ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1 9 7 8 Coal-derived
ether derivatives suggest the presence of phenols, hindered phenols, and benzylic hydroxyls.
liquid
~
Pentone
ACKNOWLEDGMENT I
I Insoluble
Soluble
Benzene
0;Is
I
The authors acknowledge the cooperation of Sayeed Akhtar and Nestor Mazzocco in providing the coal liquefaction products and associated process data.
I
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I
Tetrahydrofuran
LITERATURE CITED
Soluble
Arpholtenes
lnsolubles Pre-osphaltenes
3
371
Acidheutrol
PA
Figure 2. Separation scheme for coal-derived liquids
derivatives. We have observed a similar distribution of hydroxyl T M S ethers from fractions derived from Figure 2. T h e degree of spectral complexity increases as one explores fractions 1-3. Table I1 summarizes the pertinent physical and chemical properties of each of the fractions. The remaining oxygen contained within the coal-derived samples may be assigned to ether linkages (e.g., furan) because the infrared data do not show a carbonyl band. If we examine Figure 1 in more detail, we can distinguish two major, but overlapping, areas of T M S ethers of the acid components of a coal-derived asphaltene. The chemical shifts of the model compounds allow us to assign portions of the spectrum as being alcoholic or hindered phenolic, 0-50 Hz. or simple phenols, 45-100 Hz. This is the first reported observation of a distinct distribution of hydroxyls in coalderived oils and asphaltenes. The NMR data of the T M S
(1) "Handbook of Analytical Chemistry", L. Meites Ed., McGraw-Hill Book Company, New York, N.Y., 1963, Section 6, pp 1-101. (2) N. Linnet, "A Discussion of the ASTM Method for Determination of Acid and Base Number in Oils", Radiometer, Denmark, ST43. (3) R. W. Martin, J . A m . Cbem. Soc., 74, 3024 (1952). (4) M. M. Sprung and L. S. Nelson, J . Org. Cbem., 20, 1950 (1955). (5) A. G. Sharkey, Jr., R. A. Friedei, and S. H. Langer, Anal. Cbern., 29, 770 (1957). (6) S. H. Langer, P. Pantages, and I. Wender, Cbern. Ind., London, 1958, 1664. (7) A. Hase and T. Hase. Analyst (London), 97, 998 (1972). (8) . . S. Friedman, C. Zahn, M. L. Kaufman, and I. Wender, U.S. Bur. Mines, Bull., No. 609, (1963). (9) J. D. Brooks and J. R. Steven, Fuel, 46, 13 (1967). (10) S. Friedman, M. L. Kaufman, W. A. Steiner, and I. Wender, Fuel, 40 (33), 26 (1961). , (11) SIH. Langer, S. Connell, and I. Wender. J . Org. Cbem.. 23, 50 (1958). (12) J. Dakok, R. F. Sprecher, A. A. Bothner-by, and T. Link, Paper presented at the 1 lth Experimental NMR Conference, Pittsburgh, Pa., 1970. (13) J. Dadok and R. F. Sprecher, J . Magn. Reson., 13, 243 (1974). (14) F. R. Brown, S.Friedman, L. E. Makovsky, and F. K. Schweighardt, Appl. Spectrosc., 31, 241 (1977). (15) B. L. Shapiro and T. W. Proulx, Org. Magn. Reson., 8 , 40 (1976). (16) H. Suzuki, Bull. Cbem. SOC.Jpn., 32, 1350 (1959). (17) J. B. Clews and K. Lonsdale, f r o c . R . SOC.London Ser. A , 161, 493 (1957). \
~~
RECEIVED for review August 22, 1977. Accepted November 29, 1977. Use of brand names facilitates understanding and does not necessarily imply endorsement by the U.S. Department of Energy.
CORRESPONQENCE Chemical Ionization Mass Spectrometry as a Tool for the Elimination of Surface Related Phenomena in the Spectra of Unstable Compounds Sir: The occurrence of surface related reactions in mass spectrometer ion sources has long been recognized. The products of gas-surface reactions, involving surface ionization on the filament and pyrolysis on hot surfaces, often contribute minor peaks to electron impact (EI) mass spectra, and complications can be severe in the analysis of reactive or thermally unstable compounds. The resulting mass spectra may lead one to a s u m e the presence of impurities not actually in the sample, or even cause errors in the identification of unknown compounds and mixtures. The purpose of this correspondence is to present an example of such surface related phenomena and to point out how these problems may be avoided utilizing chemical ionization (CI) mass spectrometry. T h e compound used to illustrate this problem is the 0003-2700/78/0350-0371$01.00/0
brominated derivative of polymeric sulfur nitride, (SN), (I). (SN), has been previously found to produce a noncyclic (SN)4 species upon sublimation ( 2 ) . The brominated derivative, of empirical formula SNBro,4(I),was expected to yield (SN), plus an unknown brominated species upon sublimation; hence, identification of the bromine containing species was of particular interest. In work reported elsewhere ( 3 ) ,we have discussed the chemistry and characterized the vapor phase species sublimed from brominated (SN),. The electron impact mass spectrum of SNBro,4obtained a t 125 "C is given in Figure la. This spectrum was obtained on a Hewlett-Packard 5980A mass spectrometer, equipped with a dual EI-CI source, using a 70-eV electron energy during the slow sublimation of brominated (SN), from a solids probe over a 40-min period. Comparison to previously published C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
372
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W e Flgure 1. Comparison of mass spectra of SNBro,4.(a) 70-eV E1 spectrum, (b) 70-eV E1 spectrum obtained using a modulated beam mass spectrometer sampling an effusive beam from a Teflon Knudsen cell, (c)CH, CI spectrum, and (d) Ar CI spectrum. The high mass peaks in the conventional 7 0 4 E1 spectrum have their oriiin in surface related phenomena in the mass spectrometer ion source. All spectra were obtained at 120 to 130 O C . All ions in the CI and modulated beam spectra may be attributed to either S,N,, SNBr, HBr, Br,, or S8 spectra of (SN), (2) suggests the presence of (SNI4 (MW = 184), Br2, and several additional bromine containing compounds (3). (A small amount of water present during the synthesis also results in a small HBr impurity (3).) The spectrum of brominated (SN), shows distinct multiplets centered a t approximately m / e 358,438, and 519, with the series of peaks around m / e 358, the most intense. These multiplets may be ascribed to one or more compounds containing 3,4, and 5 bromine atoms, respectively. Although it was not possible to identify the other elemental constituents of these species, they do contain elements other than sulfur, nitrogen, hydrogen, and bromine. T h e relative importance of these high mass clusters may be reduced somewhat by shutting off the source heaters and allowing the source to cool, or simply running the solids inlet probe a t a lower temperature. (Modulated molecular beam experiments, discussed below, clearly show this behavior is not due to changes in the vapor composition resulting from temperature changes ( 3 ) . ) However, these approaches were plagued by increased background, rapidly decaying and changing source conditions due to the build up of surface contaminants, and a greatly reduced signal intensity due to the decreased rate of sublimation and to condensation on the cooler parts of the source. To determine the relative importance of surface related reactions, the conventional E1 spectrum (Figure l a ) were compared to E1 spectra obtained using a modulated molecular beam sampled from a Teflon Knudsen cell a t temperatures of 70 to 150 "C. The molecular beam, effusing through a 1-mm orifice in the Knudsen cell into a differentially pumped region (maintained a t
