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b A new set of low-voltage calibration data is presented for aromatic hydrocarbons, including compound types from alkylbenzenes to chrysenes. Separate data are provided for alkylbenzenes with different degrees of substitution and for average coefficients for these compound types obtained by using detlmnined and calculated average isomeric distribution values. Ionization-efficiency curves also have been obtciined for a number of alkylbenzenes in an effort to correlate ion current magnitudes at a given ionizing energy level with molecular structure. Preliminary data indicate that the degree of substitution on the aromatic ring affects the slope o f the ionization-efficiency curves.HE LOW-VOLTAGE METHOD for identi-T fication and analysis of hydrogendeficient materials has continued to be one of the most powerful techniques in mass spectrometry since its inception in 1955 (3). A set of calibration data for a number of olefinic and aromatic compound types published in 1958 (6) is widely used in industrial research.Revision and extension of the coefficients obtained originally are justified by the availability of a larger number of pure compounds and increased knowledge about the constitution of petroleum. The rectnt shift in emphasis to chemicals and chemical precursors makes a recalibration on a weight, rather than volume, per cent basis desirable. Concurrently, the scope of low-voltage analysis is being augmented by the advent of instruments featuring resolving powers of the order of 1 in 5000 and above arid thus capable of differentiating among the parent peaks of more than seven compound types. These factors furnished the incentive for obtaining a new and consistent set of calibration coefficients for aromatic hydrocarbons, whic i is presented and discussed here.Correlations between molecular structures and low-voltage ion current intensities have been sought and established for several cltsses of compounds by Crable, Kearns, and Sorris (2). Their data have been corroborated and extended by the presentations of ionization efficiency curvw for a number of alkylbenzenes. Table I. Reproducibility of Ethylbenzene Sensitivities Divisions/pmole 7 . 6 volts 15.0 volts 7 0 . 0 volts Range for 21 runs 18.2-19.6 128.5-135.6 241.6-256.2 Av . 18.73 132.53 247.6 Std. dev. kO.55 f 2 , 8 5 h 3 . 6 6 Rel. std. dev. 1 2 . 9 1 1 2 . 1 5 3~1 . 4 8 EXPERIMENTAL Calibration data were obtained on a 180' magnetic deflection (Consolidated Electrodynamics Corp. Model 21-103) mass spectrometer, adapted for lowvoltage work as reported previously (8). The instrument was operated in the focused mode. The level of the magnetic field was about 3800 gauss, and the repeller plates were kept a t about 2.5 volts. Solid materials were introduced through the solids inlet system devised by Lumpkin and Taylor (7). The availability of this system made calibration possible for four-and five-ring condensed aromatics; 62 alkylbenzenes, nine indanes and Tetralins, 22 naphthalenes, five noncondensed 2-ring aromatics, 20 condensed three-, four-, and five-ring...

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There are many advantages of using on-line liquid chromatography/mass spectrometry (LC/MS) to characterize complex mixtures. By incorporating low-voltage electron-impact ionization/highresolution MS with moving belt LC/MS, differentiation can be made between naphthenoaromatics and alkylaromatics and between aromatic hydrocarbons and "difficult-to-resolve" thiophenes. Alternative on-line LC/MS techniques for heavy hydrocarbon characterization are also discussed.

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