A Simple Multi-Turn Time of Flight Mass Spectrometer MULTUM II

Transcription

1 J. Mass Spectrom. Soc. Jpn. Vol. 51, No. 2, 23 REGULAR PAPER A Simple Multi-Turn Time of Flight Mass Spectrometer MULTUM II Daisuke O@JBJG6, a) Michisato TDND96, a) Morio IH=>=6G6, a) and Itsuo K6I6@JH: a) (Received September 6, 22; Accepted November 13, 22) Anew multi-turn time-of-flight mass spectrometer MULTUM II was constructed. The ion optical system was simplified comparing with MULTUM Linear plus. The multi-turn part of the new instruments consisted of only four toroidal electric sector fields. The mean radius of cylindrical electric sector was 5 mm and the deflection angle was The total flight pass length of one cycle was 1.38 m. In a preliminary experiment, the mass spectra of Xe ion were measured and enhancement in mass resolution with increasing number of cycles was obserbed. The mass resolution 51 at m/z 136 was achieved after 1 cycles. Introduction The theory of sector type of time-of-flight (TOF) mass spectrometers was proposed in 197s by Poschenrieder. Triple focusing (directional focusing and 1), 2) vanishing energy-dependent dispersion in time-offlight and imaging) was satisfied by combining linear drift space with electric sector fields. In 1985, Sakurai et al. designed and constructed a multiple-focusing TOF mass spectrometer that satisfied triple isochronous focusing and triple space focusing simultaneously with four toroidal electric sector fields. 3) Moreover, Poschenrieder also proposed a multi-turn geometry resembling a figure eight. Sakurai et al. designed and constructed a large multi-turn TOF mass spectrometer OVAL. 4) It consisted of six electrostatic analyzers. The length of the circular orbit was 7.4 m. A compact multi-turn TOF mass spectrometer MULTUM Linear plus was developed in our laboratory. 5), 6) It consisted of four electric sectors and 28 Q- lenses. The length of one cycle was m. MULTUM Linear plus was constructed as a laboratory model of the COSAC project of the ROSETTA mission. 7) However, it was not simple to carry on a spacecraft. In order to reduce weight of the instrument and simplify the instrument, we constructed MULTUM II consisting of only four toroidal electric sector fields. In this paper, we report technical data for construction and preliminary experimental data. Ion Optics of MULTUM II Detailed discussions about ion optics of a multi-turn 8), 9) TOF mass spectrometer were given elsewhere. Here we simply explain a feature of the ion optical system of MULTUM II. The perfect focusing 9) condition is very important to increase the mass resolution for multi-turn systems. If it isn t satisfied, the ion beam will diverge. Consequently the mass resolution and the ion transmission a) Department of Physics, Graduate School of Science, Osaka University (116 Machikaneyama, Toyonaka, Osaka 56 43, Japan) will decrease with increasing the number of cycles. To avoid this problem, absolute value of the position and angle at the final position should be the same as at the initial position in both the horizontal and vertical directions. This relationship is represented in a first order approximation by using transfer matrix method 1) as follows: x f 1 a f y f b f g f d f l f lg 1 x i a i y i b i g i d i 1 l i where x, y, a, andb are the position and angle dispersion and g, d, l are the mass, energy and flight pass length deviation, respectively. The subscripts i and f indicate conditions of before and after a cycle, respectively. The ion optical system of MULTUM Linear plus satisfies Eq.(1) every.5 cycle. In order to simplify the ion optical system, we researched perfect focusing ion optical systems consisting of toroidal electric fields instead of Q-lenses and cylindrical electric fields. As a result we found one such system that comprised only four toroidal electric sector fields (the c-value 11) of a toroidal electric sector field is.33). Ion trajectories of MULTUM II simulated by TRIO-DRAW 12) are shown in Fig.1. The ion trajectories of Figs. 1(a), (b), and (c) show a top, a vertical and a horizontal view, respectively. Design and Operation of MULTUM II Aschematic drawing and photograph of MULTUM II are shown in Figs. 2 and 3. In the ion source housing, there was a two-stage acceleration EI type of ion source. 13) Gaseous samples were introduced into the ion source housing and ionized. Ions ejected from the ion source housing were focused by a Q-lens doublet located in front of the multi-turn housing to improve ion transmission in linear-part. In the multi-turn housing, there were four toroidal electric sector fields. Because of small c-value.33, it (1) 349

2 D. Okumura et al. Fig. 2. Schematic drawing of MLTUM II. Fig. 1. Ion trajectories of MULTUM II simulated by TRIO-DRAW. (a) Top view, (b) x-direction, and (c) y-direction starting from s point of (a). is di$cult to make toroidal electric sector electrodes. Then four toroidal electric sector fields were produced by cylindrical electric sectors and MATSUDA plates. 14) The mean radius of cylindrical electric sector was 5 mm, the deflection angle was 157.1, the cross-sectional view of the electrodes is shown in Fig. 4. Ions were injected and ejected through a hole of outer sector electrodes. The total pass length of one cycle was 1.38 m. All elements were hanged on the top plate of the multi-turn housing. While the ions injected into the multi-turn parts, the voltage of sector IV was o#, andit was turned on before the ions returned. After the ions had undergone the desired cycles, the voltage of sector Iwas turned o# to eject the ions from the multi-turn part. Because long flight pass length can be obtained by a multi-turn geometry, ion-molecule collisions should be prevented to expand the mean free path. Therefore high vacuum is required to obtain good ion transmission in an analyzer part. The vacuum pressure in the mass spectrometer increased when samples were introduced in the ion source. Therefore a di#erential pumping system was applied. A turbo molecular pump (Turbo-25, Varian S.p.A, Italy) was attached to the ion source housing. A turbo molecular pump (PT5, Mitsubishi Heavy Industries., Ltd., Japan) was attached to the multi-turn housing. In the detector housing, there were two detectors and an ion mirror. The ejected ions flight toward a twostage micro channel plate of 14.5 mm diameter (F , Hamamatsu Photonics K.K., Shizuoka, Japan) attached at the position of detector 1 in Fig. 2. In the case of using the ion mirror, a two-stage micro channel plate of 27 mm diameter with center hole (F4294-9, Hamamatsu Photonics K.K., Shizuoka, Japan) attached at detector 2 in Fig. 2 was used. The ion mirror was used to satisfy energy focusing in linear part. It consisted of 11-ring electrodes and 85 transmission meshes are attached to the first, fourth and last electrodes. The output signals were accumulated with adigital oscilloscope (LC564DL, LeCroy Japan, Osaka, Japan). Because the light ions flight faster than the heavy ions. After certain cycles the light ions pass the heavy ions. We used an ion gate located in front of multi-turn part to over come this problem. The ion gate consisted of two parallel electrodes. One electrode was grounded and the voltage 35 V was supplied to the other electrode. In this case ions were deflected and could not enter the multi-turn part. When we injected ions into 35

3 A Simple Multi-Turn TOF MS!MULTUM II Fig. 3. Photograph of MULTUM II. Fig. 4. Cross-sectional view of cylindrical electrodes and MATSUDA plates. the multi-turn part, the other electrode was grounded. In this study, we injected only single charged xenon ions into the multi-turn part and exclude undesired ions such as multiple charged xenon ions and ions of residual gases. A block diagram of the pulse control system for measurement of xenon at 4 cycles is shown in Fig. 5. A digital pattern generator (MODEL 555 Pulse Generator, BNC, San Rafael, CA, USA) provided the timing signals for the ion source, the ion gate, the sector electrodes and MATSUDA plates I, IV and the digital oscilloscope. Experimental Apreliminary experiment was carried out by using xenon as a sample. The ion mirror was not used in this experiment. The experimental conditions were as follows: (1) The electron energy was 69 ev. (2) The electron emission current was 9 ma (3) The pulse voltage supplied to the first stage electrode of the ion source was 18 V and total acceleration voltage of ions was 1.5 kv. (4) The background pressure was Pa in the ion source housing and Pa in the multi-turn housing. The vacuum pressure in the ion source housing increased to Pa when xenon gas was introduced. The vacuum pressure was measured with ionization gage (M-43HG, ANELVA, Tokyo, Japan). We think because the pumping speed to the volume of the ion source housing was higher than that of the multi-turn housing, the vacuum in the ion source housing was higher than that in the multi-turn housing. (5) The voltage supplied to the sector electrodes were3 V. (6) The voltage supplied to the MATSUDA plates were 18 V. (7) The voltage supplied to the micro channel plate was 1.7 kv. (8) The sampling rate of the digital oscilloscope was 2 GS/s and the following TOF spectra were obtained by summing 1, spectra on the digital oscilloscope. Result and Discussion The TOF spectra of Xe acquired using di#erent flight path lengths (1, 4, 7, and 1 cycles) are shown in Fig. 6. It is clear that the mass resolution increases with increasing the number of cycles. The relation between the number of cycles and the mass resolution of 136 Xe is shown in Fig. 7. The peak widths were approximately the same in the range of 29.5 to 31. ns. Accordingly the mass resolution increased in proportion to the number of cycles. The mass resolution 51 at m/z 136 was achieved after 1 cycles. Hence, we can conclude that MULTUM II satisfies the time focusing. The variation of intensity, which is the sum of all Xe isotopes, with the number of cycles is shown in Fig. 8. The ordinate was normalized so that intensity of cycle was 1. A large number of ions can not circulate the multi-turn part owing to the gap of sector electrodes and slits in the multi-turn part. Therefore, there was great reduction of intensity after one cycle. In contrast to this, from 1 to 1 cycles no significant decrease in the ion intensity was observed. Hence we can conclude that MULTUM II has stable multi-turn orbits. Acknowledgements The authors would like to acknowledge the assistance, support and e#orts of Mr. Tatsuji Kobayashi of 351

4 D. Okumura et al. Fig. 5. Block diagram of the pulse control system for measurement xenon at 4 cycles of MULTUM II. Fig. 7. The relation between the number of cycles and the mass resolution of 136 Xe. Fig. 6. TOF spectra of some xenon isotopes. (a) 1 cycle (the total flight pass length was 2.18 m), (b) 4 cycles (6.32 m), (c) 7 cycles (9.956 m), and (d) 1 cycles (13.88 m). JEOL Ltd. Akishima Tokyo and Mr. Toshio Ichihara of Osaka University for technical support. This work was supported by Grant-in-Aid for Scientific Research (B) (No ) and Grant-in-Aid for Scientific Research (B) (No ). References 1) W. P. Poschenrieder, Int. J. Mass Spectrom. Ion Phys., 6, 413 (1971). 2) W. P. Poschenrieder, Int. J. Mass Spectrom. Ion Phys., 9, Fig. 8. The variation of intensity which was the sum of all Xe isotopes with the number of cycles. The ordinate was normalized so that intensity of the linear mode was (1972). 3) T. Sakurai, Y. Fujita, T. Matsuo, H. Matsuda, and I. Katakuse, Int. J. Mass Spectrom. Ion Proc., 66, 283 (1985). 4) T. Sakurai, H. Nakabushi, T. Hiasa, and K. Okanishi, Nucl. Instrum. Meth. A, 427, 182 (1999). 5) T. Matsuo, M. Ishihara, M. Toyoda, H. Ito, S. Yamaguchi, R. Roll, and H. Rosenbauer, Adv. Space Res., 23(2),

5 A Simple Multi-Turn TOF MS!MULTUM II (1999). 6) M. Toyoda, M. Ishihara, S. Yamaguchi, H. Ito, T. Matsuo, R. Roll, and H. Rosenbauer, J. Mass Spectrom., 35, 163 (2). 7) gwdg.de/cosacpae./index.html 8) T. Matsuo, M. Toyoda, T. Sakurai, and M. Ishihara, J. Mass Spectrom., 32, 1179 (1997). 9) M. Ishihara, M. Toyoda, and T. Matsuo, Int. J. Mass Spectrom., 197, 179 (2). 1) T. Sakurai, T. Matsuo, and H. Matsuda, Int. J. Mass Spectrom. Ion Proc., 63, 273 (1985). 11) H. Wollnik, T. Matsuo, and H. Matsuda, Nucl. Instr. and Meth., 12, 13(1972). 12) M. Toyoda and T. Matsuo, Nucl. Instrum. Meth. A, 427, 375 (1999). 13) W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum., 26, 115 (1955). 14) H. Matsuda and Y. Fujita, Int. J. Mass Spectrom Ion Phys., 16, 395 (1975). Keywords: Multi-turn time-of-flight mass spectrometer, High mass resolution, Ion optics 353