Miniaturized High-Resolution Time-of-Flight Mass Spectrometer

Sep 22, 2010 - The multiturn TOF analyzer is capable of high mass resolution because of its infinite flight path utilizing perfect space and time focu...
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Anal. Chem. 2010, 82, 8456–8463

Miniaturized High-Resolution Time-of-Flight Mass Spectrometer MULTUM-S II with an Infinite Flight Path Shuichi Shimma,†,* Hirofumi Nagao,‡ Jun Aoki,§ Keiji Takahashi,| Shinichi Miki,| and Michisato Toyoda†,‡,§,| Center for Advanced Science and Innovation, Venture Business Laboratory, Osaka University, Suita, Osaka 565-0871, Japan, Renovation Center of Instruments for Science Education and Technology, Osaka University, Toyonaka, Osaka 560-0043, Japan, Graduate School of Science, Osaka University, Osaka 560-0043, Japan, and MSI TOKYO Inc., Chofu-shi, Tokyo 182-0036, Japan A new miniature multiturn time-of-flight (TOF) analyzer “MULTUM-S II” has been designed and constructed. This instrument consists of an electron ionization source, the multiturn TOF ion optics, a detector, vacuum system, and electronic circuits. The multiturn TOF analyzer consists of four electrostatic toroidal sectors and two additional electric toroidal sectors for the purpose of ion injection/ ejection. The size and weight of the system is less than 50 cm × 57 cm × 30 cm and 35 kg (including vacuum pumps and electronic circuits). The multiturn TOF analyzer is capable of high mass resolution because of its infinite flight path utilizing perfect space and time focused closed flight orbit. To evaluate the resolution in MULTUM-S II, separation of pyridine (12C5H5N) and the isotopic component of benzene (13C12C5H6) was performed at a mass resolution of about 20 000. Another performance characteristic of the MULTUM-S II was demonstrated by the separation of the greenhouse gas doublet CO2 and N2O (∆m ) 0.0113 Da). While the mass difference is a mere 0.01 Da, the instrument could easily separate the two peaks at a calculated mass resolution of 31 600. The MULTUM-S II offers high mass resolution mass spectrometry in a miniaturized/portable enclosure. In recent years, design and development of novel miniature mass spectrometers has been at the forefront of research in mass spectrometry. These instruments have widespread applications, for example, detection and identification of chemical and biological hazards for homeland security,1 environmental monitoring,2,3 food * Corresponding author. E-mail: [email protected]. Fax: +81-6-6850-5589. Tel: +81-6-6850-5590. † Center for Advanced Science and Innovation, Venture Business Laboratory, Osaka University. ‡ Renovation Center of Instruments for Science Education and Technology, Osaka University. § Graduate School of Science, Osaka University. | MSI TOKYO Inc. (1) Contreras, J. A.; Murray, J. A.; Tolley, S. E.; Oliphant, J. L.; Tolley, H. D.; Lammert, S. A.; Lee, E. D.; Later, D. W.; Lee, M. L. J. Am. Soc. Mass Spectrom. 2008, 19, 1425–1434. (2) Eckenrode, B. A. J. Am. Soc. Mass Spectrom. 2001, 12, 683–693. (3) Schluter, M.; Gentz, T. J. Am. Soc. Mass Spectrom. 2008, 19, 1395–1402.

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safety,4 and so forth. The miniature mass spectrometers are capable of in situ analysis of these applications. Their lightweight and small size are ideal for field use. Methods to reduce weight and size were attempted by various research groups. According to the reported papers on miniaturized instruments, a wide variety of instrument types including ion traps, quadrupole mass filters (QMF),5 magnetic sector mass spectrometers,6-8 and time-of-flight (TOF) mass spectrometers9-12 were described. It is considered that ion traps or QMF are more favorable than other instruments for miniaturization. In fact, almost all commercialized miniature instruments have adopted ion traps and QMF.13-15 In addition, it can be noted that there is a large variety of ion traps: (1) three dimensional hyperbolic ion traps, (2) rectilinear ion traps,16-19 (3) toroidal ion traps,20 (4) planar electrode (Halo) ion traps, 21-23 and (5) cylindrical ion traps.24-31 (4) Garcia-Reyes, J. F.; Jackson, A. U.; Molina-Diaz, A.; Cooks, R. G. Anal. Chem. 2009, 81, 820–829. (5) Geear, M.; Syms, R. R. A.; Wright, S.; Holmes, A. S. J. Microelectromech. Syst. 2005, 14, 1156–1166. (6) Diaz, J. A.; Giese, C. F.; Gentry, W. R. Field Anal. Chem. Technol. 2001, 5, 156–167. (7) Diaz, J. A.; Giese, C. F.; Gentry, W. R. J. Am. Soc. Mass Spectrom. 2001, 12, 619–632. (8) Sinha, M. P.; Gutnikov, G. Anal. Chem. 1991, 63, 2012–2016. (9) Berkout, V. D.; Cotter, R. J.; Segers, D. P. J. Am. Soc. Mass Spectrom. 2001, 12, 641–647. (10) Cornish, T. J.; Cotter, R. J. Anal. Chem. 1997, 69, 4615–4618. (11) Cotter, R. J.; Fancher, C.; Cornish, T. J. J. Mass Spectrom. 1999, 34, 1368– 1372. (12) Ecelberger, S. A.; Cornish, T. J.; Collins, B. F.; Lewis, D. L.; Bryden, W. A. Johns Hopkins APL Tech. Dig. 2004, 25, 14–19. (13) Belanger, J. M. R.; Pare, J. R. J.; Turpin, R.; Schaefer, J.; Chuang, C. W. J. Hazard. Mater. 2007, 145, 336–338. (14) Holkeboer, D. H.; Karandy, T. L.; Currier, F. C.; Frees, L. C.; Ellefson, R. E. J. Vac. Sci. Technol. 1998, 16, 1157–1162. (15) Ferran, R. J.; Boumsellek, S. J. Vac. Sci. Technol. 1996, 14, 1258–1265. (16) Ouyang, Z.; Noll, R. J.; Cooks, R. G. Anal. Chem. 2009, 81, 2421–2425. (17) Liang, G.; Sugiarto, A.; Harper, J. D.; Cooks, R. G.; Zheng, O. Y. Anal. Chem. 2008, 80, 7198–7205. (18) Fico, M.; Maas, J. D.; Smith, S. A.; Costa, A. B.; Ouyang, Z.; Chappell, W. J.; Cooks, R. G. Analyst 2009, 134, 1338–1347. (19) Li, X. X.; Jiang, G. Y.; Luo, C.; Xu, F. X.; Wang, Y. Y.; Ding, L.; Ding, C. F. Anal. Chem. 2009, 81, 4840–4846. (20) Lammert, S. A.; Rockwood, A. A.; Wang, M.; Lee, M. L.; Lee, E. D.; Tolley, S. E.; Oliphant, J. R.; Jones, J. L.; Waite, R. W. J. Am. Soc. Mass Spectrom. 2006, 17, 916–922. (21) Austin, D. E.; Wang, M.; Tolley, S. E.; Maas, J. D.; Hawkins, A. R.; Rockwood, A. L.; Tolley, H. D.; Lee, E. D.; Lee, M. L. Anal. Chem. 2007, 79, 2927–2932. 10.1021/ac1010348  2010 American Chemical Society Published on Web 09/22/2010

The main reasons for choosing ion traps for portable instruments are that ion traps provide a relaxed vacuum condition, low power consumption, inherent tandem mass spectrometry capability, and easily miniaturized geometry for weight saving. A handheld ion trap mass spectrometer system “Mini 11” was reported by Ouyang et al.,16,32 whose total weight with batteries is 5.0 kg, power consumption is 35 W, and dimensions are 22 cm × 12 cm × 18 cm. In addition to the miniaturized characteristics, the instrument has been coupled with many kinds of ambient ionization sources, for example, desorption electrospray ionization, electrospray ionization, and atmospheric pressure chemical ionization.33 Miniaturized instruments, especially ion traps or QMF described above, appear to have usable performance in field use. However, sensitivity and mass resolution in these physically smaller devices are lower compared to laboratory instruments. To overcome the loss of sensitivity due to lower transmission of ions into and out of the traps, ion traps using array geometry were proposed. Recently, ion trap arrays have been developed with fabrication in polymer,18 in silicon,5,30 and in printed circuit board.19 By introducing these array technologies, sufficient ion current was obtained even if micrometer scale traps were produced via the microfabrication technique. However, high mass resolution cannot in principle be obtained in QMF or ion traps. The typical mass resolution in the miniaturized instruments is less than a few hundred, mass-to-charge ratio (m/z) 500, and the maximum mass resolution was reported as 1000 at m/z 390.34 We predicted that miniature instruments would be more commonly utilized in the future. In particular, their high mass resolution is important when (1) it is difficult to perform sufficient sample preparations and (2) quick and simple sample preparations are required in the field. Consequently, the demands on higher mass resolution will be increased compared to the instruments as described above. From the point of mass resolution, TOF mass analyzers, magnetic sector mass analyzers, FTICR mass spectrometers,35 and Orbitraps36 are (22) Austin, D. E.; Peng, Y.; Hansen, B. J.; Miller, I. W.; Rockwood, A. L.; Hawkins, A. R.; Tolley, S. E. J. Am. Soc. Mass Spectrom. 2008, 19, 1435– 1441. (23) Yang, M.; Kim, T. Y.; Hwang, H. C.; Yi, S. K.; Kim, D. H. J. Am. Soc. Mass Spectrom. 2008, 19, 1442–1448. (24) Misharin, A. S.; Laughlin, B. C.; Vilkov, A.; Takats, Z.; Zheng, O. Y.; Cooks, R. G. Anal. Chem. 2005, 77, 459–470. (25) Tabert, A. M.; Misharin, A. S.; Cooks, R. G. Analyst 2004, 129, 323–330. (26) Tabert, A. M.; Griep-Raming, J.; Guymon, A. J.; Cooks, R. G. Anal. Chem. 2003, 75, 5656–5664. (27) Badman, E. R.; Cooks, R. G. Anal. Chem. 2000, 72, 5079–5086. (28) Van Amerom, F. H. W.; Chaudhary, A.; Cardenas, M.; Bumgarner, J.; Short, R. T. Chem. Eng. Commun. 2008, 195, 98–114. (29) Wells, J. M.; Roth, M. J.; Keil, A. D.; Grossenbacher, J. W.; Justes, D. R.; Patterson, G. E.; Barket, D. J. J. Am. Soc. Mass Spectrom. 2008, 19, 1419– 1424. (30) Chaudhary, A.; van Amerom, F. H. W.; Short, R. T. J. Microelectromech. Syst. 2009, 18, 442–448. (31) Cruz, D.; Chang, J. P.; Fico, M.; Guymon, A. J.; Austin, D. E.; Blain, M. G. Rev. Sci. Instrum. 2007, 78, 015107. (32) Gao, L.; Li, G. T.; Nie, Z. X.; Duncan, J.; Ouyang, Z.; Cooks, R. G. Int. J. Mass Spectrom. 2009, 283, 30–34. (33) Cooks, R. G.; Ouyang, Z.; Noll, R.; Manicke, N.; Costa, A.; Wu, C.; Xia, Y.; Dill, A.; Ifa, D. Biopolymers 2009, 92, 297–297. (34) Ouyang, Z.; Cooks, R. G. Ann. Rev. Anal. Chem. 2009, 2, 187–214. (35) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. Rev. 1998, 17, 1–35. (36) Hu, Q. Z.; Noll, R. J.; Li, H. Y.; Makarov, A.; Hardman, M.; Cooks, R. G. J. Mass Spectrom. 2005, 40, 430–443.

assumed to be feasible alternatives to ion traps. However, if high mass resolution is to be achieved in magnetic sector mass analyzers, the instrument will become large and heavy because of the required increase in size of the magnets. However, in TOF mass spectrometers, the instruments have simple structure and are lightweight, which are more favorable for reduction in size. Mass resolution of TOF mass spectrometers is directly proportional to the size of the instrument, as it is in magnetic sector mass spectrometers. Therefore, high mass resolution cannot be available by simply shortening the flight length. Reported portable TOF analyzers have an extremely short flight tube of a few centimeters.11 The reported mass resolution in miniaturized TOF mass spectrometers is typically about 200 at m/z 1000,10 1600 at m/z 28, and 5500 at m/z 265, using a somewhat longer TOF tube (0.4 m).9 It is difficult to achieve very high resolution by simply extending the length of the flight tube. One solution to overcome this fundamental problem is to cause ions to fly in the same space many times. On the basis of this idea, multipass reflectron timeof-flight mass spectrometers37-41 or multiturn systems were proposed.42 In particular, multiturn ion optical geometries using electric sectors have a closed orbit. Ions are stored in the orbit and traverse the same orbit many times. Accordingly, the flight path length can be extended infinitely. In our laboratory, the multiturn time-of-flight mass spectrometer “MULTUM Linear plus” was designed and developed for the purpose of the COSAC project of the ROSETTA space mission in the late 1990s.43,44 On the basis of this instrument, other improved multiturn systems were developed (1) “MULTUM II”45,46 (multiturn TOF instrument with the simplest design), (2) “MULTUM-TOF/TOF”47 (tandem TOF instrument consisting of MULTUM II and quadratic field ion mirror), (3) “MULTUM-IMG”.48,49 (imaging instrument with stigmatic mode), and (4) “MULTUM-S”.50 (lab-built miniaturized multiturn TOF instrument). In this manuscript, the overview and performance evaluation results are described for the newly developed miniaturized multiturn TOF system “MULTUM-S II”, which has higher manufacturing precision than the “MULTUMS”. (37) Hanson, C. D. Anal. Chem. 2000, 72, 448–453. (38) Casares, A.; Kholomeev, A.; Wollnik, H. Int. J. Mass Spectrom. 2001, 206, 267–273. (39) Wollnik, H.; Casares, A. Int. J. Mass Spectrom. 2003, 227, 217–222. (40) Verentchikov, A. N.; Yavor, M. I.; Hasin, Y. I.; Gavrik, M. A. Tech. Phys. 2005, 50, 82–86. (41) Yavor, M. I.; Verentchikov, A. N.; Hasin, J.; Kozlov, B.; Gavrik, M.; Trufanov, A. Phys. Procedia 2008, 1, 391–400. (42) Poschenrieder, W. P. Int. J. Mass Spectrom. Ion Phys. 1972, 9, 357–373. (43) Matsuo, T.; Ishihara, M.; Toyoda, M.; Ito, H.; Yamaguchi, S.; Roll, R.; Rosenbauer, H. Adv. Space Res. 1999, 23, 341–348. (44) Toyoda, M.; Ishihara, M.; Yamaguchi, S.; Ito, H.; Matsuo, T.; Roll, R.; Rosenbauer, H. J. Mass Spectrom. 2000, 35, 163–167. (45) Toyoda, M.; Okumura, D.; Ishihara, M.; Katakuse, I. J. Mass Spectrom. 2003, 38, 1125–1142. (46) Okumura, D.; Toyoda, M.; Ishihara, M.; Katakuse, I. J.Mass Spectrom. 2004, 39, 86–90. (47) Toyoda, M.; Giannakopulos, A. E.; Colburn, A. W.; Derrick, P. J. Rev. Sci. Instrum. 2007, 78, 074101. (48) Hazama, H.; Aoki, J.; Nagao, H.; Suzuki, R.; Tashima, T.; Fujii, K.; Masuda, K.; Awazu, K.; Toyoda, M.; Naito, Y. Appl. Surf. Sci. 2008, 255, 1257– 1263. (49) Hazama, H.; Nagao, H.; Suzuki, R.; Toyoda, M.; Masuda, K.; Naito, Y.; Awazu, K. Rapid Commun. Mass Spectrom. 2008, 22, 1461–1466. (50) Ichihara, T.; Uchida, S.; Ishihara, M.; Katakuse, I.; Toyoda, M. J. Mass Spectrom. Soc. Jpn. 2007, 55, 363–368.

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Figure 1. Photograph of MULTUM-S II system.

INSTRUMENTATION Overview of the Miniaturized Multiturn TOF Mass Spectrometer “MULTUM-S II”. Photographs of the developed “MULTUM-S II” system are shown in Figure 1. The developed system consists of the following: ion source multiturn mass analyzer, vacuum system, and high voltage circuit unit (Figure 1A). The complete mass spectrometer weighs 35 kg (including electric circuits and vacuum pumps). The total size of the instrument is 50.4 cm × 58.4 cm × 27.3 cm (Figure 1B). The equipped ionization source is a two-stage acceleration ion source of electron ionization (EI) type introduced by W. C. Wiley and I. H. McLaren.51 The accelerated ions are focused using the Einzel lens. After focusing, the ions are injected into the multiturn TOF mass spectrometer. The size of the multiturn TOF mass spectrometer is less than 20 cm × 20 cm. After separation in the multiturn TOF mass spectrometer, the ion signal is detected using a secondary electron multiplier (14880, ETP Electron Multipliers, Ermington, Australia). Ion Optics of Miniaturized Multiturn TOF Mass Spectrometer. In multiturn TOF mass spectrometer geometries, it is essential that ion packets focus in space and time during the closed orbit flight. If the ion beam is divergent, both mass resolution and ion transmission decrease as the number of cycles increase. Therefore, our multiturn system is imposed on “perfect space and time focusing” conditions. The detailed discussion on this theory was described elsewhere.45,52 A brief descriptions of “perfect space and time focusing” is described in the Supporting Information, Figure S1. Ion Optics of Ion Injection/Ejection Components. It is an important issue on how ions are introduced with high efficiency into the mass analyzer from the ion source. Additionally, introduced ions need to travel with stability in the closed orbit to obtain high mass resolution in multiturn TOF instruments. In the previous MULTUM system (MULTUM Linear plus and MULTUM II), the ion beam passes through small holes in the outer electrodes of two of the electric sectors. When ions were injected or ejected, the voltages applied to the sector electrodes were switched. To prevent the reduction of resolution due to instability of the power supply for switching, we offered stability of better than 50 ppm.47 The power supply was complicated and became large to satisfy this requirement. In order to overcome the problem of ion injection/ejection and stable traveling, MULTUM-S II has two additional sectors. Since these sectors specialized in ion injection/ejection, it is unnecessary (51) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150–1157. (52) Ishihara, M.; Toyoda, M.; Matsuo, T. Int. J. Mass Spectrom. 2000, 197, 179–189.

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Figure 2. Photograph of inside of MULTUM-S II.

to apply pulsed voltages to the sectors in the closed orbit. Hence, static voltage is simply applied to the sector electrodes, through which ions pass during cycles. In this concept, the stability of applied voltages to the electrodes in the multiturn part is maintained without difficulty. Furthermore, the stability is not essential for ion injection/ejection sectors, because ions only pass through each switching sector after ionization and before detection. For all of these reasons, the electrical circuits for the multiturn TOF mass spectrometer could be miniaturized. In our previous work, a few ion injection/ejection electrode geometries were proposed. The example of the proposed ion injection/ejection method is shown in Figure S2, Supporting Information. With respect to the alignment space for the ion source, the detector, and other components, the S-type method was chosen for our instrument. Ion trajectories for the ion injection/ejection assemblies were simulated using software. This data is included in the Supporting Information. Design and Manufacture of MULTUM-S II System. Two small turbomolecular pumps (TF70G, Osaka Vacuum Ltd., Osaka, Japan), which have a pumping speed of 70 L/min and robustness for vibration in field use, were selected. Each manifold of the ionization source and the mass analyzer is evacuated by the turbomolecular pumps. Each backing pressure is provided with single-stage diaphragm pumps (N84.3ANDC, KNF Neuberger Inc., Trenton, NJ). The background pressure in the analyzer part of 2 × 10-5 Pa can be attained. All pumps are driven by a 24 V dc power supply. Differential pumping is achieved by an orifice plate, which is placed between each manifold of ionization source and mass analyzer. The opening in the orifice plate has a diameter of 4.5 mm. In addition, the isolation valve is assembled in front of the orifice plate. A picture of aligned sector electrodes on a base plate is shown in Figure 2. The size of the sector electrode, which is based on ion optics calculations, is shown in Figure S1 and Figure S3, Supporting Information. The height of all electrodes is 26 mm. These electrodes are installed in a 20 × 20 cm2 vacuum chamber which is made of aluminum alloy. The analyzer consists of a symmetrical array of four static units and two units for ion injection and ejection. Each sector electrode consists of two

Table 1. Basic Condition of the EI Ion Source

Figure 3. (A) Timing chart of the experimental events. Definition of measurement mode: (B) half cycle mode (low resolution) and (C) multiturn mode (high resolution). Number of cycles are controlled by changing timing of the ejection sector switch.

cylindrical electrodes (inner/outer) and MATSUDA plates.53 This combination of electrodes produces a toroidal electric sector field. To reduce thermal expansion coefficient, the sector electrode and base plate were made of titanium alloy. Operation. A block diagram of the timing control system is shown in Figure 3A. To evaluate mass resolution and variation of peak intensity, a digital pattern generator (Model 555 pulse/delay generator, Berkeley Nucleonics, CA, USA) supplied the timing signals to the ion source, the injection/ejection electric sector, and the digital oscilloscope (LC564DL, LeCroy Japan, Osaka, Japan). In another experiment, a high speed digitizer (D-Flex, MSI TOKYO Inc., Tokyo, Japan) and a self-developed data acquisition software package were utilized. Since our target mass resolution was a few tens of thousands (i.e., the maximum TOF < 1 ms), the repetition rate of data acquisition was set to 0.1-1 kHz. Here, the ions were extracted from the EI ion source by applying the pulsed voltage to the push plate. The voltage applied to the injection sector was turned on when ions were injected into the multiturn assembly. After the ions had been injected into the MULTUM part, the voltage applied to the injection sector was turned off. On the other hand, the voltage to the ejection electrode was initially switched off; thus, the ion packets flew in the closed orbit. After a preset number of cycles, the voltage of ejection sector was switched on, and the ion packets were ejected from the closed orbit. The ions were then detected. Operation modes were defined as (1) the first mode, “halfcycle mode” (Figure 3B), where the ions passed through the MULTUM part without prescribing a closed trajectory and (2) the second mode, “multi-turn mode” (Figure 3C), where the ions flew in the same orbit of the MULTUM part many times. Under experimental conditions discussed in this paper, generated ions in the EI source remain within the electron flow region of the ion source. This is due to the fact the high voltage (HV) switch “for both the injection and ejection sectors” is cycled off during a multiturn cycle; consequently, a large number of ions may survive during this period. However, to reduce the loss of ions, an enhanced repetition rate of data acquisition (DAQ) and HV switching is required. Since our target mass resolution was a few tens of thousands (total TOF < 1 ms), the repetition rate was (53) Matsuda, H.; Fujita, Y. Int. J. Mass Spectom. Ion Process. 1975, 16, 395– 404.

electron energy

80 eV

emission currents pulse voltage extraction pulse length total acceleration voltage background pressure (w/o gas) pressure gases (w/ gas)

100 µA 1.5 kV 450 ns 5.5 kV 1.5 × 10-4 Pa 2.2 × 10-4 Pa

accordingly set to 1 kHz. We consider that the efficiency of our system under this condition is equivalent to an orthogonal acceleration method used in current commercial TOF instruments. Experimental Summary. To evaluate variations of mass resolution and ion transmission in a MULTUM-S II system, gaseous samples were introduced via a needle valve which connected directly to the ionization chamber. For the evaluation of dynamic range and limit of detection (LOD), a commercial gas chromatograph (Agilent 6890N GC) was connected to the EI source via a heated GC interface embedded with ceramic heaters. In this experiment, a capillary column of BPX5 (12 m × 0.22 mm i.d.; film thickness 0.25 µm; ETP Analytical Science Pty Ltd., Australia) was included in the GC system. For the evaluation of resolution, two doublets were measured as follows: (1) pyridine/benzene (12C5H5N: 79.0422)/(13C12C5H6: 79.0503) and (2) carbon dioxide/nitrous oxide (CO2: 43.9898)/ (N2O: 44.0011). Liquid samples of pyridine and benzene were introduced into the heated sample reservoir. The evaporated samples were then introduced into the EI ion source. The mixture was prepared at the ratio of 1:10, and 1 µL of mixture was injected into the reservoir with a microsyringe. The mixture of ultrapure CO2 and N2O (49.4%:50.6%) was purchased from DAIHO SANGYO Inc. (Minato-ku, Tokyo, Japan). Perfluorotributylamine (PFTBA), “common calibration compound for mass spec”, was purchased from Sigma-Aldrich (St. Louis, MO) and used as a calibration standard. Octafluoronaphthalene (OFN), “fluorinated compound used as a mass spec standard”, and methyl-stearate, “commonly used mass spec standard used for testing sensitivity in EI mode”, were purchased from Tokyo chemical industry Co. Ltd. (Tokyo, Japan) and used as an evaluation of LOD and signal linearity. Detailed EI ion source conditions are summarized in Table 1. RESULTS AND DISCUSSION Mass Resolution in the Half Cycle Mode. A TOF spectrum of PFTBA obtained in the half cycle mode is shown in Figure 4. The spectrum was an accumulation of 500 individual spectra. Enlarged spectra around 4.8 and 11.8 µs correspond to m/z 69 (CF3) and m/z 414 (C8F16N), respectively. The obtained peak width (full width at half-maximum, fwhm) of m/z 69 and m/z 414 was 8.8 and 13.5 ns; therefore, calculated mass resolutions were 280 and 440, respectively. This result confirmed that unit mass resolution was possible during half cycle mode (flight path length: 44 cm). Mass Resolution and Ion Transmittance in the Multiturn Mode. Evaluations of mass resolution, peak width, and ion transmission were performed using air introduced via the needle valve into the EI ion source. By controlling the timing of the high Analytical Chemistry, Vol. 82, No. 20, October 15, 2010

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Figure 4. Time-of-flight mass spectrum of perfluorotributyl amine (PFTBA) in the half cycle mode. Enlarged spectra correspond to peaks m/z 69 and m/z 414, and unit mass resolution was verified in this mode.

voltage switch for the injection sector, only nitrogen ions (N2+: m/z 28) were introduced into the multiturn part. Variations of peak width and resolution from a half cycle to about 100 cycles were shown in Figure 5A. After a few cycles, ions having large energy distribution hit against the electrodes; the energy distribution of ions was limited to less than 10%. After two cycles, peak width became a constant value (approximately 6.5 ns) and mass resolution increased linearly with the number of cycles. In this experiment, mass resolution greater than 20 000 was achieved after 100 cycles. Given that ion packets had initially large space, angular distributions, and energy distributions in the initial 10 cycles, the ions were lost by hitting against the electrodes. Then, signal intensity was decreased by half after a few cycles. The ion transmission dependence on the number of cycles after 10 cycles is shown in Figure 5B. The vertical axis in Figure 5B represents the ion intensity (peak area) normalized by the intensity of 10

cycles. The decrease was caused by the small-angle scattering with neutral particles which were attributed to residual gas. The intensity around 500 µs (142 cycles) was decreased to 0.05% of initial intensity. This result indicated that approximately 98% ions remained in each cycle. This transmittance was comparable with previous reports on MULTUM.45 We have estimated the number of ions in an ion packet exiting the source region at approximately 300 (data not included). We predicted that keeping a relatively small number of ions per packet would keep space charge effects to a minimum. Furthermore, it was known that if space charge effects existed, the peak will have a broader fwhm and transmittance will decrease, especially in the multiturn mode. As shown in Figure 5, such trends were not observed. In our previous work, the ion intensity in the first prototype of MULTUM-S declined dramatically.50 The previous instrument was manufactured using a wide-use lathe and milling machine, resulting in a lack of machining precision and assembly accuracy (>50 µm). We attempted to recover the ion transmission by adjusting the applied voltages. Nonetheless, very few ions could be observed after 10 cycles due to low intensity. According to our previous work,47 incorporating tighter tolerances during machining and assembly ( 30 000, which is unheard of for an instrument of this size. Dynamic range of 3 orders of magnitude is good, but with LOD in linear mode at 1 and 100 ppb at 10 cycles, we understand it is essential for future field applications that this is improved upon. It was shown that high accurate mass is also possible from this instrument, with mass accuracies down to 2.8 ppm. Detection of the CO2 and N2O doublet separately has proven one real time field application that this instrument can be utilized for, but we feel there will be many more as its operation is improved in the future. ACKNOWLEDGMENT The authors thank Mr. Toshio Ichihara of Osaka University for his technical support. The authors also thank Prof. Yasuyuki

Hashidoko and Prof. Ryusuke Hatano in Hokkaido University for their fruitful discussion of environmental application. This work was supported by a Supporting Program for Creating University Ventures, from the Japan Science and Technology Agency (JST). One of the authors (M.T.) was supported by Grant in Aid for Young Scientists (A) (21685010) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. One of the authors (S. S.) was partially supported by a Grant in Aid for Young Scientists (start-up) (08106916) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review April 20, 2010. Accepted September 3, 2010. AC1010348

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