Handheld Rectilinear Ion Trap Mass Spectrometer - Analytical

Aug 5, 2006 - This instrument has evolved from a decade-long experimental and simulation program in mass spectrometer miniaturization. The rectilinear...
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Anal. Chem. 2006, 78, 5994-6002

Handheld Rectilinear Ion Trap Mass Spectrometer Liang Gao,† Qingyu Song,† Garth E. Patterson,‡ R. Graham Cooks,*,† and Zheng Ouyang*,†

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, and Griffin Analytical Technology, Inc., West Lafayette, West Lafayette, Indiana 47906

A shoebox-sized, 10-kg, handheld mass spectrometer, Mini 10, based on a rectilinear ion trap mass analyzer has been designed, built, and characterized. This instrument has evolved from a decade-long experimental and simulation program in mass spectrometer miniaturization. The rectilinear ion trap has a simplified geometry and high trapping capacity, and when used with a miniature and ruggedized pumping system, it allows chemical analysis while the instrument is being carried. Compact electronics, including an air core RF drive coil, were developed to control the instrument and to record mass spectra. The instrument runs on battery power, consuming less than 70 W, similar to a laptop computer. Wired and wireless networking capabilities are implemented. The instrument gives unit resolution and a mass range of over m/z 500. Tandem mass spectrometry capabilities are implemented using collision-induced dissociation, and they are used to provide confirmation of chemical structure during in situ analysis. Continuous monitoring of air and solution samples is demonstrated, and a limit of detection of 50 ppb was obtained for toluene vapor in air and for an aqueous naphthalene solution using membrane sample introduction.

Miniature mass spectrometers are of growing interest due to their potential for in situ chemical analysis combined with rapid response, high sensitivity, and high specificity. These characteristics are especially applicable in public safety, environmental protection, and industrial process monitoring. The goal of miniaturization is to retain the existing advantages of mass spectrometry while enhancing instrument portability by minimizing size, weight, and power consumption. Almost all types of mass spectrometers have been the subjects of miniaturization, with the effort being concentrated on designing, building, and characterizing miniature mass analyzers.1-10 Although relatively little effort * Corresponding authors. (Cooks) Tel: (765) 494-5262. Fax: (765) 494-9421. E-mail: [email protected]. (Ouyang) Tel: (765) 496-1539. Fax: (765) 494-9421. E-mail: [email protected]. † Purdue University. ‡ Griffin Analytical Technology, Inc. (1) Kogan, V. T.; Kazanskii, A. D.; Pavlov, A. K.; Tubol’tsev, Y. V.; Chichagov, Y. V.; Gladkov, G. Y.; Il’yasov, E. I. Instrum. Exp. Tech.. 1995, 38, 106110. (2) Sinha, M. P.; Tomassian, A. D. Rev. Sci. Instrum. 1991, 62, 2618-2620. (3) Sinha, M. Presented at the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, May 31-June 4, 1998. (4) Bryden, W. A.; Benson, R. C.; Ecelberger, S. A.; Phillips, T. E.; Cotter, R. J.; Fenselau, C. Johns Hopkins APL Tech. Dig. 1995, 16, 296-310.

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has gone into building miniature mass spectrometer systems, as opposed to miniature mass analyzers, there is a strong connection between the size of a mass analyzer and the size of the whole mass spectrometer system. This is the result of connections between analyzer size and that of the vacuum system, control electronics, and power system. The complex interplay of factors that affect mass spectrometer performance as a function of the size of the mass analyzer and other components has seldom been systematically explored. Exceptions include (i) the work of Ferran on the quadrupole mass filter11 and (ii) that on ion traps in this lab;12 these studies led to the multiplexing of individual small analyzers13,14 to achieve high system performance in a small package. Other exceptions are (iii) the work of Cotter and co-workers on short drift field time-offlight mass analyzers,5 and (iv) recent investigations of the performance of micrometer-sized cylindrical ion traps by the Purdue/Sandia group15 and that of Pau, Ramsey, and co-workers.16 The simulation of ion trap performance as a function of size and operating parameters, done using the multiparticle ion motion simulation program, ITSIM, is noteworthy. ITSIM has been shown to allow mass spectra to be predicted with reasonable accuracy,17 and hence, it forms an appropriate vehicle for enquiry into the effects of miniaturization on performance.18 Quadrupole ion traps have significant advantages as platforms for instrument miniaturization. The operating pressure of traps can be 100 times higher than that for other mass analyzers, which is highly desirable for miniature instruments with reduced (5) Cotter, R. J.; Fancher, C.; Cornish, T. J. J. Mass Spectrom. 1999, 34, 13681372. (6) Ferran, R. J.; Boumsellek, S. J. Vac. Sci. Technol. A 1996, 14, 1258-1265. (7) Orient, O. J.; Chutjian, A.; Garkanian, V. Rev. Sci. Instrum. 1997, 68, 13931397. (8) Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H. Int. J. Mass Spectrom. Ion Processes 1991, 106, 79-115. (9) Kornienko, O.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Rapid Commun. Mass Spectrom. 1999, 13, 50-53. (10) Miller, G.; Koch, M.; Hsu, J. P.; Ozuna, F., Proc. 45th ASMS Conf. Mass Spectrom. Allied Top., Palm Springs, CA, June 1-5, 1997. (11) Boumsellek, S.; Ferran, R. J. J. IEST 1999, 42, 27-31. (12) Badman, E. R.; Johnson, R. C.; Plass, W. R.; Cooks, R. G. Anal. Chem. 1998, 70, 4896-4901. (13) Boumsellek, S.; Ferran, R. J. J. Am. Soc. Mass Spectrom. 2001, 12, 633640. (14) Badman, E. R. Purdue University, West Lafayette, IN, 2000. (15) Blain, M. G.; Riter, L. S.; Cruz, D.; Austin, D. E.; Wu, G.; Plass, W. R.; Cooks, R. G. Int. J. Mass Spectrom. 2004, 236, 91-104. (16) Pau, S.; Pai, C. S.; Low, Y. L.; Moxom, J.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Phys. Rev. Lett. 2006, 96, 120801. (17) Ouyang, Z.; Wu, G.; Song, Y.; Li, H.; Plass, W. R.; Cooks, R. G. Anal. Chem. 2004, 76, 4595-4605. (18) Wu, G.; Ouyang, Z.; Plass, W. R.; Cooks, R. G. 50th ASMS Conf. Mass Spectrom. Allied Top., Orlando, FL, 2002. 10.1021/ac061144k CCC: $33.50

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pumping capability. Even in a miniature ion trap, tandem mass spectrometry can still be performed.19,20 This is so because in ion traps of all types,21,22 23 frequency of ion motion is the basis for mass analysis. In the readily miniaturized quadrupole ion trap mass analyzers, performance, especially resolution and dynamic range, improve with increasing size although the mass/charge range increases as the radius is decreased.12 Even though the mass analyzer in a trapping mass spectrometer is usually small in comparison to the size of the instrument, the electronics, vacuum, and power equipment are all significantly and favorably impacted by the selection of smaller mass analyzer sizes.12,24 On the other hand, shrinking the size of the mass analyzer increases the precision requirements for machining. Simplified geometries have been developed for 3D25 and 2D17,26 ion traps, and new methods 15,27-29 have been explored for fabricating traps on the millimeter and micrometer scales, where traditional precision machining methods are not applicable. Shrinking the size of the ion trap mass analyzer has a direct impact on the RF system in an ion trap mass spectrometer. Once the required RF amplitude is lower than 1500 V0-p, the RF voltage can be generated using air core coils that can be quite compact.30 Further reduction of the trap size would allow the use of even lower RF voltages, but a higher RF frequency would then be needed to retain the analytical performance,9,12,28 which consequently requires a higher RF amplitude to cover the same m/z range. In our laboratory, we have been investigating the performance of traps with various geometries and configurations of sizes over the wide range from 10 mm to 1 µm. For the portable trap instruments so far constructed, ion traps with an r0 (internal radius of the ring electrode of the cylindrical ion trap) or x0 (half interdistance between x electrode in the case of the rectilinear ion trap) of several millimeters are preferred over larger and smaller sizes due to their demonstrated performance as mass analyzers17,31 with moderate requirements as to RF circuit specifications.32 (19) Riter, L. S.; Peng, Y.; Noll, R. J.; Patterson, G. E.; Aggerholm, T.; Cooks, R. G. Anal. Chem. 2002, 74, 6154-6162. (20) Riter, L. S.; Meurer, E. C.; Handberg, E. S.; Laughlin, B. C.; Chen, H.; Patterson, G. E.; Eberlin, M. N.; Cooks, R. G. Analyst 2003, 128, 11121118. (21) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282-283. (22) March, R. E. J. Mass Spectrom. 1997, 32, 351-369. (23) March, R. E., Todd, J. F. J., Eds. Practical Aspects of Ion Trap Mass Spectrometry; CRC Press: Boca Raton, FL, 1995; Vol 1. (24) Rimkus, W. V.; Davis, D. V.; Gallaher, K. Novel Miniature Ftms for Analysis of Corrosives and Chemical Warfare Agents. 51st ASMS Conf. Mass Spectrom. Allied Top.; Montreal, Canada, 2003. (25) Wells, J. M.; Badman, E. R.; Cooks, R. G. Anal. Chem. 1998, 70, 438444. (26) Song, Y.; Wu, G.; Song, Q.; Cooks, R. G.; Ouyang, Z.; Plass, W. R. J. Am. Soc. Mass Spectrom. 2006, 17, 631-639. (27) Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S. Electron. Lett. 1996, 32, 2094-2095. (28) Chaudhary, A.; van Amerom, F. H. W.; Short, R. T.; Bhansali, S. Int. J. Mass Spectrom. 2006, 251, 32-39. (29) Yu, M.; Fico, M.; Cooks, R. G.; Chappell, W. J. 2006, submitted. (30) Patterson, G. E.; Grossenbacher, J. W.; Wells, J. M.; Knecht, B. A.; Rardin, B.; Barket, D. J. J. 51st ASMS Conf. Mass Spectrom. Allied Top., Montreal, Canada, 2003. (31) Ouyang, Z.; Badman, E. R.; Cooks, R. G. Rapid Commun. Mass Spectrom. 1999, 13, 2444-2449. (32) Patterson, G. E.; Guymon, A. J.; Riter, L. S.; Everly, M.; Griep-Raming, J.; Laughlin, B. C.; Ouyang, Z.; Cooks, R. G. Anal. Chem. 2002, 74, 61456153.

With the advent of compact RF circuits that we and others have developed and used in portable ion mass spectrometers, the vacuum system becomes the major contributor to both the size and the weight of the instruments.20,30,33,34 In conventional lab instruments, the combination of a rotary vane mechanical pump and a turbo molecular pump forms the standard vacuum system. To obtain high vacuum, 10-4 Torr or lower, in some portable instruments, the manifold has been prepumped and ion pumps have been used to maintain the vacuum for the period of analysis.24,35 However, when relatively large flow intakes are needed, like those used for air analysis, continuous pumping is necessary, and turbo molecular pumps are still the only choice. With the addition of the drag stage, turbo molecular pumps can tolerate backing pressures of several Torr, and this makes small diaphragm pumps suitable as backing pumps for small instruments. The compact size and the oil-free nature of diaphragm pumps make them highly suited to use with portable instruments. Turbo pumps with relatively small sizes are commercially available, but their robustness has been a major concern for use in any portable instruments. The rotors spin at speeds higher than 60 000 rpm, making them extremely vulnerable to sudden inertial changes. Small rotor inertia and durable bearing mechanics are highly desirable features for turbo pumps used for portable instruments. Among the miniature ion trap mass spectrometers 20,30,33 previously developed in this lab was a system with a total weight of 40 lbs and a power consumption of 135 W. This, the Mini 7 instrument, formed the basis for a successful commercial mass spectrometer. Instruments of this size are transportable for infield analysis; however, to allow the operators, especially first responders, to perform fast in situ analysis while hand-carrying the instrument, both the size/weight and the power consumption needed to be further reduced. With this as the basis of a major effort, a handheld mass spectrometer, termed the Mini 10 rectilinear ion trap, has now been developed. It is shoebox sized and consumes less power than a laptop computer, and its performance characteristics are described in this paper. Instrument Description. The configuration of the Mini 10 handheld mass spectrometer is shown in Figure 1. All components, including the electronics and vacuum systems, are assembled into an aluminum case 32 cm in length, 22 cm in width and 19 cm in height. The total weight of the instrument, including batteries, is 10 kg. The maximum power consumption, when both rough and turbo pumps are running and the RF is continuously being scanned to its maximum amplitude, is below 70 W. The Mini 10 instrument has an on-board computer as well as network capabilities and can communicate with remote computers and with other mini instruments. Programs have been written to transfer data recorded by Mini 10’s through the network and to process them using appropriate algorithms to provide the intelligence needed for a reliable alarm. Mass Analyzer, Sample Introduction, and Ionization Source. The vacuum manifold was machined from aluminum with outside (33) Laughlin, B. C.; Mulligan, C. C.; Cooks, R. G. Anal. Chem. 2005, 77, 29282939. (34) Zhang, C.; Chen, H.; Guymon, A. J.; Wu, G.; Cooks, R. G.; Ouyang, Z. Int. J. Mass Spectrom., in press. (35) Meuzelaar, H. L. C.; Dworzanski, J. P.; Arnold, N. S.; McClennen, W. H.; Wager, D. J. Field Anal. Chem. Technol. 2000, 4, 3-13.

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Figure 1. Mini 10 mass spectrometer and its components.

Figure 2. In-vacuum components, including rectilinear ion trap mass analyzer, ionization source, electron multiplier, and sample introduction system.

dimensions of 12.7 cm × 5.7 cm × 6.5 cm and a wall thickness of 5.3 mm. All in-vacuum components and feedthroughs were mounted on the manifold for ease of assembly (Figure 2). Instead of the cylindrical ion traps used in previous transportable instruments,20,30,33 a rectilinear ion trap (RIT)17 was used as the mass analyzer of the Mini 10. The RIT is a linear ion trap with simplified geometry, and its improved ion trapping capacity has been demonstrated.17 An RIT geometry with interelectrode distances of 5.0 and 4.0 mm was selected, since its analytical performance had been demonstrated previously.17 Helium is usually used as the buffer gas for ion trap instruments. However, portability problems make helium unsuitable for portable instruments, so air was used as the buffer gas for the RIT in the Mini 10 mass spectrometer.36 Three vacuum feedthroughs were inserted into the manifold cover using Swagelok 1/16-in. tubing connectors (SS-100-1-OR, Swagelok Fluid System Technologies, Solon, OH) to allow easy (36) Lammert, S. A.; Wells, J. M. Rapid Commun. Mass Spectrom. 1996, 10, 361-371.

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coupling of various sample introduction systems. Sample introduction is based on a membrane inlet in the instrument described here, but a solid-phase sorption system has been tested and chromatographic inlets will be explored in the future. For the in situ chemical analysis of volatile organic compounds, a 15.0-cm PDMS (poly(dimethylsiloxane)) membrane tubing (o.d. 1.19 mm, i.d. 0.64 mm, Dow Corning Corporation, Midland, MI), was used to connect two of the feedthroughs. A miniature KNF Neuberger 1.3-1.6 l/min sample pump (UNMP015D, KNF Neuberger, Inc., Trenton, NJ) was used to pull sample through the membrane tubing. Liquid samples can also be forced through the membrane tubing using a syringe or peristaltic pump. Volatile organic compounds in the gaseous samples permeate the membrane and enter the vacuum manifold where they are ionized and mass analyzed. Other sample introduction systems have been developed to allow the analysis of involatile compounds, and these trap and thermal release systems will be reported elsewhere. Internal electron impact (EI) ionization was implemented. A traditional filament electron source was first implemented, and a glow discharge electron source was later developed as an alternative, as shown in Figure 3. For the filament electron source, a filament control board was built to provide power, and typically, 5 W was used to generate an emission electron current of 5 µA, which can be regulated through filament control electronics. For the glow discharge source, a 10-cm fused-silica capillary with a 20-µm i.d. was used to connect the inside of the discharge cell to the atmosphere via one of the vacuum feedthroughs. Air was used to support the discharge, and the pressure in the discharge cell was estimated to be ∼0.5 Torr. The distance between the two discharge plates (diameter of 22.0 mm) is 6.5 mm. The diameter of the electron exit hole was 100 µm. A DC voltage between 370 and 400 V was applied on the discharge plate to initiate and maintain the discharge. A stable discharge current of 0.5 mA was obtained, and the electron current emerging from the cell for intrap ionization was measured to be ∼5 µA. The use of a filament does not require careful balance of the pressures in different regions. Additionally, the emission current can be easily adjusted and regulated via electronic control. The glow discharge cell has this pressure balance requirement, but it can be implemented with

Figure 3. Ionization sources developed for the Mini 10 mass spectrometer: (a) filament electron source and (b) glow discharge electron source.

much simpler electronics, and the power consumption for generating the 5 µA emission current is as low as 0.2 W. An additional significant advantage of using a glow discharge cell as the electron source is that the source cannot be damaged at high pressure. Both sources were used in the characterization of the instrument. Vacuum System. A miniature rough pump and a miniature turbo pump were used in the Mini 10 to achieve an ultimate vacuum below 1 × 10-5 Torr. A two-stage KNF Neuberger diaphragm pump (1091-N84.0-8.99) with a pumping speed of 5 L/min was used as the rough pump to provide a backing pressure below 2 Torr for the turbo pump. This pump weighs 1.35 kg and consumes 10 W continuously. A Pfeiffer TPD 011 (Pfeiffer Vacuum Inc., Nashua, NH) turbo pump was used as the main pump. At a rotation speed of 60 000 rpm, an ultimate vacuum below 1 × 10-5 Torr was obtained. As discussed above, the robustness of turbo pumps has been of great concern for handheld instruments due to potential damage to the rotor during movement of the instrument. The Alcatel ATH 31 series miniature pumps (Alcatel Vacuum Technology Corporation, Hingham, MA) are used in many transportable ion trap mass spectrometers.20,30,33 We selected the TPD 011 because it has a rotor with a relatively small diameter,

which may help to reduce the rotary inertia at high speed. To test the robustness of the Mini 10 pumping system, the instrument has repeatedly been carried by hand and moved around while operating and performing chemical analysis. No damage occurred to the vacuum pumps, and no significant differences in analytical results were found in operation between while moving and when stationary. Control Electronics. The electronics control system of the Mini 10 is shown in Figure 4. This system is a modified form that was developed for the Minotaur 300 transportable CIT mass spectrometer (Griffin Analytical Technologies, Inc., West Lafayette, IN 47906). The control/DAQ board is the brain of the instrument, on which a Xilinx XCV200E field-programmable array (Xilinx, Inc., San Jose, CA) was programmed to run the procedures for output control and data acquisition. A variety of signals were generated in time sequence to control the instrument. An RF signal with amplitude adjustable between 0 and 5 V was used as the source of the main RF for ion trapping; the signal is amplified (up to 2,600 Vp-p) and applied to the y electrodes17 of the RIT. A frequency of 1.0 MHz was used for the RF drive. The differential outputs from another AC channel were connected directly to the x electrodes17 of the RIT. Sinusoid signals with designated frequencies and arbitrary waveforms with peak-to-peak amplitudes up to 30 V could be generated and output through this channel to effect resonance ion ejection, ion isolation, ion excitation, etc. A DSP (TMS320C671, Texas Instruments Inc., Dallas, TX) on the control/ DAQ board was programmed to process the data acquired in the format of intensity as a function of time. Functions including dataaveraging, background-subtraction, and centroid peak-listing can be applied in real time while the instrument is scanning. A multichannel DAC on the control/DAQ board was used to generate a series of DC signals for the control of the lens voltages, electron multiplier voltage, filament and heater currents, and sampling pump operation. A high voltage board was developed to amplify these DC signals from the control/DAQ and to provide the appropriate voltages to the components in a vacuum. The output status of these voltages was read back through ADCs and displayed on the user interface to allow monitoring of instrument status.

Figure 4. Schematic diagram of the control electronics of the Mini 10 mass spectrometer.

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Figure 5. Mass spectra of toluene collected using (a) filament electron source and (b) glow discharge electron source. The RIT mass analyzer was operated using resonance ejection at 350 kHz and 0.6-1.1 V0-P.

The control/DAQ board can communicate with other computers via a TCP/IP port. A user interface program was developed to allow the operator to control the Mini 10 through a computer running Microsoft Windows XP Professional (Microsoft Corporation, Redmond, WA). When working as a sensing unit at a fixed location, the instrument can be connected into a wired network through this TCP/IP port, and operation of multiple units can be controlled by a single remote computer. When working as a handheld instrument, a handheld Sony computer (U71P, Sony Corporation, Tokyo, Japan) is mounted on the Mini 10 to run the user interface. The commands and settings can be input through the touch-screen of the computer. With the wireless communication capability of the handheld computer, the Mini 10 can communicate with remote computers through a wireless network. Instrument Power. A truly handheld instrument needs to be powered by rechargeable batteries. Two types of the rechargeable batteries were considered for the Mini 10, lithium ion and nickel-metal hydride batteries. Lithium ion batteries have a relatively good capacity-to-weight ratio, a great advantage for portable instruments. However, lithium ion batteries cannot provide surge power like metal hydride batteries do. In the Mini 10, the power consumption is between 30 and 70 W (the latter when all the electronics and the pumps are on and the RF is scanning), requirements which are easily handled with a lithium ion battery. However, when the rough pump is first started, an initial current which is 10 times that in the continuous mode is required for a period of ∼100 ms. The nickelmetal hydride batteries were chosen for this reason, and three 7.2 V, 3300 mAh rechargeable battery packs (Powerizer, Richmond, CA) were used. This power pack weighs ∼1.1 kg and provides power for 1 h when the instrument is constantly scanning RF for chemical analysis, although more typically 3-4 h of operation is achieved. Intelligent power control is being developed to allow some components to be powered-off at times to increase battery life without affecting the chemical analysis. Various measurements were made to characterize the power consumption of the turbo and diaphragm pumps, two of the highest power-consuming components. For those applications when continuous monitoring is not necessary, the pumps, along with the RF circuit and high 5998 Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

Figure 6. Mass spectrum of PFTBA showing a mass/charge range over m/z 500; filament electron source, resonance ejection at 350 kHz and 0.7-1.3 V0-P.

voltage board, can be power off to minimize the power consumption. The diaphragm pump holds seal well when it is off. When the instrument is turned off after an initial pump-down, the pressure in the vacuum manifold can be kept below 10 Torr for more than 5 h and go back to 10-5 Torr within 3 min after the pumping system is turned on again. Spectra with low background can be obtained several seconds after the filament is turned on. For continuous analysis using the membrane tubing inlet, both pumps can be turned off periodically for ∼2 min before the vacuum goes higher than 1 × 10-4 Torr. During these power-off intervals, chemical analysis can continue to be preformed, and good quality spectra can be recorded. However, the turbo pump consumes twice as much power when speeding up than during continuous running at full speed, so it is not beneficial to turn the turbo pump on and off for power saving unless the interval between each two analysis operations is significantly longer than the speedup time of the turbo pump, which is ∼1 min. The diaphragm pump can be turned off or slowed periodically using real-time feedback of the turbo pump current draw as the control signal. Analytical Performance. The analytical performance of the Mini 10 handheld mass spectrometer has been characterized using various gaseous and liquid samples containing volatile organic compounds. As shown in Figure 5, the headspace air of

Figure 7. MS3 data for methyl salicylate, recorded using filament electron source, (a) MS spectrum of methyl salicylate; (b) molecular ion m/z 152 isolated using a SWIFT, with a notch between 100 and 110 kHz at 2 V; (c) product ion spectrum, excitation at 105 kHz, 0.1V; (d) sequential product ion spectrum of isolated m/z 120, excitation at at 140 kHz, 0.1 V.

a vial containing toluene was pumped through the membrane tubing, and mass spectra were recorded using both filament and glow discharge electron sources. Similar peak intensities were obtained for the glow discharge source operated at 370 V (5-µA emission current) and the filament source operated at the same 5-µA emission current. The peaks due to the molecular ion m/z 92 and the fragment ion m/z 91 are well-separated, indicating unit resolution at this m/z value. The mass range available was tested using perfluorotributylamine (PFTBA). The headspace air of a vial containing liquid PFTBA was pumped through the membrane tubing, and a mass spectrum was recorded (Figure 6). This experiment was performed while applying an AC signal between the x-electrodes of the RIT to establish resonance ejection conditions using a frequency of 350 kHz, which corresponds to operation at a nonlinear resonance ejection point,37 q ) 0.80. The best ejection conditions are usually dependent on the mass-tocharge ratio of the ions. To optimize resonance ejection for ions covering a wide mass range, it is desirable to adjust the amplitude of the resonance AC. The AC amplitude therefore was ramped in parallel with the RF ramp. During the scan from m/z 60 to 520, the AC amplitude was ramped from 0.7 to 1.3 V. Peaks of PFTBA fragment ions up to m/z 502 were recorded under these conditions. False alarms are of great concern in in situ applications when corresponding actions need to be taken immediately on the basis of the results of the analysis. Due to the requirements for short (37) Franzen, J.; Gabling, R. H.; Schubert, M.; Wang, Y. In Practical Aspects of Ion Trap Mass Spectrometry; March, R. E., Todd, J. F. J., Eds.; CRC Press: Boca Raton, FL, 1995; Vol. 1, pp 49-167.

Figure 8. Ion monitoring chronogram for naphthalene vapor in air, membrane temperature 25 °C, 1.3-s sampling every 360 s.

overall analysis time as well as limited availability of equipment in the field, only limited sample preparation or preseparation steps are practical. Multistage tandem mass spectrometry can provide additional information about the chemical nature of the analyte and, therefore, help to minimize the false alarm rate. The limit of detection can also be improved with tandem mass spectrometry, especially for analytes present in complex mixtures due to the elimination of the chemical noise.38 It has also been demonstrated that tandem mass spectrometry experiments involving (38) Busch, K. L.; Cooks, R. G. In Tandem Mass Spectrometry; McLafferty, F. W., Ed.; John Wiley and Sons: New York, 1983; pp 11-39.

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Figure 9. Analysis of air sample containing DMMP, n-butylbenzene, methyl salicylate, and 1,3-dichlorobezene using tandem mass spectrometry.

ion/molecule reactions can greatly enhance the selectivity of detection of targeted compounds. 20, 39 The MSn capability of the Mini 10 was demonstrated using methyl salicylate, present in the vapor released from a wintergreen flavored mint, which was held close to the inlet where air flowed into the membrane tube. The molecular ion m/z 152 was isolated and then fragmented by applying SWIFT (stored waveform inversion Fourier transform) waveforms and excitation AC, respectively, between the x electrodes of the RIT. A SWIFT waveform, with amplitude 2.0 V and a frequency notch between 102 and 105 kHz, was precalculated and stored in memory on the control/DAQ board. During ion isolation, the RF amplitude was adjusted to set the secular frequency of the precursor ions within the frequency notch, and the SWIFT signal was applied for 5.0 ms to eject all other ions from the trap. Then an AC signal, amplitude of 0.1 V and frequency 105 kHz, was applied for 15.0 ms to excite the parent ions of m/z 152 via collisions with background air molecules and so induce fragmentation. As shown in Figure 7, the isolated molecular ion of methyl salicylate m/z 152 (Figure 7b) could be fragmented to give m/z 137, 120, and 92 (product ion spectrum, Figure 7c). The fragment ion m/z 120 was subsequently isolated and in turn fragmented via CID to m/z 90 (Figure 7d). The fragmentation patterns obtained via the MS/MS and MS3 experiments serve for structural confirmation and can be used to improve the confidence of identification of methyl salicylate from complex mixtures. Membrane sample introduction serves as a convenient method for direct analysis of both air and water samples when using the handheld MS instruments in a continuous mode. The response and clear time of the air sampling using the Mini 10 was (39) Chen, H.; Zheng, X.; Cooks, R. G. J. Am. Soc. Mass Spectrom. 2003, 14, 182-188.

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Figure 10. Calibration curve of naphthalene in water with concentrations from 10 to 1000 ppb using membrane inlet.

characterized using air containing naphthalene vapor. Untreated room air was pumped through the membrane tubing using the KNF Neuberger sample pump. A mothball was put into a glass vial and the headspace air was exposed to the membrane tubing inlet for 1.3 s every 360 s. The total ion abundance recorded as a function of time is shown in Figure 8. An increase in response was obtained 7 s after the beginning of sampling, and the signal rose from 10 to 90% within ∼22 s. The average clear time, from 90 to 10%, was ∼175 s. Using the membrane sample introduction system, the analysis of complex mixtures was demonstrated with the aid of tandem mass spectrometry. Dimethyl methylphosphonate (DMMP), n-butylbenzene, methyl salicylate, and 1,3-dichlorobezene in equal volumes were mixed, and the headspace vapor of the mixture was sampled with the Mini 10 for 1.0 s. The MS spectrum of the mixture is shown in Figure 9a; product ion MS/MS spectra were acquired for precursor ions m/z 125, 134, and 152 via collisioninduced dissociation (CID). Typical fragmentation patterns were

Figure 11. Schematic of control program for the sensing network using Mini 10 mass spectrometers as sensors.

obtained for these characteristic ions of DMMP, n-butylbezene and methyl salicylate (Figure 9b-d). The limit of detection and the sensitivity of analysis using the Mini 10 were characterized using aqueous solutions of naphthalene. Aqueous solutions with naphthalene concentrations from 10 ppb to 1 ppm were prepared. They were pumped through the membrane tubing at flow rates of 20 mL/min at room temperature. The mass spectra were recorded and averaged for 5 s for each sample. After the introduction of each sample solution, a blank solution was pumped through the membrane tubing for 5 min to clean the membrane. The intensity of the total ion current plotted as a function of the solution concentration is shown in Figure 10. An LOD of 50 ppb was obtained for this experiment, and a linear response with an R2 of 0.999 was observed in the concentration range from 50 ppb to 1 ppm. A sensing network scheme was also developed on the basis of using the Mini 10 mass spectrometers as sensors. This capability was demonstrated in a rudimentary fashion with two Mini 10 instruments and one remote computer, communicating through a wireless network. The data collected by each Mini 10 were first processed by the program running on the DSP of the control/DAQ boards in real time. The background-subtracted spectra were averaged. A peak list was then generated to decrease the load for data transfer and saved in a file on the handheld computer, which was updated after a given number of scans. A program was developed and run on the remote computer to access this file from each Mini 10 every 3 s and to transfer the peak lists to the remote computer. This program had a user interface to allow the operator to select target compounds to be monitored by each Mini 10 while the locations of the instruments were shown on a map. A database containing the standard peak lists for all the target compounds was built. The peak lists collected from each Mini 10 were compared against those stored in the database in

real time to determine whether targeted compounds were being detected. An algorithm using a spectral contrast angle40 technique was implemented for spectral comparison. Audio alarms were set off when matches were found, with the information including the names of the compounds and the locations where they are detected (Figure 11). CONCLUSION A handheld shoebox-sized rectilinear ion trap mass spectrometer has been developed. This instrument is battery-powered and can perform chemical analysis on air or water samples while being hand-carried. Its performance as a mass spectrometer is compromised, although not severely, by the significant decrease in the size of the instrument compared to benchtop mass spectrometers. Unit mass resolution at m/z 100 and a mass range over m/z 500 have been achieved. Multiple-stage tandem mass spectrometry was demonstrated to provide additional confirmation during in situ analysis. The Mini 10 mass spectrometer has wired and wireless networking capabilities and can serve as a node in a sensing network. A program has also been developed to control multiple Mini 10 instruments and acquire data through the network and to process and analyze the data in real time to support an alarm system. In previous work, we have used a different approach to building a handheld mass spectrometer, choosing to build a micromachined mass analyzer using standard silicon fabrication technologies and compensating for the small size (1, 2, and 5-micrometer internal radius) of the individual cylindrical ion traps by using a massively parallel array.15 The same approach, using larger analyzers, has been used by Pau et al.16 In contrast to this revolutionary approach, the evolutionary approach selected here (40) Wan, K. X.; Vidavsky, I.; Gross, M. L. J. Am. Soc. Mass Spectrom. 2002, 13, 85-88.

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is (so far) much more successful, at least in our hands. The total system size is about the same but the performance of the singlemillimeter-scale RIT analyzer system is much superior to that of any micrometer-sized individual analyzer system yet described.

Pevtsov for help in software development, and support from the Homeland Security Advanced Research Projects Agency (HSHQPA-05-9-0033).

ACKNOWLEDGMENT The authors acknowledge Brent D. Rardin and Brent A. Knecht for help in instrument development, Irina Fedulova and Sergey

Received for review June 23, 2006. Accepted July 25, 2006.

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