Comprehensive two-dimensional gas chromatography for fast characterization of oil contamination

Most analytical methods for the characterization of oil contamination rely on gas chromatography. Capillary GC offers a high separation power (peak capacity typically 100 - 500), and by using a boiling point column a volatility based distribution is obtained. However, petrochemical mixtures like mineral oil easily contain more then 5000 compounds, and capillary GC does not succeed in separating such mixtures up to the level of individual components. Analytical results are therefore limited to a classification into boiling point (or equivalent-carbon) fractions. Recent methods refined the classification by fractionation in aromatic and aliphatic hydrocarbons prior to GC-analysis (TPH method, Weisman 1998), and by incorporating water solubility (TTE and OK method, for a detailed description we refer to the Consoil 2005 lecture by R. Lookman 'Oil characterization: assessment of composition, degradation and remediation potential of total petroleum hydrocarbons in soil'). However, besides analytical difficulties and time consuming protocols related with these methods, the resulting classification still remains coarse and the various groups are not univocally chemically identified. Recently, comprehensive two-dimensional GC (GCXGC) was introduced. This technique combines two GC-columns with different separation mechanisms. The two columns are connected by a modulator, a device that traps, focuses and re-injects the peaks that elute from the first column into the second column. Each peak from the first column (representing a number of overlapping peaks) is further separated on the second column. The very high separation power of GCXGC (peak capacity 2000 - 10000) is ideal for the analysis of complex mixtures like mineral oil. Another important feature is the generation of structured chromatograms. Isomers appear as distinct groups in the chromatogram as a result of their similar interaction with the 2D-column phase. For chemical identification comprehensive GC can be coupled to a mass spectrometer. However, the peaks eluting from the second dimension column are very narrow (typically 100-200 ms) and only a time-of-flight MS (TOFMS) can deliver the high acquisition rates necessary for quantitative description of the peaks. In the present study comprehensive GC was evaluated as a tool for fast oil characterization in the context of soil remediation projects. For this routine application TOFMS was considered too expensive, and therefore a method was developed based on GCXGC/FID. The method was applied to oil-polluted soil samples and the results were compared with the TPH method. Only the extractable petroleum hydrocarbons (from n-C8 to n-C40) were quantitatively analyzed. Compared with the TPH method, the major advantage of GCXGC is that a more detailed chemical description of the oil pollution is obtained. The method is not limited to 'mineral' oil but can be applied to different types of pollution, including polar components