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S. Drozdova, W. Ritter, B. Lendl, E. Rosenberg:
"Challenges in the determination of total petroleum hydrocarbons (hydrocarbon index) by gas chromatography";
Poster: HTC12 - 12th International Symposium on Hyphenated Techniques in Chromatography, Brügge, Belgium; 31.01.2012 - 03.02.2012.

Petroleum hydrocarbons represent one of the most important sources of large-scale environmental pollution. Accurate measurement of petroleum hydrocarbon contaminations is thus an important task which is nowadays done mostly by gas chromatographic methods, such as the standard ISO 9377-2:2000. Although a standard method should provide a sufficient degree of robustness, the correctness and precision of results for the petroleum hydrocarbon determination strongly depends on the proper choice of measurement parameters whose correct selection is left to the judgement of the analyst. The influence of these parameters on the results and what challenges the determination of TPH poses in general will be discussed.
No single method gives a precise and accurate measurement of the true total petroleum hydrocarbon (TPH) concentration for all types of contamination, because petroleum and petroleum products are very complex mixtures and different methods are designed to extract and measure slightly different subsets of petroleum hydrocarbons. The primary advantage of GC-based methods over e.g., a gravimetric method is that they do not only provide quantitative, but also qualitative information which in the ideal case even allows the understanding of the source of contamination. However, as experiments show, different types of oil lead even for equal amounts of oil in water to qualitatively and quantitatively strongly differing results.
The GC-based method ISO 9377-2:2000 for TPH determination involves a clean-up step using Florisil and sodium sulphate (anhydrous) which essentially aims at removing the polar, non-petroleum hydrocarbons of biological origin and remaining traces of water. We have shown, however, that depending on the type of oil up to 16% of the hydrocarbons are lost in this step. The recovery of sample from the clean-up step clearly depends on the chemical composition of oil sample. It can thus be concluded that there will always be a sample specific bias between the `true´ TPH value and the produced result.
As classical (one-dimensional) gas chromatography does not allow to understand which fraction of hydrocarbons are lost in the clean-up step due to the enormous complexity of the sample, we have also used comprehensive two-dimensional gas chromatography (GCxGC or 2D-GC), which provides sufficient resolution power to identify individual compound classes and to relate the losses of certain hydrocarbon fractions to the particular source of petroleum hydrocarbons.