Application of a logarithmic Transfer Response Circuit to the Recording of Mass Spectra and Especially Peaks from Metastable Ions Robin T. Aplin, Herbert Budzikiewicz, Harrison S. Horn, and Joshua Lederberg, Department of Chemistry, Stanford University, Stanford, Calif., and Department of Genetics, Stanford Medical School, Palo Alto, Calif.
in the application C of computer techniques to the plotting (9) and analysis (4, 8, 11, 12, URRENT INTEREST
IS) of mass spectra has led to the development of various systems for a readout and data transfer. One, for use in conjunction with a mass spectrometer employing photographic plate recording, has recently been described ( 5 ) . Mass spectrometers equipped with electric readout are usually implemented by an oscillograph, a strip chart recorder, or a multipen recording galvanometer, the last giving the widest dynamic rangee.g., the CEC 21-103C instrument employs five galvanometers with sensitivities ranging from 1 to 100. In addition, systems for digitization of mass spectral data have become commercially available. However, all these systems do not offer the advantages discussed below for the recognition of metastable ions. The problem of recording such data on tape, because of limitations in the useful dynamic range of analog recorders and analog-to-digital converters, has been simplified by the development of a logarithmic converter. A circuit has recently been developed (10) (Figure 1) with a logarithmic transfer response over a dynamic range of nine decades. It has now been incorporated into a data acquisition system for both low and high resolution mass spectra with electrical readout. This system has been used in the authors’ laboratory in conjunction with an Atlas C H 4 mass spectrometer, whose usual recording system requires manual attenuation and several scans to
structure determination, and detailed interpretation of the fragmentation mechanisms of organic conipounds (3, 6, 7 , 14). Figure 2 shows the direct record of the spectrum of n-decane. To observe even the very intense metastable peaks at m/e 33.0, 44.5, and 90.0, it is necessary to use a much greater recorder sensitivity (Figure 3). Figure 4 gives the logarithmic transfer plot recorded simultaneously with the record produced in Figure 2; not only does this show a considerable number of metastable peaks not apparent in Figure 3, but it also produces a spectrum which is far easier to count. Since this technique is currently being applied in our laboratory for the detection of metastable ions to faciliate the elucidation of fragmentation mechan-
obtain accurate intensities of both strong and weak peaks. This is especially disadvantageous if one is trying to observe metastable ions which appear as weak broad peaks. These metastable ions originate from the decomposition, occurring beyond the ionizing region of to the source, of a charged species (mi) give a daughter ion (m2) and a neutral species (n)(Equation 1 ) . mi
-
mz
+n
(1)
The metastable peak for the above transition mill be found a t mass (m*) given by Equation 2 (2).
m*
(2)
= m22/ml
Metastable ions are of great importance in the qualitative identification, DUCCIR UI?WILR
7 Figure 1 .
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Figure 2.
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ANALYTICAL CHEMISTRY
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Logarithmic transfer circuit
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n-Decane, normal plot
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n-Decane, normal plot
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isms, experiments are also in progress to adapt it to an A. E. I. MS-9 high resolution mass spectrometer, which has been shown to possess considerable advantage over other instruments in the observation of these features ( 1 ) . LITERATURE CITED
(1) Barber, M., Elliot, R. XI., Twelfth Annual Sympositrm on &lass Spectrometry, Montrea;: 1964. ( 2 ) Beynon, J. H., hIass Spectrometry and Its Application to Organic Chemistry,” p. 251 $. Elsevier, Amsterdam, 1960. ( 3 ) Bi:mann, K., “Mass Spectrometry,” p. 103, -McCraw-Hill, S e w York, 1962. ( 4 ) Biemann, K., Bommer, P., Desidero, D. M., Tetrahedron Letters 1964, 1725.
100
90
110
120
130
140
n-Decane, log transfer plot
K., Desidero, D. M., Twelfth Annual Symposium on >lass Spectrometry, XIontreal, 1964. ( 6 ) Budzikiewicz, H., Iljerassi, C., Williams. D. H.. “InterDretation of Mass Spectra of Organic Compoiinds,” Holden-Day, San Francisco, 1964. (7) Budzikiewicz, H., Djerassi, C., Williams. D. H.. “Strrictiire Elucidation of S a t w a l Products by Mass Spectrometry, Vols. I and 11, Holden-Day, San Francisco, 1964. (8) Burlingame, A. L., Conference on Spectroscopy, Instrrimentation and Chemistry, San Framicisro, 1964. (9) Elman, C:. J., 1)igital Computer Analysis of Spectrophotometric Data, h1.S.thesis, Stanford I-niversity ( 1964). (10) Gibbons, J. F . , Horn, H. S., I.E.E.E. Transactions on Circuit Theoru. 3. 378 11964). (11) Kennicott, P., Twelfth Annual Sym( 5 ) Biemann,
I
posium on Mass Spectrometry, Montreal, 1964. (12) Lederberg, J., “Cornpatation of Ylolecnlar Formulas for Alass Spectrometry,” Holden-Day, San Francisco, 1964. (13) Lederberg, J., Wightman, lf,, “A Siibalgol Program for the Calciilation of Moleciilar Compositional Formulas from Mass Spectral Data,” NASA Scientific and Technical Aerospace Report No. 0000 (1964). (14) h‘IcLafferty, F. W., Gohlke, R. S., Golesworthy, R. C., Twelfth Anniial Symposirim on Mass Spectrometry, hlontreal, 1964. WORK sirpported by the Kational Institutes of Health of the U. S. Priblic Health Service (Grant7 S o . AM 04257 and S B 04270) and the National Aeronautics and Space Administration (Grant NO. NsG 81-60). VOL. 37, NO. 6, M A Y 1965
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