Gas phase studies on terpenes by ion mobility spectrometry using different atmospheric pressure chemical ionization techniques

The ionization pathways and drift behavior were determined for sets of constitutional isomeric and stereoisomeric non-polar hydrocarbons (unsaturated monocyclic terpenes, unsaturated and saturated bicyclic terpenes) using ion mobility spectrometry (IMS) with different techniques of atmospheric pressure chemical ionization (APCI) to assess how structural and stereochemical differences influence ion formation. Depending on the structural features, different ions were observed for constitutional isomers using ion mobility spectrometry with photoionization (PI) and corona discharge (CD) ionization. Photoionization provides ion mobility spectra containing one major peak for saturated compounds while at two peaks were observed for unsaturated compounds, which can be assigned to product ions related to monomer and dimer ions. However, differences in relative abundance of product ions were found depending on the position of the double bond. Although IMS using corona discharge ionization permits the most sensitive detection of non-polar hydrocarbons, the spectra are complex and differ from those obtained using photoionization. Additional cluster ions and fragment ions were detected. Only small differences in ion mobility spectra were observed for the diastereomers while the enantiomers provide identical spectra. The structure of the product ions formed was checked by investigations using the coupling of ion mobility spectrometry with mass spectrometry (IMS-MS).