Identification of Monoterpene Isomers by Collision-Induced Dissociation Mass Spectrometry (CID-MS)

Abstract

Scent affects our mind and body, so that it plays an important role in cosmetics. It is difficult to understand which components in scent affect dominantly, because scent contains many components with low concentrations. Gas chromatography mass spectrometry (GC/MS) has been widely used for scent analysis: however, this method requires sample preparation and accumulation over some periods of time, meaning that we cannot trace instantaneous change in the concentration of each component. Furthermore, since scent components contain many structural isomers as represented by monoterpenes, it is also important to distinguishably measure isomers in real time in scent research. In this study, atmospheric pressure corona discharge ionization (APCDI) coupled with collision-induced dissociation mass spectrometry (CID-MS) was used to develop a method to distinguish monoterpene isomers (C₁₀H₁₆) in real time. Three monoterpene isomers（i.e., α-pinene, limonene and myrcene） were protonated instantaneously, and then were selected as precursor ions at product-ion scan and as monitoring ions at precursor-ion scan. In the case of product-ion scan, similar product ions (e.g., C₆H₉⁺ at m/z 81 and C₇H₁₁⁺ at m/z 95) were observed for all the analytes. These results are likely attributable to the isomers having similar structure to protonated molecules. In contrast, precursor-ion spectra showed a specific pattern for each isomer, which can result from the difference in formation of covalent-bonded and/or hydrogen-bonded dimers depending on isomeric structure. Consequently, it was found that precursor-ion scan is useful for identification of monoterpene isomers in real time.