Analysis of coal tar and petroleum pitches by computerized pattern recognition

Abstract

SummaryPyrolysis gas chromatography (P/GC) has been successful in differentiating coal tar pitches which are used in the manufacture of high temperature graphite materials [1,2]. In the present study, coal tar and petroleum pitches received from five suppliers were readily differentiated using three distinct methods of polymer analysis: dynamic headspace and pyrolysis gas chromatography mass spectrometry (DH/GC/MS and P/GC/MS, [3]), and pyrolysis mass spectrometry (P/MS, [4,5]).All of these methods are appropriate for the analysisand pattern recognition study of coal and petroleum products; however, methods which incorporate GC usually necessitate long analysis times and are subject to problems associated with gas chromatographic reproducibility (e.g. active sites [6], and nonconstant retention times). As a fingerprint analysis method, P/MS is rapid and ideally suited to computerized pattern recognition study of pitches due to the simple format of P/MS data (computer tabulation of normalized intensities versus masses); whereas, in the present study, the normalized peak area versus retention time data from DH/GC/MS and P/GC/MS analyses were not suitable for direct input to a computerized pattern recognition analysis program, (due to non-constant retention times and improper data format). One limitation is that P/MS does not provide positive identification of thermally evolved products (as can DH/GC/MS and P/GC/MS) due to the complexity of the spectra of a mixture of compounds produced by PIMS. Within the limits of the commercial instrumentation and computer programs used in this study, a combination of all three techniques provided the greatest capability for the differentiation of the five coal tar and petroleum based pitches studied. Experimental Dynamic Headspace and Pyrolysis Gas Chromatography Mass SpectrometryA pyrolyzer (Chemical Data Systems Pyroprobe, Model 1000) is attached to the injector assembly of a gas chromatograph/mass spectrometer (Finnigan, Model 4021). The inlet of a polyimide-coated fused silica capillary column (J&W Scientific, Model DB5-30N with bonded SE54 stationary phase at 1 pm film thickness) is directly coupled to the capillary injector. This open tubular column is reduced to 20 meters length and is 0.25 mm ID. The outlet of the column is threaded into the ion source region of the mass spectrometer. At a pressure of 10 psig, helium carrier gas sweeps the pyrolyzer and enters the injector, where the flow is split so that 35 mllmin is carried off and approximately 1 ml/min enters the GC column (split = 25 mllmin, sweep = 10 mllmin). The products emerging from the column are ionized by electron impact in the EI/SEP mode with 0.5 mA of 70 eV electrons. The masslcharge range of the mass spectrometer is scanned from mlz 33 to 533 in 0.95 s with a hold at the low mass end of 0.05 between scans. The conversion dynodes are run at 3000 V and electron multiplier at 1200 V. The analysis of a typical sample is conducted in two consecutive steps: (1) analysis of volatile compounds desorbed from the ...