Matrix-assisted laser desorption/ionization Fourier transform mass spectrometric analysis of oxygenated triglycerides and phosphatidylcholines in egg tempera paint dosimeters used for environmental monitoring of museum display conditions.

Oxidative changes in triacylglycerols and diacylphosphatidylcholines in egg tempera paint strips are used for chemical dosimetry of the quality of the museum environment. High-resolution matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS) was used as a rapid method for the determination of the exact elemental composition of the alteration products from diacylphosphatidylcholines and triacylglycerols. Light exposure of the egg tempera paints yields oxygenated diacylphosphatidylcholines and triacylglycerols. In the latter multiple incorporation of oxygen was observed as a recurring mass difference of 15.995, the exact atomic mass of oxygen. Owing to the high resolution of the FTMS data (routinely 20 000 at m/z 1000 in broadband mode), oxidation products with different elemental compositions but identical nominal mass could be distinguished. Products of oxidative cleavage of triacylglycerols were observed in samples exposed for longer times. The relative intensities of the peaks of singly and multiply oxygenated triacylglycerols were used to derive the degree of oxygenation of the egg lipids in the tempera paint dosimeters. The degree of oxygenation was found to be directly related to the light exposure time. Exposure to elevated temperature (60 degrees C) for a period of 21 days did not lead to oxygenation of the triacylglycerols and diacylphosphatidylcholines. Exposure to NO(x) and SO(2) in the dark greatly increased the degree of oxygenation. Addition of lead- or copper-containing pigments to the egg binding medium (and subsequent storage for 6 months in the dark) led to accelerated conversion of egg lipids to oxidised products.

Benzoolysis of diacylglycerophosphocholines: dephosphorylation and sequential formation of isomeric reaction products.

Benzoolysis experiments are reported in which diacylglycerophosphocholine is heated at 100 degrees C with benzoic anhydride for variable periods of time. It is shown that more than 90% of the phospholipid is dephosphorylated after 5 h of heating. Lipid extracts of the reaction mixture contained 1,2- and 1,3-diacylglycerobenzoate and 1,2- and 1,3-diacylglycerol in nearly constant isomer ratios of about 3:1 and 1:2, respectively, independent of the heating and extraction time. The total amount of isomeric diacylglycerobenzoates increased more slowly with increasing heating time that corresponded with the dephosphorylation rate, complete benzoylation being attained only after a 15 h heating period. The total amount of isomeric diacylglycerols went through a maximum after about 4 h and vanished after 15 h of heating. Addition of 4-dimethylaminopyridine subsequent to the heating period resulted in rapid and complete benzoylation of dephosphorylated phospholipid. However, the ratio of 1,2- to 1,3-diacylglycerobenzoate then found in the lipid extract depended on heating time, changing from less than 1:1 to about 3:1 upon an increase of heating time from 1 to 15 h. The results are interpreted in terms of two consecutive reactions. In a relatively fast first step, a dephosphorylated intermediate is formed, which in the molten benzoic anhydride, is slowly benzoylated. The intermediate yields diacylglycerols upon extraction in the absence of 4-dimethylaminopyridine and diacylglycerobenzoates upon extraction in the presence of 4-dimethylaminopyridine.