carrier gas stream as shown in Figs. 1 and 2. An asymmetric RF electric field EŽ t . Žoriented at 908 to the gas flow. is produced by applying a voltage between two, parallel plate, ion filter electrodes. The RF electric field Ž Emax ) 10,000 Vrcm. causes the ions to oscillate in a direction transverse to the carrier gas flow during ion transport through the length of the drift tube. Since the amplitude of the RF field is asymmetric, an ion velocity
301
Fig. 1. Schematic of the FA-IMS system showing the arrangement of electrical controls, gas flows, and drift tube.
towards the top electrode as described in Eq. Ž1. will differ from its velocity towards the bottom ion filter electrode: Õ Hs K Ž E . E Ž t .
Ž 1.
here Õ H is the ion velocity perpendicular to the gas flow direction, K Ž E . is the field dependent mobility, and EŽ t . is the electric field strength. This difference in ion velocities is due not only to the differences in electric field strengths Emin - Emax but also to the dependence of ion mobility on electric field strength at fields greater than 10,000 Vrcm w14,15x. Therefore, for each period of applied RF field, the ion is drawn towards one ion filter electrode by a net amount D h and total displacement of the ion in space includes the number of RF periods as nD h. The average value of "D h for an ion species is determined by the duty cycle of the RF field and the field dependence of the mobility K Ž E .. The resultant motion of the ion along the drift tube under the influence of field and flow yields a trajectory that is specific to each ion species, and only ions with a total transverse displacement less than the drift channel height can reach the detector. All other ions collide with the filter electrodes are neutralized and are removed from the drift region by the gas flow. While all ions interact with the applied RF field and are drawn toward the drift tube walls, select ions can be kept in the flowing gas by applying particular low DC voltages to the ion filter electrodes. The voltages generate fields and subsequently forces on the ions that oppose their net RF-induced transverse motion. Thus, an ion can be passed between the ion filter electrodes and transported to the detector and this DC Žor compensation. voltage is a characteristic of the ion species. Sweeping the compensation voltage over a range of voltages produces a high-field equivalent of a mobility spectrum of the different ion species present in the gas stream. The unique aspect of this approach to ion characterization is that part of the ion interaction with the RF field occurs under high-field conditions where ion mobilities are dependent upon the electric field w13x. In traditional lowfield IMS, ion mobilities are independent of electric field and are reasonably well described through the Mason–
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Fig. 2. Diagram of the anticipated motion or trajectory of ions in a FA-IMS with the DC compensation voltage superimposed on the RF field.
Schamp equation. The Mason–Schamp equation links mobilities to reduced mass and cross-section areas for ion– molecule collisions w14,15x. In contrast, the relationship between molecular structure and the coefficient of mobility in the high electric field for multi-atomic organic ions is not well-described and the relationship between conventional terms of reduced mass, collisional cross-section, and the high-field influence is unknown. Nevertheless, the limited publications on large, conventionally machined FA-IMS analyzers suggest that different ions species appear at characteristic compensation voltages. Additionally, since the FA-IMS operates in a regime that differs from the low-field, conventional IMS, some ion species that cannot be resolved in the conventional IMS appear well resolved in the FA-IMS w7–12,16,17x. In short, the RF field interacts with ions in ways that allow ion separations based upon ion molecular interactions, which are not present in mobility spectrometers operating in low-field regimes.
wanted metal residing on top of the photoresist while leaving behind the metal electrodes on the Pyrex. Once metal electrodes have been formed, the Pyrex wafers are diced into rectangular pieces. For spectrometers intended to be used with photo-ionization sources, holes are drilled into the top Pyrex pieces to provide a means for the photons to enter the ionization region. The silicon wafer thickness defines the gap between the top and bottom electrodes of the ion filter. In the present method, the silicon wafer is diced into strips that are 25 mm long, 2 mm wide, and 0.5 mm thick. The silicon strips are aligned with the metal electrodes and bonded to the Pyrex as shown in Fig. 3. The anodic bonding process uses a combination of high electric field Ž; 1000 V., heat Ž3508C. and applied pressure to permanently attach the Pyrex pieces to the silicon strips. Metal electrodes also make contact to these silicon strips allowing control of their potential. A Teflon test fixture provides a means for interfacing a vapor generator to the spectrometer for experimental measurements. The RF asymmetric field required to filter the ions is produced by a soft-switched semi-resonant circuit which incorporates a flyback transformer to generate the high voltage pulses, similar circuits have been described by Krylov w18x. The circuit, shown in Fig. 4, provides a peak-to-peak RF voltage of 1400 V at a frequency of 2 MHz with a duty cycle of approximately 30%. This RF voltage is superimposed onto the compensation voltage. In these studies, sample ionization was accomplished using photo-ionization discharge lamps. A miniature 10.6 eV Ž l s 116.5 nm. photo-discharge lamp was positioned above the hole in the top Pyrex wafer of the spectrometer to permit the photons to enter the drift tube. The bottom ion filter electrode was grounded while the high voltage asymmetric field and compensation voltages were applied to the upper filter plate. An amplifier was used to detect the charge collected at the detector plate. A polynomial waveform synthesizer model 2920 was used to sweep the compensation voltage and a Tektronics model TDS340A
3. Experimental 3.1. Instrumentation A schematic of the micromachined drift tube is shown in Fig. 3 and consists of an ion filter, a detector electrode, and a deflector electrode. The spectrometer is fabricated from two Pyrex wafers and one heavily boron doped silicon wafer. Metal electrodes are formed on the top and bottom Pyrex wafers defining the ion filter, deflector, and detector plates. The metal electrodes are fashioned on the Pyrex wafers by first coating the wafers with Shipley 1822 photoresist and patterning the resist using a photolithographic process. The patterned photoresist is then coated ˚ gold on 400 A˚ titanium. by sputter with metal Ž1200 A deposition. A lift-off process is used to remove the un-
Fig. 3. Photograph of a micromachined field asymmetric ion mobility spectrometer.
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4. Results and discussion 4.1. Influence of applied RF fields on ion behaÕior in drift tube
Fig. 4. Schematic of circuit used to generate the high voltage, asymmetric, RF electric field.
storage oscilloscope was used to record the signal from the amplifier and produce a spectrum for the sample gas. 3.2. Sample generation and gas supply Clean air was provided using a pure air generator Žmodel 737, Addco, Miami, FL. with gas scrubbing through beds of activated charcoal and 13X molecular sieve. Purified air was split into three independently controlled parts with 1–4 lrmin used as carrier gas flow, 10–300 mlrmin used to operate a sample generator based upon diffusion, and 0–300 mlrmin used as a dilution gas flow with the sample flow. In experiments with mixtures of chemicals, the dilution gas line was also used for preparing and introducing an additional vapor sample. Sample vapors were prepared in the vapor generator using glass diffusion tubes or Teflon permeation cells. All three parts of the flow were combined and entered the drift tube shown in Fig. 3. The vapor generator is a hermetically sealed, glass container Ž200 ml. located in a thermostated aluminum block at 35–808C via a Minco CT 137 controller. All supply lines after the vapor generator were kept at about 508C to minimize wall-adsorption. This flow system allowed a constant flow to be delivered to the drift tube while permitting changes in vapor concentration by adjusting the ratio of sample to dilutent flows. The diffusion or permeation tubes were weighed over several days to gravimetrically determine concentrations of the sample flow.
The mobility spectrum for acetone obtained using the micromachined high-field asymmetric ion mobility spectrometer is shown in Fig. 5, where the DC voltage is swept from y15 to q15 V, in the absence Ždashed line. and presence Žsolid line. of an applied RF field. In the spectrum without the RF field applied, ions arrived at the detector through bulk flow in the drift gas and there was no need for a compensation voltage to control ion motion. Consequently, the ion peak appears at a compensation voltage at or very near 0 V DC. The slight shift from zero compensation voltage is believed due to the manner in which the vapor generator was interfaced to the FA-IMS. This caused a slight asymmetry in the gas flow, which tended to deflect the ions preferentially toward one of the ion filter electrodes. Here, a slightly positive compensation voltage was required to overcome this deflection. The baseline for this spectrum is constant and low throughout the remainder of the sweep including the extremes beyond "5 V DC Žnot shown.. The intensity of this peak was about 700 mV with a near Gaussian peak shape and peak width at half height of 0.5 V DC. Application of a high-field RF waveform affected ion behavior and the ion was detected at a compensation voltage of y3.5 V DC. At values lower than y4.5 V DC, the baseline was flat and no ions were detectable. At values ranging from y2.8 V to about q1.5 V DC, the baseline was elevated slightly suggesting the presence of ions. This baseline perturbation was dependent on, and proportional to, the concentration of acetone in the gas flow and can be explained by ion–molecule cluster reactions that likely occur in the drift tube between the ion source and the detector. Such reac-
3.3. Chemicals Chemicals used in this work were acetone, benzene, and toluene. All three chemicals were purchased from Aldrich Chemical ŽSt. Louis, MO. and used without further purification.
Fig. 5. FA-IMS spectra for acetone in air with and without the RF field applied. The product ion was created by photo-ionization and ions created in the same source and gas were identified using mass spectrometry as MHq.
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tions, as shown in Eqs. Ž2. and Ž3., would lead to association and dissociation reactions throughout this region since the ions and neutral species would move together in the drift gas flow. Acetone clusters are well known and dimers and trimers may be anticipated to form at ambient temperature.
lM M lM
Mq q M ion
Mq 2 q dimer
neutral
neutral
q 2
Ž 2.
dimer q 3
Ž 3.
trimer
Although peak intensities with and without the RF field applied differed by nearly 30%, the areas under the total spectrum including the baseline perturbations were found to be the same. Thus, the transmission coefficient for ions in the drift region under the influence of the RF and DC fields is close to 100%, implying ion losses to surfaces were minor or not detectable. Minor Ž20%. broadening of the peak shape was observed with the application of the RF field to the drift tube. The reason for the peak broadening is not known yet, though may be attributed to increased ion paths from the oscillations. The flow rate of the carrier gas affects both peak intensity and peak area. Without an RF field, 2.5 lrmin of gas flow provides the largest peak heights and areas. At higher flows, the intensities decrease. This decrease was at first attributed to turbulent flow caused by the hole in the Pyrex wall used for ionization. However, finite-element analysis did not confirm this hypothesis and showed laminar flow at flow rates in excess of 3 lrmin. Further investigations are being performed to determine the cause of the decrease in intensity at higher flows. The maximum amplitude of the RF waveform also affects the appearance of spectral profiles and 1000 V Žor ; 20,000 Vrcm. showed optimum yield for ions during passage down the drift tube. The ability to predict the value of the compensation voltage for different chemicals is limited at present by inadequate understanding of the effect of field strength on mobility. This is true for all further discussions below and constitutes a main theme for future research in FA-IMS analyzers. 4.2. PositiÕe and negatiÕe ions in micromachined high FA-IMS Mobility spectra for benzene in Fig. 6 Žtop two traces., showed field dependent behavior similar to acetone. The product ion for benzene Žtentatively identified as Mq Ø from separate PID-MS studies. appeared as expected near zero DC compensation voltage without applied RF fields. A compensation voltage of y10.5 V DC was needed, with the RF voltage applied, in order to register product ions at the detector. As with acetone, a broad shift of the baseline between the product ion and zero compensation voltage was observed and here extended from y8 to q2 V DC. The intensity of this shift was proportional to benzene concentration and the broad spectral band is consistent
Fig. 6. FA-IMS spectra for benzene vapor in air without and with RF field applied. The top spectrum is without RF field and a with positive deflector plate bias. The middle spectrum shows spectra with positive deflector plate bias and with the RF field applied. The bottom spectrum shows response with the RF field and a negative deflector plate bias.
with the formation of ion–molecule clusters per Eqs. Ž2. and Ž3.. This interpretation, though tentative, is supported by evidence from high-pressure mass spectrometry where cluster ions of benzene have been measured. The presence of negative molecular-based ions, along with positive ions in the drift gas during transport toward the detector was initially considered implausible owing to the presence of oxygen with a high electron affinity in the drift gas. However, reversal of the deflector electrode polarity without any other changes to the drift tube or electronics allowed negative ions to be detected ŽFig. 6 bottom trace.. The intensities of the negative ions, however, were substantially lower in comparison to the positive ions Žroughly 10% of the positive ion intensity.. Still, this signal demonstrates that negative ions were passed through the drift region and two peaks appeared at compensation voltages of q3 and q0.5 V DC. Negative ions were not observed with other vapors used in this study and should best be explored with another ion source such as a beta-emitter. The location and peak shape is beyond current models and the identity of this ion was not determined. The proportional response with vapor concentration of benzene is not informative and further conclusions must await mass spectral analysis. Nonetheless, the ability to detect simultaneously positive and negative ions is an unexpected possible benefit of the micromachined FA-IMS design that has not been previously recognized. 4.3. Response to sample Õapor concentrations The response of the analyzer to increases in concentrations of sample vapors in air is shown in Figs. 7 and 8. Photo-ionization detectors in gas chromatography are known to provide working ranges of 10 3 or better, so proportional response was anticipated and was seen in the
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the range of concentrations down to 0.1 ppm. Apparent limits of detection ŽLOD. based upon signal to noise calculations are estimated at 60–70 ppb but may be lower since the response curve becomes shallow below 0.1 ppm. At present, causes for the deviation from linearity in Fig. 8 for very low concentrations are unknown. 4.4. Resolution of multiple ion species in mixtures
Fig. 7. Mobility spectra for FA-IMS for toluene with increasing concentration from bottom to top Ža. without RF field and Žb. with the RF field.
high-field mobility spectra. Spectra for toluene without and with the RF field are shown in Fig. 7a and b, respectively with vapor concentrations ranging from 0.1 to 0.7 ppm. As seen in both sets of spectra, the product ion intensity was proportional Žabove the detection limit. to vapor concentrations. The peak heights and areas in Fig. 7b are plotted in Fig. 8 as a function of concentration and appear linear over
Fig. 8. Peak areas and intensities at various concentrations of toluene vapor.
The ability to distinguish between multiple ion species in a drift tube is a measure of the resolution or analytical selectivity of the device. The practical analytical selectivity or resolution in IMS is dictated not only by the number of discrete peaks that can fit within the total spectrum band width but depends on the ion generation process as well. This fact has permitted the IMS to be successfully applied and dominate the chemical agent and explosives detection arenas. Nonetheless, a measure of instrument performance is how well ion species can be separated. Fig. 9 shows typical spectra from a traditional low-field time-of-flight drift tube for the small organic molecules, benzene, toluene, and acetone w19x. The product ion peaks are all close to the reactant ion peak and have nearly the same drift times Žthe source of charge used for these spectra is a radioactive ion source.. For example, drift times from these spectra are 4.44 ms for the reactant ion peak, 4.86 ms for acetone, 5.04 ms for toluene and 4.84 ms for benzene. In a mixture, these chemicals would be difficult to resolve using a low-field drift tube. In contrast, good separation of product ions in the FA-IMS drift tube is evident for the pairs of chemicals shown in Fig. 10. In the top spectrum, benzene and acetone are well separated at compensation voltages of y10.5 and y3.5 V DC, respectively. In a mixture of toluene and acetone two distinct peaks are observed Žbottom trace. at y7.0 and y3.5 V DC. These peaks occur at compensation voltages identical to those measured for toluene and acetone individually. In addition, a peak at a compensation voltage of y9.5 V is also present. The presence of this unexpected peak at y9.5 V DC is be-
Fig. 9. Mobility spectra from conventional time-of-flight IMS drift tube for acetone, toluene, and benzene. Source: Ref. w20x. The product ions all appear between 4.8–5.0 ms.
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Acknowledgements The authors would like to thank Dr. William P. Kelleher for helpful discussions, Brenda A. Hugh for her assistance with the fabrication, and Mr. John T. Kauppinen Jr. for his assistance with spectrometer characterization. Support from the FBI through contract no. J-FBI-98-111 to GAE and EGN, is gratefully acknowledged.
References Fig. 10. FA-IMS spectra for chemical pairs of benzene–acetone and toluene–acetone. The peak for acetone is at y3.5 V.
lieved due to interaction between the acetone and toluene as the ions and vapors move through the drift region. In addition, the baseline perturbations increase for both mixtures with increased vapor levels and the peaks for benzene and toluene occur at slightly more negative compensation voltages. Both of these effects are presently ascribed to the formation of ion–molecule clusters in the drift gas flow. The presence of sample neutrals with sample ions in the flowing gas stream must be rectified in future drift tube designs. This has been recognized by others and corrected by isolating the ion formation region from ion identification region, and performing the identification in clean air w11,12x. Similar modifications will be incorporated into future designs of the micromachined FA-IMS. This will lead to reduced clustering and to the realization of robust, miniature, low cost FA-IMS.
5. Conclusions A micromachined FA-IMS has been microfabricated and successfully demonstrated with the detection of acetone, toluene, and benzene. The parallel electrode design of this analyzer allows mass-production methods to be used in the construction of the analyzer. The ion separations were distinct and well resolved for the chemical pairs acetone–benzene and acetone–toluene which are not easily separated using low-field mobility spectrometry. A detection threshold of about 60 ppb was demonstrated for toluene with an UV photo-discharge lamp. The response of the spectra to concentration variations suggests that the ion characterization should be performed in a carrier gas free of sample neutrals. Findings suggest that sample neutral ion clusters in the drift region lead to baseline perturbations. Through planned modifications to the drift tube design, the drift gas will be kept free of sample neutrals in future spectrometers.
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