Aromatic ring separation via online LC-LC-GC-FID
Aromatic hydrocarbons (MOAH fraction) are considered to be contaminants of concern due to their potential human toxicity. According to the EFSA assessment updated in 2023 [1], the MOAH with three or more aromatic rings (3 - 7 ring MOAH) are of particular relevance, as they may have DNA-damaging and carcinogenic effects. Two-dimensional gas chromatography (GCxGC) was established as the reference method for characterising MOAH with regard to the number of aromatic rings. With the help of conventional HPLC-GC-FID routine analysis, however, it is not possible to differentiate MOAH according to this important aspect.
Considering published preliminary work [2], we have developed an alternative analytical method for the advanced characterisation of aromatic hydrocarbons regarding their ring number and published the results 2024 in a peer-reviewed journal [3]. Briefly, the conventional online HPLC-GC-FID was extended by an additional, aromatic-specific separation column enabling a further separation of MOAH into the mono- and di-aromatics fraction (MDAF) and the tri- and poly-aromatics fraction (TPAF). Based on this online coupled LC-LC-GC-FID, the differentiation of positive MOAH findings can also be performed.
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The MDAF/TPAF approach can provide valuable additional information during the characterisation of petrochemical products and printing inks. The possibility of false-positive findings for 3-7 ring MOAH in specially refined products is considerably reduced compared to the established GCxGC analysis [3].
MOAH subfractionation using advanced LC-LC-GC-FID
We developed an instrumental method for the automated extension of classic MOSH/MOAH analysis using HPLC-GC-FID [3]. After the already established silica gel separation of the hydrocarbons into the MOSH and MOAH fractions on the first column, the latter is transferred to an aromatics-specific second separation column and separated into the mono- and di-aromatics fraction (MDAF) and the tri- and poly-aromatics fraction (TPAF). The specific separation principle of this innovative sub-fractionation is based on the concept of donor-acceptor complex chromatography (DACC).
​Characterisation of petrochemical products, printing inks & contaminated foodstuffs
The advanced LC-LC-GC-FID analysis enables the reliable differentiation of aromatic hydrocarbons into 1-2 ring MOAH (MDAF) and 3-7 ring MOAH (TPAF) according to their toxicological potential as demanded by the EFSA [1]. Our examples show a batching oil, as used for softening jute bags, with an MDAF-TPAF ratio of approx. 4:1 and a contaminated rice sample with an MDAF-TPAF ratio of approx. 7:1.
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As part of the compliance assessment of printing inks and printed FCMs, TPAF analysis can be used to ensure the requirements of the French decree of 13 April 2022 on the restriction of the use of mineral oils in printing inks [4]. Accordingly, printing inks and printed products intended for the public must not contain 3-7 ring MOAH above 1 mg/kg.
Verification via GCxGC-MS/FID
The advanced LC-LC-GC-FID method for MDAF/TPAF was compared with the established GCxGC-MS/FID analysis for method validation. For most samples, results were well in line regarding 1-2 ring and 3-7 ring MOAH, respectively. Furthermore, the LC-LC-GC-FID approach showed significant advantages for samples with partially hydrogenated aromatic hydrocarbons, for which GCxGC analysis tends to systematically overestimate the ≥3 ring aromatics [3]. LC-LC-GC-FID analysis can also be used as a valuable analytical tool to further confirm positive GCxGC results.
References
[1] 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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[3] M. Lommatzsch, M. Eckardt, J. Holzapfel, S. Säger, T. Simat (2024)
Advanced separation of mineral oil aromatic hydrocarbons by number of aromatic rings using donor-acceptor-complex chromatography to extend on-line coupled liquid chromatography-gas chromatography. Journal of Chromatography A. 1715: 464600.
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​DOI: 10.1016/j.chroma.2023.464600
[2] M. Koch, E. Becker, M. Päch, S. Kühn, E. Kirchhoff (2020)
Separation of the mineral oil aromatic hydrocarbons of three and
more aromatic rings from those of one or two aromatic rings. Journal of Separation Science. 43(6):1089-1099.
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​DOI: 10.1002/jssc.201900833
[4] Decree of the French Ministry of the Environment on the restriction of MOH in printing inks (2022)
Arrêté du 13 avril 2022 précisant les substances contenues dans les huiles minérales dont l’utilisation est interdite sur les emballages et pour les impressions à destination du public. JOURNAL OFFICIEL DE LA RÉPUBLIQUE FRANÇAISE.
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