use of high ionizing voltages (about 70) permits detailed structural study as well as the determination of molecular weight (3). Further development along these lines could be very useful in characterizing small quantities of organic compounds, particularly in the fields of biology and medicine.
ACKNOWLEDGMENT
The authors are grateful to David Fukushima of the Sloan-Kettering Institute, who supplied many of the samples.
(2) Brown, R. *4.,Taylor, R. D., MeE polder, F. W., Young, W. S., ANAL. CHEM.20, (1948). (3) De Mayo, P., Reed, R. I., Chem. & Ind. (London)1956. 1481.
LITERATURE CITED
(1) Benyon, J. H., Jfzkrochzni. .Icta 1956,
437.
RECEIVED for review ilugust 27, 1957. Accepted August 8, 1958. Work supported by the American Cancer Society.
Derivation of Mass Spectra of Individual Compounds from the Spectra of Mixtures SEYMOUR MEYERSON Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
b Analysis of mixtures by mass spectrometry requires the resolution of the spectra into the contributions of the individual components. Such resolution can often b e obtained, even with no prior knowledge of composition, from the spectra of two different mixtures of the same two components. By subtracting all the peak heights of one spectrum, multiplied b y an appropriate factor, from the corresponding peak heights of the other, a difference spectrum for either individual component can b e derived. The method can b e extended to mixtures of three or four components in favorable cases. It has been particularly useful in correcting reference spectra for persistent unknown impurities in calibrating materials,
A
NALYSIS by mass spectrometry, as
by other spectroscopic methods, entails a comparison of the spectra of unknown materials with those of known compounds. Published reference spectra (1) and empirical correlations (5-8, 10, 12) enable the identification of a large number of organic compounds in the absence of interfering substances. I n mixtures, however, the contributions of any individual compound t o the spectrum may be so heavily masked that recognition is impossible. Interpretation of the spectrum then requires that it first be resolved into its component parts. Preliminary separation of complex mixtures by distillation, chromatography, or other techniques often produces fractions with few components and correspondingly simplified spectra, If the spectra of two such fractions contain in common a pair of peaks that are almost certainly due to different components (7, 11), these peaks may
174
ANALYTICAL CHEMISTRY
enable the spectroscopist to separate the contributions of the individual compounds, even with no prior knowledge of sample composition. Table I shows partial spectra of two arbitrary mixtures, I and 11, of benzene and cyclohexane. Even without this qualitative information, it could be inferred immediately that the peaks a t masses 78 and 84, separated by a mass interval of 8 units, are due to different components. By subtracting all the peak heights in one spectrum, multiplied by an appropriate factor, from the corresponding peak heights in the other spectrum, a difference spectrum in which the peak a t either 78 or 84 is entirely absent can be derived. Thus, spectrum I minus 87.5/156 spectrum I1 equals spectrum 111, and spectrum I1 minus 430/645 spectrum I equals spectrum IV. Spectra I11 and IV have been normalized to relative intensities of 100.0 a t masses 78 and 84, respectively. The derived spectra correspond to compositions having one component less than the original samples. Comparison with reference spectra of benzene and cyclohexane, obtained from the pure compounds, completes the identification. The agreement is such that the difference spectra could serve as reference spectra if pure calibrating materials mere unavailable and the identities of the compounds were known from other information. Not only the relative intensities but also the sensitivities of the individual compounds can be computed from the spectra of the mixtures and these values permit quantitative analysis. Mixtures I and I1 mere introduced from the same constant-volume micropipet; thus, sensitivity may be defined as the number of divisions of peak height per volume per cent. (The use of volume sensi-
tivities implies that the sample is an ideal solution. This approximation could be avoided by expressing sensitivities as divisions per micron pressure. However, the possible advantage of such a procedure appears slight, and often negligible, in view of the accuracy limitations of most pressure measurements in the range usually employed, 20 to 50 microns.) Solution of the simultaneous equations
+ 87.5/Srv = 100 430/8111 + 156/S1v = 100
645/S111
and
gives numerical values for SI11 and SIV, the sensitivities associated with spectrum 111, benzene, a t 78 and IV, cyclohexane, a t 84. The computed sensitivities and the sample analyses derived from them agree well with the values obtained from the reference spectra. The method is generally applicable to the spectra of binary mixtures, provided the spectra of two different mixtures of the same components are available and the spectrum of each individual component contains a t least one peak to which the other does not contribute. If the spectrum of only one component contains a peak to which the other does not contribute, a difference spectrum can still be computed for the second component, though not for the first. To try the method, it must be assumed that the necessary conditions hold and the peaks to be used in the computation selected accordingly. An incorrect assumption or a wrong choice of peaks will nearly always be revealed by negative intensities in the difference spectra obtained. When the choice of peaks is not obvious, the intensity ratios of the peaks a t corresponding masses in the two spectra
can be listed and the peaks that give the highest, and the lowest ratios selected; thcse peaks contain the least contributions from other components. The method was applied in the study of two simples of comniercial n-butyl disulfide shon n by a wet-chemical method ( 2 ) to contain 30 and 99+% disulfide. Bcsides the expected disulfide parent peak a t mass 178, both spectra contained a peak a t 146 vihicli suggesttd a butyl sulfide impurity. The peak heights a t 179 and 180 and a t 147 and 148 agreed closely uith the values expected for CsHj8S2+ and CsH&+ ions containing carbon-13, sulfur-33, and sulfur-34 in their natural abundances. The spectra gave no obvious indication of other components Assuming the two samples to be niixturrs of the sime two components, a butyl disulfide and a butyl sulfide, difference spectra w r e derived. Resulting negative intensities showed that this asumption was not valid. However, the spectra obtained from successive introductions of either sample to the spectrometer differed from one another. The composition was evidently changing in the sample-introduction system, presumably as a result of sorption on the walls (9). A modified approach was then tried, in which the t n o samples were treated independently in the initial computations. On the assumption that both samples n cre binary mixtures even though they might not contain the same components, difference spectra were derived from pairs of spectra obtained upon successive introductions of each individual sample. I n each case, the mass-178 peak was assumed to be due solely to one component and the mass-146 peak solely to the other. The spectra so obtained, shown in Table 11, contain no negative intensities. The two butyl disulfide spectra differ sharply and show that the disulfides in the two samples are not the same, That in sample I is dissociated by electron impact far more extensively than the one in sample 11. It nould be inferred that the former is more highly branched than the latter, which may be n-butTl disulfide; no reference spectrum is available to confirm this. The butyl sulfide spectra agree fairly well with each other and n i t h the recently issued API spectrum of n-butyl sulfide ( 1 ) . The differences among these spectra may be due to small amounts of additional components in the samples, or they may have resulted from the computation, which was based on the small peaks a t 178 and 146. Doubtless, the sulfide in both samples is mainly, if not solely, the n-butyl isomer. Although this technique has been used chiefly with binary mixtures, it can also be applied to mixtures of three or four components in favorable cases.
Table I.
Partial Spectra of Benzene and Cyclohexane
Spectra of Mixtures Peak Heights Mass,
w e
I 47.1 140.1 3.8 4.3 19.1 1.0 1 0 29 8 25.0 96.3 645 5.6 87.5
I1 83.8 249 2.8 62 3.1 63 13.4 61 0.9 1 7 65 69 53.0 76 16.5 77 68,4 78 430 83 9.8 84 156 Sensitivity Composition, vol. yo Mixture I 70.1 Mixture I1 46.8 .issumed zero in deriving difference spectra.
55 56 61
a
Difference Spectra Reference Spectra Relative Intensities CycloI11 IT‘ Benzene hexane 0.0 53.6 0.0 54.3 0.1 159 0.0 158 0.5 0.3 0.6 0.1 0.6 0.2 0.7 0.2 2.9 0.7 2.9 0.6 0.1 0 2 0.1 0.1 1.0 0.0 0.0 0.9 33.9 0.0 0.0 34 2 3.9 -0.2 3.6 0.1 14.8 14.7 1.2 1.0 100 100.0 0.4 0.0 6.2 0.0 6.3 100,o 0.0 100.0 9.20 8.97 2.93 2.98
Table II.
0
(1
29.9 53.2
71.0 47.8
29.0 52.2
Partial Spectra of Butyl Disulfide and Butyl Sulfide
Difference Spectra Butyl Disulfide Butyl Sulfide Relative Intensities Mass, m/e I I1 I I1 55 a2 10 21 25 56 197 9 97 98 57 100 100 35 60 2 4 58 13 5 59 8 2 3 4 60 15 2 4 5 61 151 4 100 100 62 26 0 2 4 63 5 0 5 4 89 53 2 6 11 90 6 1 21 22 10 12 9 3 131 3 0 0.2 0.3 146 26 31 D 178 2 19 a Assumed zero in deriving difference spectra. (I
0
Extension to such complex materials requires that enough different mixtures of the same components be available and that the spectra show enough unrelated peaks. Additional mixtures can sometimes be obtained, as with the butyl disulfide-butyl sulfide samples, by a crude separation in the sample introduction system itself (9). Highly purified materials which might be used as spectroscopic standards occasionally show persistent small peaks that can not reasonably be attributed to the desired compounds. With this method, impurities thus revealed can often be removed from the spectra, even when they cannot be identified. LITERATURE CITED
(1) American Petroleum Institute, Research Project 44, “Catalog of Mass
Reference Spectrum ( I ) , n-Butyl Sulfide 24 91 31 2 4 5 100 4 5 12 23 11 13 5 0.0 0.3
31 0
Spectral Data,’’ Carnegie Institute of Technology, Pittsburgh, Pa., 1947. (2) Earle, T. E., ANAL. CHEM.25, 769 ( 1953). (3) Friedel, R Zbid., 28, 94 (4). FrJed,el, R.
(8j Ibid , 29, 1782 (1957). (9) Meyerson, Seymour, Ibid., 28, 317 (1956). (10) Meyerson, Seymour, A p p l . Spectroscopy 9, 120 (1955). (11) Rock, S. M., ANAL.CHEM.23, 261 (1951). (12) Sharkey, A. G., Jr., Shultz, J. L., Friedel, R. A., Zbid., 28, 934 (1956).
RECEIVEDfor review February 17, 1958. Accepted September 12, 1958. American Society for Testing Materials Committee E-14 on Mass Spectrometry, New Orleans, La., June 1958. VOL. 31, NO. 2, FEBRUARY 1959
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