Ozone in ambient air as a source of adventitious oxidation. A mass spectrometric study

This paper reports on an oxidation of biomolecules that can occur during sample preparation for analysis by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). In measurements performed over several years, the oxidation artifacts were detected on methionine and tryptophan side chains in peptides containing these residues, as well as at the carbon-carbon double bonds of unsaturated phospholipids. A distinct hallmark of the artifacts is their seasonal nature: the highest levels of oxidation are typically seen in samples prepared for MALDI analysis during daytime hours and warmer months. The artifacts appear strongest when samples are allowed to air-dry in the absence of MALDI matrix and can be minimized when sample preparation is carried out in a nitrogen atmosphere or under vacuum. It is proposed that ambient ozone (originating from outdoor air taken into the laboratory through the air handling system) is acting during the obligate MALDI sample dry-down and matrix crystallization steps. To confirm the hypothesis, ozonolysis experiments as well as direct measurements of ambient ozone levels were performed. The hypothesis is reminiscent of the recent finding that ozone is the source of degradation of self-assembled alkanethiol monolayers that are exposed to ambient air (Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502-4513). Thus, caution should be taken in handling ozone-reactive substances that are placed thinly on surfaces and exposed to ambient air, whether for MALDI analysis or for other increasingly popular applications involving the microdeposition of liquids.