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Metastable Ions in a MS-30 Double Beam Mass Spectrometer J. M. Miller1, J. Ross, J. Rustenburg, and G . L. Wilson Department of Chemistry, Brock University, S t . Catharines, Ontario, Canada
THE IMPORTANCE of “normal” metastable ions has long been recognized in the interpretation of mass spectra ( I ) ; n double focusing instruments of Nier Johnson geometry hese arise in the second field free region between the electric md magnetic sectors, where the parent ion ml, decomposes o give a daughter ion m2,and a neutral fragment, and where he metastables are observed at the apparent mass m* = n22/ml. More recently several metastable “defocusing” echniques have been introduced, which give enhanced metatable ion sensitivity by removing the normal ions. The Address correspondence to this author. ~
1 ) J. H. Beynon, ANAL.CHEM., 42 (l), 97A (1970).
first of these, the Barber and Elliott technique and its refinements (2-4), permits metastable ions formed in the first field free region between the source and ESA to be focused on the collector. After tuning the magnet to a particular ion, the accelerating voltage is increased while the ESA voltage is held constant, and the collector output is recorded as a function of accelerating voltage. By this means the precursors to the ion tuned in by the magnet are observed, Le., m2Jm1 (2) M. Barber and R. M. Elliott, ASTM E-14 Conference on Mass Spectrometry, Montreal, 1964. ( 3 ) M. Barber, W. A. Wolstenholme, and K. R. Jennings, Nature, 214,664 (1967). (4) T. W. Shannon, T. E. Mead, C. G. Warner, and F. W. McLafferty, ANAL.CHEM., 39,1748 (1967). ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
627
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Figure 3. MIKES scans for A . (CH3)3SiC6F5. B. (CHa)zSiCsF6+ C. (CH3)*SiF+
Figure 1. MS-30 ion optics schematic
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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= V,/V,, where m land rn2 are the masses of the precursor and daughter ions and V, and VI are the initial accelerating voltage and that required to tune the precursor ion to the energy of the ESA. A later development of this technique, referred to as “Ion Kinetic Energy (IKE) Spectroscopy” by Beynon (5, 6), involves an ESA voltage scan with the accelerating voltage held constant. Now all the first drift region metastables are recorded at the total ion monitor between the ESA and magnet. Interpretation of these results requires trial fits to all possible transitions. The most recent metastable “defocusing” technique, which is referred to as “Mass Analyzed Ion Kinetic Spectroscopy” or “MIKES” by Beynon (7), and as “DADI” or “Direct Analysis of Daughter Ions” by Maurer (8) requires the interchange of the source and collector positions in a double focusing mass spectrometer of Nier Johnson geometry. With this configuration, if the magnet is tuned to a particular ion and the ESA voltage scanned while keeping the accelerating voltage constant, the collector output as a function of ESA voltage gives all the daughter ions formed in the region between the magnet and ESA. To obtain MIKES involves either extensive mechanical modification of conventional commercial instruments (7, 9), i.e., physically interchanging the source and collector ends of the instrument, or the purchase of a machine constructed with reverse geometry (8). Such reversed geometry instruments have the disadvantage, for a normal mass spec service lab, of surpressing “normal” metastables. If the non-expert doesn’t see “normal” metastables, he has nothing to entice him into further metastable ion studies. It now turns out that the recently introduced AEI MS-30 double beam, double focusing mass spectrometer combines in one instrument the potential benefits of normal and reversed Nier Johnson geometry. The ion optics schematic (Figure l), shows that two independant ion sources, monitors and collectors, with independent slits, share the same electric and magnetic analyzers. Beam 1 is of essentially conventional Nier Johnson geometry, “normal” metastables being observed in good abundance. The first two metastable “defocusing” techniques are also available. Le., ESA and accelerating voltage scans, the latter being offered as an accessory by AEI. Beam 2 has the same geometry, except that following the collector slit, there is a small electrostatic sector which deflects this beam into its collector multiplier. This sector effectively surpresses the “normal” metastables, so that by selection of beams used for sample and reference, one can obtain spectra either rich or deficient in metastables. However, since in Beam 2 there is this small electric sector after the magnet, we have a situation similar to the reversed Nier Johnson geometry required for MIKES or DADI scans. This technique is then available as well as IKES and BarberElliott defocusing without any mechanical modification. ( 5 ) J. H. Beynon, J. W. Amy, and W. E. Baitinger, Cliem. Corn-
miui., 1969, 723. (6) J. H. Beynon, R. M. Caprioli, W. E. Baitinger, and J. W. Amy, I m . J. MCISS Specrrom. 1017Pliys., 3, 31 3 (1969). (7) J. H. Beynon and R. G. Cooks, RmiDecelop., 26, Nov. (1971). (8) K. H. Maurer, C. Brume, G. Kappus, K. Habfast, U. Schroder, and P. Schulze, 19th Conf. on Mass Spec., Atlanta, 1971, Paper K-9. (9) T. Wachs, P. F. Bente, and F. W. McLafferty, h r . J . Mass Specrrom. loti Pliys.,9,333 (1972).
All that is required is the appropriate electronics to scan either of the two electric sectors or the accelerating voltage. Figure 2 shows the output stage of a simple bilateral ramp generator which we have built, capable of scanning from zero to a syrnmetrical plus and minus seventy volts, and which we use to scan the Beam 2 deflector plates. A similar circuit with high voltage transistors and a higher voltage transformer will be used to scan the main electrostatic sector. Since we have been interested in halogen transfers in the mass spectra of perhalogenoaromatic derivatives (10, I I ) , we used C6F5Si(CH3)3 as a test compound for trial MIKES from Beam 2 deflector plate scans. Beam 2 collector output was connected to the vertical axis of an X-Y recorder, while the ramp generator was fed to the deflector plates and the horizontal axis of the recorder. The results of these trials are shown (Figure 3) for the parent ion and the two most abundant ions in the spectrum, C6F5Si(CH&- and the base peak, (CH3)SiF+. As can be seen, many metastables are observed, and can be readily assigned to the major daughter ions (11). The very large metastable ion (Figure 36) for the reaction
-
+
(CH3)2SiC6F5- C6F2CH3- CH3SiF3 corresponds to the only “normal” metastable observed (m* = 69.4). The C6F2CH3+ion is also formed from the parent ion, via loss of CH3SiFaand CH3’simultaneously. Although the resolution is not high, it can be seen that it is sufficient to provide useful information on daughter ions. The relatively low resolution may be attributed to several factors; the size of the sector is fairly small, there is an entrance slit but no exit slit for the sector, and the geometry of the sector with respect to the collector slit may not be ideal. As seen in Figure 1, the direction of the deflector is perpendicular to the plane defined by the ion trajectories through the ESA and magnetic sectors. Thus, the collector slit, which serves as the entrance slit to the beam 2 deflector sector, is rotated 90’ from that required for a resolution slit. Since the MS-30 uses disposable slit cassettes, we plan to modify one of the fixed slit positions of the Beam 2 collector slit by means of a Z restriction which should improve resolution of the MIKES scan. The fact that the deflector plates operate perpendicular to the ESA/magnet plane may offer possibilities of studying internal energy release in two perpendicular directions. We have thus shown that in a commercial instrument with all the advantages of conventional Nier Johnson geometry, it is also possible to use the MIKES or DADI technique of metastable “defocusing” without any mechanical modification. ACKNOWLEDGMENT
We wish to thank AEI engineers for helpful discussions and the National Research Council of Canada and the Ontario Department of University Affairs for financial support. RECEIVED for review September 28, 1972. Accepted November 20, 1972. (10) G. F. Lanthier and J. M. Miller, Org. Muss. Specrroni.. 6 , 89 (1972). (11) J. M. Miller, J. Cliem. Soc. A : 1967,828.
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