Usefulness of field desorption mass spectrometry in determining molecular masses of carotenoids, natural carotenoid derivatives and their chemical derivatives

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Abstract
Field desorption (FD) mass spectrometry was applied to the determination of the molecular masses of carotenoids, natural carotenoid derivatives and their chemical derivatives. All the carotenoids examined gave the molecular ion as the base peak with negligible fragment ions. Carotenoid glucoside and its fatty acid monoester were successfully determined without acetylation, whereas carotenoic acids (carboxylate and sulphate) needed to be converted into methyl esters prior to analysis. The applicable ranges of molecular masses and polarity were very wide. In addition, carotenoid glycoside gave only [M]+˙ without [M + H]+˙ and [M + cation]+˙. The numbers of carbonyl groups, primary and/or secondary hydroxyl groups and total hydroxyl groups could be directly determined according to the increase in mass units of the carotenoids after chemical reduction, acetylation and trimethylsilylation, respectively. Owing to the negligible fragment ions, FD analysis was also suitable for carotenoids containing small amounts of impurities or other carotenoids. Hence this technique is useful for determining the molecular masses of carotenoids and the number of modifiable groups in carotenoids.