Laboratory studies in support of the detection of sesquiterpenes by proton-transfer-reaction-mass-spectrometry

The effects of the ratio of the electric field strength to the buffer gas number density (E/N) in the drift tube of a proton transfer reaction mass spectrometer on the product ion distributions of the sesquiterpenes β-caryophyllene, α-humulene, α-cedrene and longifolene have been investigated.Chemical ionization of the sesquiterpenes resulted in important fragmentation of the nascent excited ion/molecule complex at the highest E/N values. The most important fragment ions were common to all sesquiterpenes and therefore cannot be used as fingerprints for specific isomers. The yield of the protonated molecule increased on average by a factor 1.6 by decreasing E/N from 140 to 80 Td. Taking into account the influence of E/N on the reaction time and on the reactant ion mobility, it is estimated that this decrease in E/N may lead to an overall increase in the PTR-MS detection sensitivity of sesquiterpenes (based on the ion signal at m/z 205) by a factor 3.5.Product ion distributions of α-cedrene and longifolene have also been determined at different water vapour pressures. No substantial influence of the water vapour pressure on the product ion yields was observed, which is an advantage when quantifying sesquiterpenes by PTR-MS in samples of varying relative humidity.