Anal. Chem. 1993, 65, 2116-2118
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Characterization of a Small FT-ICR Mass Spectrometer Based on a Permanent Magnet Loreen C. Zeller, James M. Kennady, and Hilkka Kenttamaa' Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393
Joseph E. Campana Extrel FTMS, Millipore Corporation, Madison, Wisconsin 53711 -2424
We report here preliminary results on a mediumperformance FT-ICRmass spectrometer, based on a 0.4-Tpermanent magnet, that could easily be configured into bench-top proportions. Among other properties, we have examined the mass range, mass resolution, and mass accuracy of this mass spectrometer, as well as the stability of the magnetic field over time. Further, the use of this device in studies of gas-phase ion/molecule reactions has been briefly explored. The data presented here demonstrate that this simple mass spectrometer can perform a variety of useful experiments, including the measurement of EIMS, MS/MS, and MS/MS/MS spectra, the determination of reaction rate constants for bimolecular ion/molecule reactions, and the measurement of the exact mass of ions. INTRODUCTION During the past 10 years, bench-top mass spectrometers have become common in many applications involving environmental, biomedical, and other analytical problems. Fourier transform mass spectrometers, introduced commercially just over 10 years ago, have used electromagnets or high-field superconducting magnets in their design in order to obtain their high- and ultrahigh-performance characteristics. The magnets used in these instruments constrain them to floorstanding configurations, which is typical of high-performance mass spectrometers. While researchers have experimented with permanent magnet FT-ICR systems,l no systematic characterization of the performance of such instruments has been published thus far. We report here our preliminary results on a medium-performance FT-ICR mass spectrometer, based on a permanent magnet, that could be easily configured into bench-top proportions. Among other properties, we have examined the mass range, mass resolution, and mass accuracy of this mass spectrometer, as well as the stability of the magnetic field over time. Further, multiple-stage mass spectrometry experiments and the measurement of a rate constant for a bimolecular reaction have been demonstrated.
EXPERIMENTAL SECTION The FT-ICR mass spectrometer discussed here is composed of a vacuum chamber that fits snugly between the pole faces of a0.4-T permanent magnet and is pumped with a 300 L/sdiffusion pump to a nominal base pressure of 1 2 X 1O-g Torr. The magnet (Extrel FTMS, Millipore Corp., Madison, WI; Part No. F-6660) is not thermally regulated. Samples were introduced into the (1) See, for example: Elling, J. W. Ph.D. Thesis, University of Wisconsin, 1991. Hearn, B. A. Ph.D. Thesis, University of Florida, 1989. 0003-2700/93/0365-2116$04.00/0
Table I. Mass Accuracy Based on External Calibration (PFTBA). calcd measd error, mass, Da mass f u, Dab PPm 68.994 67 130.991 49 218.985 11 263.985 69 413.977 03 501.970 66
68.995 03 f 0.001 02 130.993 84 f 0.004 87 218.952 78 f 0.019 30 264.029 83 f 0.020 66 414.013 89 f 0.047 32 501.921 22 f 0.074 90
5 18 147 16'lC 89 99
All of these measurements were made without the use of an internal mass reference compound. The mass was measured against calculatedmass values, and a computer-based calibration table was generated for each of the 20 experiments. For the determination of u, n = 20. The low abundance of this fragment ion makes the measurement less accurate.
instrument through one of two leak valves or a heated solids probe. The instrument is controlled by an Extrel FTMS-2000 data station (with Nicolet 1280 computer) with an Extrel FTMS SWIFT module (stored-waveforminverse Fourier transform).z The instrument has a single, nominally cubic 3.0-cm cell that was used with screened3as well as unscreened trapping plates. The overall performance of the instrument was found to be dramatically improved by the use of screened trapping plates.' Grounded screens placed in front of the trapping plates decrease the electricfield gradients insidethe cell.3fi Thisservesto decrease space-chargeeffects and the effects of magnetron motion, which, in a small cell, are a significant consideration. The screens were constructed from tungsten wire woven to make a mesh of 3 wires/ cm in both the z and y directions (the x,y plane is defined as the plane that is perpendicular to the magnetic field lines). Ceramic spacers (2 mm long)separate the screensfrom the trappingplates. The screens can be operated either at instrument ground or with an applied potential (either positive or negative). Under normal operating conditions, the best performance was achieved when the screens were grounded.
RESULTS AND DISCUSSION A permanent magnet-based FT-ICR mass spectrometer equipped with a screened cell (grounded screens) can be used to detect ions with mass-to-charge ratios greater than 1400 Da. The heaviest ion studied thus far was generated by electron ionization of 2,4,6-tris(perfluorononyl) l,3,btriazine, and it has a mass value of 1466 Da (C3,4?aN3+). The mass accuracy of the mass spectrometer was determined from 20 measurements (without an internal reference compound (2) Marshall, A. G.; Wang, T.-C. L.; Ricca, T. L. J. Am. Chem. SOC. 1985,107,7893. Marshall,A. G.;Ricca,T. L.; Wang,T. C.-L. U.S.Patent 4,761,545, 1988. (3) Wang, M.; Marshall,A. G. Anal. Chem. 1989,61,1288. Marshall, A. G.; Wang, M. U.S. Patent 4,931,640, 1990. (4)Zeller, L. C.: Kennady, J. M.; Campana, J. E.: Kenttiimaa, H. I., in preparation. (5) Zeller, L. C.; Kennady, J. M.; Kenttiimaa, H. I., submitted.
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
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Flgure 1. (a) Sample of acetone Ionized by uslng 5 0 4 electron ionization and (b) fragment ion of mlz 43 Isolated by using a SWIFT waveform and (c) albwed to react wkh the neutral acetone In the cell. Variation of the final time parameter allowed the calculation of the reaction rate. Comparison of thls rate to the known rate constant albwed the calculatron of an ionlzatlon gauge correction factor for small ketones. The resolution of these spectra Is on the order of lo3.
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Flgure 2. Rate of reaction of lonlzed cyclobutanone wlth neutral cyclobutanone measured in two instruments. Electron ionization (70 eV) was used to generate the molecular ion in both Instruments: (a) The low-fieid (0.4 T) Instrument yields a rate constant of 4.6 X 10-lo cm3 mobcule-l s-1 (the nominal reagent pressure was 2.2 X lo-' Torr). (b) The high-field (3 T) Instrument yields a rate constant of 4.8 X 10-10 cm3 molecule-1 s-1 (the nominal reagent pressure was 1.1X 10-7 TOV).
present during the measurement) of the electron ionization mass spectrum of perfluorotributylamine (PFTBA) over the courae of 1 month (Table I), and it was found to vary from 5 to 150 ppm over the mass range of 69-502 Da. Since highmass resolution is one of the characteristics which FT-ICR mass spectrometry is well-known for, it is of special interest here. Our preliminary results show that the permanent magnet-based FT-ICR can achievea resolvingpower of 53 OOO
112
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Flours 3. MS3 experiment performed in the low-field Instrument: (a) generation of the radical cation of cyclobutanone, mlz 70, with 5 0 4 EI, (b) Isolation of the Ion of mlz 70 by using a SWIFT waveform, (c) reaction of the Isolated radlcal catlon with neutral cyclobutanone for 1 sat a reagent pressure of 2.4 X Torr, (d) isolatlon of the product ion of m/z 112 by using a SWIFT waveform, and (e) reaction of the product Ionof mlz 112 for 2 s with neutral cyclobutanone (at the same pressure). The product Ions are discussed in the text.
for the PFTBA fragment ion of mlz 69 (the nominal sample pressure was 5 X 10-8 Torr, 16K data points were collected over a bandwidth of 5.16 kHz a t a heterodyne frequency of 91,090 kHz). The resolution was calculated by using the full width at half-maximum (fwhm) definition. The ability to study ion/molecule reactions was demonstrated for two different chemical systems. One of these systems is the well-characterized6 reaction of the acetone fragment ion CHsCO+ (m/z 43) with neutral acetone. The fragment ion of mlz 43 was generated by 50-eV electron ionization of acetone and was isolated by using a SWIFT waveform. This ion was then allowed to react with neutral acetone in an MS/MS experiment (the spectra are illustrated in Figure 1). The fragment ion protonates neutral acetone, producing the ion of mlz 59, as reported earlier.6 The rate of this reaction was measured and compared to the rate constant reported in the literature. The ratio thus obtained was used as a geometry correction factor for the pressure reading of the ionization gauge in the experiment discussed below. Another reaction studied was that of ionized cyclobutanone with neutral cyclobutanone. Upon reaction of these two species, two primary product ions were observed: an ion of m/z 84 from transfer of CH2*+to cyclobutanone (presumably from loss of CO and C H p C H 2 ) and an ion of mlz 112 from transfer of an ion of mlz 42 (either C3Ha'+ or CH2CO'+) to cyclobutanone. A t longer reaction times, an ion of mlz 71, the result of proton transfer to cyclobutanone, was also observed. A similar product distribution was observed in a high-field FT-ICR instrument' (a commercial Extrel Model 2001 FT/MS system equipped with a dual cell and a 3-T superconducting magnet). By using the geometry correction factor determined from the acetone experiment described ( 6 ) Gas phase ion-molecule reaction rates constants through 1986; Ikezoe, Y., Matauoka, S., Takebe, M., Viggiano, A., Eds.;Maruzen Co.: Tokyo, 1987.
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above, a reaction rate constant of 4.6 X W0cm3 molecule-' s-1 was calculated (Figure 2a). The reproducibility of these measurements was found to be f5 % The rate constant for this reaction measured in the high-field FT-ICR mass spectrometer' (Figure 2b: 4.8 X cm3 molecule-' s-1; reproducibility 1 5 % ) is within experimental error of the rate measured in the low-field instrument. The capability of the low-field instrument for additional stages of mass spectrometry (MS") was demonstrated by using the cyclobutanone system. Cyclobutanone was ionized by 50-eV electron ionization (Figure 3a); the molecular ion was isolated by using a SWIFT waveform (Figure 3b) and allowed to react with neutral cyclobutanone (Figure 3c). A SWIFT waveform was used to isolate one of the product ions described above (mlz 112, Figure 3d). This ion was then allowed to react with neutral cyclobutanone for 2 s (Figure 3e). The structure of the ion of mlz 112 is unknown. However, on the basis of some recent work by Stirk and K e n t t h a a ? it could be postulated that the ion is an electrostatically bound
.
complex of ionized cyclobutanone and ketene. If this is true, then neutral cyclobutanone should be able to replace ketene in the ion and produce a product ion of mlz 140. The absence of this product (Figure 3e) suggests that the structure of the ion of mlz 112 is not that proposed. The structure of the ion is currently under investigation.
CONCLUSION The preliminary data presented here demonstrate that an FT-ICR mass spectrometer based on a 0.4-Tpermanent magnet can perform a variety of useful experiments, including the measurement of EIMS, MSIMS, and MSIMSIMSspectra, the determination of reaction rate constants, and the measurement of the exact mass of ions. Ongoing studies show promise in improved mass resolution, mass accuracy, and mass range.
ACKNOWLEDGMENT We thank the Lubrizol Corp. and the National Science Foundation (CHE-9107121)for partial support of this work.
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(7) Farrell, J. T., Jr.; Lin,P.; Kenttiimaa, H. I. Anal. Chim. Acta 1991,
246, 227.
(8)Stirk, K. M.; KenttEunaa, H. I. Org.Moss Spectrom. 1992,27,1153.
RECEIVED for review February 8, 1993. Accepted May 5, 1993.
