Analysis of Volatile Esters and Alkanoic Acids by an Atmospheric Pressure Corona Discharge Ionization Collision-Induced Dissociation Mass Spectrometry in Positive-Ion Mode: Ionization, Fragmentation Patterns, and Discrimination between Isomeric Compounds.

The ionization and fragmentation patterns of 24 compounds with elemental composition of CmH2mO2, including isomeric esters and alkanoic acids, were investigated by atmospheric pressure corona discharge ionization collision-induced dissociation (CID) mass spectrometry in the positive-ion mode. All compounds were ionized as protonated molecules and NH4+ adducts. In addition, fragment ions were observed in mass spectra of esters other than methyl esters, which are owing to the dissociation of the alkyl group in the alcohol side from the protonated molecules. In CID spectra, protonated alkanoic acids/methyl esters split off H2O/CH3OH and CO or the alkyl group in the acid side, depending on the carbon chain length. Protonated esters (other than methyl esters) mainly fragmented the alkyl group in the alcohol side. These general rules on ionization and fragmentation patterns can provide relevant information on the discrimination of isomers.