Metabolomics approach to the identification of emerging unknown contaminants

Another sophisticated means of identification of unknown contaminants is based on a metabolomics-like approach. Again the sample should be extracted and purified in such a way to maintain all relevant contaminant classes of interest. Next the sample, still being a highly complex mixture, is analysed by a highresolution chromatographic technique such as ultra performance liquid chromatography (UPLC), GC or even comprehensive GC x GC, combined with a sensitive full-scan MS technique such as TOF, ion trap or FT Orbitrap. The data from the sample replicates are aligned in the retention time domain and compared with the data from a set of reference sample replicates. Finally uni- or multi-variate statistics are applied in order to assess the significant differences between the suspect sample and the regular reference situation. By using appropriate data analysis software contaminants could be retrieved automatically from an oily preparation, drinking water and grass samples [52].

Successful application of this approach requires the availability of a clean, stable and highly reproducible chromatography system, including reproduci-bility in solvent and column impurities. Moreover, the reference situation is crucial, requiring a more or less reproducible sample matrix background. For homogeneous samples such as drinking water this can be relatively easily achieved but an adequate reference for inherently inhomogeneous samples having a fluctuating composition such as feed will be very difficult to obtain. Last but not least, intelligent data analysis software is required which can automatically correct for small changes in retention time and/or mass accuracy and is capable of keeping the underlying raw data accessible for retrospective analysis, reprocessing and evaluation in its original software format. Note that a GC x GC/TOFMS analysis creates a challenging four-dimensional dataset. Ignoring all these factors will yield many irrelevant data from system impurities, bulk composition changes, etc., and probably not identify the unknown emerging contaminant.

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