Determination of methanol-derivatives in drying oils after metal oxide-based dispersive solid phase extraction/QuEChERS clean-up.

The determination of secondary volatile degradation products in drying oil extracts is substantial to prevent formation of undesirable metal formates in paintings and/or other artefacts. This study develops a simple, cost-effective, and reliable, high-performance liquid chromatography with diode array detector (HPLC-DAD) method to determine three secondary volatile degradation products (methanol, formaldehyde, and formic acid) in drying oils, including linseed, poppy-seed, and walnut oil. Extraction of analytes was performed using QuEChERS-based procedure followed by metal oxide-based dispersive solid-phase extraction (d-SPE) clean-up and presented a good performance for all of the volatile analytes of interest with recoveries in the range of 90-120% after application of the nanostructured cerium oxide-based (CeO) and zirconia-based (ZrO) sorbents prepared by favorable and ecological-friendly methods. With a new clean-up solution for samples with high-fat content, it was possible to achieve higher recoveries than with commercial Z-Sep/C18 sorbent. In all cases, relative standard deviations (RSD) of less than 10% were achieved. No significant matrix interference was observed due to the application of effective sorbents in nanostructured form. The developed method was applied to samples of drying oils, and it was found that after storage for three months, all methanol was most likely oxidized to formaldehyde and formic acid. The concentrations of formaldehyde were in the range of 260 - 304 μg∙g, while formic acid concentrations ranged between 72 - 386 μg∙g. The highest concentration of formaldehyde (304 μg∙g) and formic acid (386 μg∙g) was found in linseed oil.