Determination of the degree of hydrolysis of oil paint samples using a two-step derivatisation method and on-column GC/MS

An elegant method is described for the determination of the degree of hydrolysis of (unsaturated) triacylglycerols, and their oxidation products, present in linseed oil based paints. The analytical strategy, a transethylation of esterified fatty acids followed by a trimethylsilylation of free fatty acids and their salts, is first tested on reference materials comparable or identical to the compounds present in fresh and aged oil paints. Reproducibility and repeatability are examined on free- and methylated fatty acids, a sodium- and lead salt and several triacylglycerols. Unwanted trimethylsilylated products could be observed for the triacylglycerols and other esterified fatty acids up to a maximum of 6%. Free fatty acids and their salts are shown to be completely trimethylsilylated but incomplete derivatisation was observed for azelaic acid. Glycerol, liberated upon transethylation was (partially) trimethylsilylated. However, the recovery was not uniform and the results could not be used for quantitative determination of the amount glycerol in the paint sample. Studies on the influence of the pigments indigo, lead white, and prussian blue and glycerol showed there is no adverse influence on the analytic result, except for lead white and indigo doped reference material. The addition of these last compounds led to increased amounts of hydrolysed products up to 37%. The repeatability of the determination of the degree of hydrolysis within oil paint systems was tested on a number of paints. It is shown for 5-year-old test paints with a relatively homogeneous composition that the method is reproducible. For less defined paints consisting of multi-layered systems it is shown that the position and way of sampling can have a significant influence on the spread in the analytical result. This is caused by the large variations that are possible in the sampled material in these inhomogeneous systems. The degree of hydrolysis of an oil paint is taken as the average of the values obtained for azelaic-, palmitic- and stearic acid, based on the results presented. Overall, it is observed that the relative amount of hydrolysed fatty (di)acids increases in time. Surprisingly, in contrast to the results obtained on reference material, for all lead white pigmented paints lower degrees of hydrolysis were found relative to other paints from that particular set.