AN OBJECTIVE METHOD FOR COMPARING UNRESOLVED COMPLEX MIXTURE (UCM) 'HUMPS' IN GAS CHROMATOGRAMS

摘要

Petroleum and many of its products often contain complex mixtures of hydrocarbons, which are only partially resolved when analysed by gas chromatography (GC), producing chromatograms with distinctive 'humps', usually referred to as 'unresolved complex mixtures' (UCM).1 In such chromatograms, additional resolved compounds, such as n-alkanes, appear as rider peaks on the UCM. The shape of an UCM on the horizontal (time) axis is determined by the relative proportions of the individual compounds which contribute to the UCM, but conventional GC methods, such as GC-flame ionisation detection (GC-FID), are not capable of identifying and quantifying these components. Despite considerable progress having been made in determining UCM composition through prior separation of components into different compound classes by liquid chromatography, and the use of gas chromatography-mass spectrometry (GCMS) in selected ion mode and two-dimensional gas chromatography (GC x GC),2 complete characterisation of many UCMs has still not been achieved. However, consideration of UCM shapes obtained from different samples, without identification of the components, may be useful in the comparison of environmental samples to establish if petroleum contaminants had originated from different sources or had received differing degrees of weathering. Whilst obvious differences in the retention times of the UCM maxima clearly indicate differences in their composition,1 visual comparisons of the UCM shapes are more difficult to make where samples have many rider peaks and similar UCM retention times. This paper describes a method for comparing the shapes of UCMs from different samples.