Improvement of the saponification process for mineral oil analysis using microwave-assisted saponification/extraction

Mineral oil hydrocarbons are a group of contaminant considered possibly toxic, especially the MOAH fraction composed of aromatic compounds, mainly alkylated. The analytical determination is however linked to a lot of variability among them mainly the sample preparation, when enrichment and purification is needed, and the chromatogram interpretation. Recently, work has been done to automate more the chromatogram interpretation thus reducing the variability. However, regarding the sample preparation, two interlaboratory trials done last year (JRC for infant formula and DGF for oil and fat) pointed to the difficulty of having a low sensitivity, the LOQ reaching 2 mg/mL for the MOAH fraction in fat samples due to a lack of reproducibility. One of the issue highlighted was the discrepancy in the recovery of the internal standards. Indeed, in both cases, TBB has a greater recovery than the MN standards with an average ratio of 1.15 instead of 1. The problem was that in one case, the recovery results were better using TBB and in the other using 2MN, suggesting that the contamination itself could have different recovery depending on its composition. In order to reduce this variability, a new saponification method was developed using microwave-assisted saponification/extraction and testing the partition of the internal standards in different matrices and conditions. In conclusion, the ratio discrepancy was reduced from between 1.11 and 1.18 to between 1.02 and 1.08 for all the matrices tested.