Quantitative Analysis of Aromatic Hydrocarbons in Complex Hydrocarbon Mixtures by High Resolution Capillary Gas Chromatography

Natural and synthetic hydrocarbon mixtures are complex samples containing hundreds of different aliphatic and aromatic components in widely differing concentrations. The number of components depends on a group-type composition and the range and distribution of boiling points of the components present in the mixture . Hydrocarbon mixtures are characterised in two ways, by structural group analysis and boiling range determination (by distillation or gas-chromatographically simulated distillation). The principál techniques employed for determining the content of paraffins, naphthenes, and aromatic compounds are mass spectrometry (MS) , high performance liquid chromatography (HPLC)", and capillary-column gas chromatography". Whereas the accuracy of MS methods for PNA (paraffin, naphthene, aromatic) determination has been shown to be sufficient for most applications, the technique does not provide information about individual components. The quantitative results obtained by HPLC are not always acceptable because of the variation of response factors for the given hydrocarbon in the sample matrix of various hydrocarbons. The HPLC technique also lacks the capacity to furnish carbon number distribution and component identification. High resolution capillary gas chromatography (HRCGC) is the most useful generál method for the analysis of complex hydrocarbon mixtures with the ultimate aim of complete component analysis. The disadvantage of this method is that every peak must be identified, or at least classified, according to the chemical group to which it belongs. Peak identification in a chromatogram is performed on the basis of retention data and the information from gas chromatography coupled with mass spectrometry (GC-MS)". Quantitation in the capillary GC by fláme ionisation detection (FID) is more accurate than other techniques. The particular advantage of GC analysis is that the quantitative response of the FID is approximately the samé for equal weights of any hydrocarbon, so that, to a first approximation, relative peak areas can be ušed directly for the determination of weight percent values ' . Aromatic hydrocarbons are an important constituent of various petroleum products and a detailed information on their composition in feed materials, intermediates, and commercial products is required for process development and quality control programs. Low molecular weight alkylbenzenes (C 7-C 1 5) represent the whole or main part of the monoaromatic fraction which has been most frequently studied in hydrocarbon mixtures. The identification and determination of aromatic compounds is also important in environmental analysis and geochemistry. Aromatic hydrocarbons in hydrocarbon samples may generally be analysed directly by HRGC, under isoťhermal and/or temperature-programmed conditions, on a single column or by multidimensional GC (ref.). The concentrates of aromatic compounds from liquid chromatography fractions can be successfully analysed using a single column containing a stationary phase of polarity suitable for obtaining the optimum resolution of the compound of interest. The author shows in review papers", that the detailed analysis (separation, identification, and quantitation) of individual aromatic hydrocarbons in hydrocarbon mixtures has been most frequently performed with single columns. Highly efficient non-polar phases are applied when information is required about all components of the mixture. Moderately polar or highly polar stationary phases háve been employed for the estimation of aromatic com-