Pulsed Direct-Current Glow Discharge Mass Spectrometry for Fragmentation Control and Structural Classification of Sesquiterpenes via 2D Isotope-Ratio Mapping

Structural characterization of volatile terpenoids, including sesquiterpene isomers represented by the chemical components of Cannabis sativa L. remains a significant challenge in forensic drug detection. In this study, a novel classification principle for these compounds is presented using direct current pulse discharge soft plasma ionization mass spectrometry. By modulating the discharge duty ratio (D), a method has been developed to actively control the transition between maintaining molecular-related information for individual characterization and generating functional-group-specific common indicators. At a low D (5 %), fragmentation was significantly suppressed compared to that at a high D (50 %), leading to the stable formation of molecular-related ions for oxygenated sesquiterpenes such as farnesol. In contrast, at D = 50 %, a characteristic fragment at m/z 117 ([C5H9 + O3]+) was observed as the base peak specifically for oxygenated sesquiterpenes. This phenomenon is attributed to a unique fragmentation pathway (Path A), reported here for the first time to our knowledge, involving the addition of neutral ozone—whose density is maximized under a reduced pressure of 2.5 kPa—to a hydroxy-initiated allyl cation fragment. Our analysis achieved high sensitivity with limits of detection ranging from 0.2 ppbv to 5.3 ppbv. This remarkable sensitivity enabled the quantitative evaluation of low-abundance natural isotopic ratios, facilitating the visual and statistical differentiation of oxygenated species from hydrocarbons like longifolene through a two-dimensional isotope correlation map. This approach, when combined with chromatography, would provide a practical classification scheme that allows for the reliable characterization of specific chemical structures using a simplified instrumental setup.