The technique of gel‐permeation chromatography (GPC) has been developed as a method for measuring molecular weight distribution in pitch materials. Molecular weight calibration data were obtained from measurements made on GPC fractions collected from a standard pitch. By solubilization of the polymeric portion of pitch through a reduction with lithium in ethylenediamine, the molecular weight range for analysis was extended to in excess of 3000. Mass spectroscopy has been used to further analyze some of the GPC fractions. The GPC calibration data can be employed, with the aid of computer analysis, to determine quantitatively number‐average molecular weights M̄n weight‐average molecular weights M̄w, and molecular weight distribution D (= M̄w/M̄n) in pitch materials.

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Information on the molecular size or weight distribution of pyrogenous raw materials that are complex mixtures of aromatic hydrocarbons has been difficult to obtain. The relatively new technique of gel permeation chromatography (GPC), augmented by the excellent apparatus made available by Waters Associates, offers the opportunity to measure routinely such distribution curves. For years we studied the composition, properties, and thermal reaction mechanisms of complex polynuclear aromatic mixtures such as coal tars, pitches, and heavy petroleum residues. A great deal of work has also been done on pure compounds, with the result that we have accumulated a large number of high‐purity reference aromatic and heterocyclic compounds. We recently determined the elution behavior of more than a hundred polycyclic compounds and also fractionated a variety of thermally produced aromatic residues, using a Model 200 Waters Gel Permeation Chromatograph. Our results with model compounds in tetrahydrofuran solutions indicate that GPC separations occur, not as a function of a single parameter, but as a complex function of molecular size, shape, and polarity. The simplicity of operation and reproducibility of the GPC method will make it extremely useful both as a means of studying the composition of thermal residual products and as a routine screening technique.