​Hydrocarbon characterisation via GCxGC-MS/FID
In the conventional, one-dimensional analysis of hydrocarbons using online coupled HPLC-GC-FID, substances are primarily separated according to their volatility. However, the limited resolution is leading to unresolved humps of complex substance mixtures in the chromatogram. A clear distinction of mixed hydrocarbon contaminations from different sources (e.g. mineral oil-based lubricants, oligomers from polyolefin packaging and resin oligomers from adhesives) or in the presence of biogenic interfering substances (e.g. terpenes) can usually not, or only inadequately, be realised using one-dimensional analysis.
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Two-dimensional gas chromatography (GCxGC) is the state-of-the-art technique for separating complex substance mixtures by implementing a second analytical separation column [1]. In the first dimension (x-axis), the usual GC separation occurs due to the different volatility of the substances (from volatile to semi-volatile). In the second dimension (y-axis), separation occurs according to polarity (from polar to apolar). The result of the analysis is a chromatographic plane (plot) in which substances are not represented as peaks, but as three-dimensional blobs whose concentration is visualised by means of colour coding (from light blue to red).
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Individual hydrocarbon subgroups are eluting in different regions of the GCxGC plot due to their specific properties. GCxGC analysis can therefore provide valuable information about the origin of hydrocarbon contamination or can contribute to the characterisation of hydrocarbon products. Among other things, a reliable distinction can be performed between the following hydrocarbon subgroups:
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MOSH subgroups: n-alkanes, iso-alkanes, cycloalkanes, hopanes, steranes
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MOAH subgroups: alkylated mono-, di-, tri- and poly-aromatics (3-7 ring MOAH) [2]
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Oligomers from polyolefins: POSH (POSHoc, POSHcy), POMH, POHox
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Oligomers from poly-alpha-olefins: PAO
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Oligomers from resins (ROSH, ROAH)
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Biogenic terpenes, squalene, carotenes
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Detection in GCxGC analysis is performed either using a flame ionisation detector (FID) for quantification or using a time-of-flight mass spectrometer (TOF-MS) for identification and confirmation.
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GCxGC analysis is a comprehensive method for characterisation of complex substance mixtures. We therefore provide a detailed explanation and interpretation of the GCxGC plots.
Comprehensive gas chromatography (GCxGC)
Complex hydrocarbon mixtures can be chromatographically separated more specifically with the help of two-dimensional gas chromatography (GCxGC). We use the so-called reversed setup for our hydrocarbon analyses. Saturated hydrocarbons elute in the upper half of the GCxGC plot and aromatic hydrocarbons in the lower half. To avoid overlapping regions, we usually analyse the individual fractions (MOSH fraction/MOAH fraction) pre-separated by HPLC according to conventional MOSH/MOAH analysis.
​Saturated hydrocarbons: MOSH
Saturated hydrocarbons of mineral oil origin (MOSH) consist predominantly of unbranched and branched alkanes and cycloalkanes (naphthenes) and form corresponding cloud formations in the GCxGC plot. Typical marker substances (e.g., pristanes, phytanes, stearanes and hopanes) confirm the mineral oil origin. The further distinction between MOSH and biogenic or synthetic saturated hydrocarbons (PAO, POSH, ROSH) can be achieved by the individual cluster formation and the elution region.
Aromatic hydrocarbons: MOAH
Because of their toxicological concern [3], the analytical confirmation of mineral oil aromatic hydrocarbons (MOAH) and their characterisation regarding their number of aromatic rings is of major importance. In particular, the determination of MOAH with three or more aromatic rings (3 - 7 ring MOAH) is in the main focus due to their potential genotoxicity and carcinogenicity [3]. Using GCxGC analysis, aromatic hydrocarbons can be reliably separated according to their number of rings [2]. In addition to the predominantly alkylated MOAH also non-alkylated polycyclic aromatic hydrocarbons (PAH) can be determined simultaneously via GCxGC-MS/FID
Biogenic and synthetic hydrocarbons
Due to their characteristic composition, non-MOH hydrocarbons form typical clusters and patterns in the GCxGC plot, which enable a clear distinction from MOSH or MOAH as well as their separate quantification.
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We have already spent many years on the interpretation of GCxGC data sets and have built up an extensive database of biogenic and synthetic hydrocarbon types, which enables a reliable characterisation of your sample.
References
[1] K. Grob, M. Biedermann (2015)
Comprehensive two-dimensional gas chromatography for characterizing mineral oils in foods and distinguishing them from synthetic hydrocarbons. J. Chromatogr. A 1375 (2015) 146–153
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[3] EFSA Panel on Contaminants in the Food Chain (CONTAM) (2023)
Update of the risk assessment of mineral oil hydrocarbons in food. EFSA Journal 2023;21(9):8215.
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​DOI: 10.2903/j.efsa.2023.8215
[2] M. Biedermann, A. Eicher, T. Altherr, G. McCombie (2022)
Quantification of mineral oil aromatic hydrocarbons by number of aromatic rings via comprehensive two-dimensional gas chromatography: First results in food. J. Chromatogr. Open 100072
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​DOI: 10.1016/j.jcoa.2022.100072