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The effects of amines on the formation of molecular adduct ions of methyl hydroperoxyoleates (OMH) in atmospheric pressure chemical ionization mass spectrometry were investigated. The short-chain primary amines such as methylamine and ethylamine were more effective than ammonia on suppressing the thermal decomposition of OMH, and provided abundant [M+H+L]+ type ions (L: each amine molecule). The secondary and tertiary amines were unsuitable for the purpose in spite of their high basicity, probably due to the steric interference. Aromatic and heterocyclic amines were also ineffective. The formation of amine adduct to OMH in liquid and/or condensed phases depends on both basicity and stereochemistry of each amine.

The method for observing molecular related ions of methyl hydroperoxyoleates (OMH) by atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) was investigated. Since thermally labile OMH is decomposed in the APCI nebulizer and/or desolvation chamber heated to a high temperature, it is difficult to observe significant peaks of their protonated molecules. The addition of ammonia water to the mobile phase (ethanol) was quite effective to suppress the thermal decomposition of OMH. As the result the mass spectra exhibited the intense peaks of ammonium adduct ions of OMH, [M+NH4]+, at m/z 346. The nebulizer temperature at 170°C and the drift voltage at 170 V were adequate for observing the molecular ion species of OMH.

Effects of additives, introduced to a sample through vapor phase, on the formation of protonated molecules and/or molecular adduct ions were investigated in liquid ionization mass spectrometry for the analysis of methyl hydroperoxyoleates. Water and formic acid having small proton affinities were useful for increasing ion abundance of protonated molecules [M+H]+. 2-Aminoethanol and pyridine having slightly greater proton affinities were useful for forming molecular adduct ions [M+H+L]+ (L: additive molecule). The use of additives, acting as matrix for producing [M+H]+ and/or as reagent for forming [M+H+L]+ in liquid ionization mass spectrometry, provided useful mass spectral characteristics of the molecular weight and structure of unknown compounds.

A mixture of isomeric methyl 9-, 10-, 11-, and 12-hydroperoxyoleates was analyzed by liquid ionization (LI) mass spectrometry. LI mass spectra of the hydroperoxides exhibited [M+H]+ peaks clearly at m/z 329, while both chemical ionization and fast atom bombardment mass spectra did not show ions directly related to the molecular weight. The fragment peaks of the hydroperoxides observed at m/z 295, corresponding to the loss of H2O2 from [M+H]+, were useful to discriminate the hydroperoxides from structurally isomeric hydroxyepoxides. In addition, several characteristic fragments were observed at m/z 173, 187, 199, 201, and 213, which were also indicative of the position of the hydroperoxy group in the hydroperoxides.