Addition of a fast GC to SIFT-MS for analyses of individual monoterpenes in mixtures

Soft chemical ionization mass spectrometry (SCI-MS) techniques can be used to accurately quantify volatile organic compounds (VOCs) in air in real time; however, differentiation of isomers still represents a challenge. A suitable pre-separation technique is thus needed, ideally capable of analyses in a few tens of seconds. To this end, a bespoke fast GC with an electrically heated 5 m long metallic capillary column was coupled to a selected ion flow tube mass spectrometry (SIFT-MS) instrument. To assess the performance of this combination a case study of monoterpene isomer (C10H16) analyses was carried out. The monoterpenes were quantified by SIFT-MS using H3O+ reagent ions (analyte ions C10H17+, m/z 137, and C6H9+, m/z 81) and NO+ reagent ions (analyte ions C10H16+, m/z 136, and C7H9+, m/z 93). The combinations of the fragment ion relative intensities obtained using H3O+ and NO+ were shown to be characteristic of the individual monoterpenes. Two non-polar GC columns (Restek Inc.) were tested: the advantage of MXT-1 was shorter retention times whilst the advantage of MXT-Volatiles was better temporal separation. Thus, it is possible to quantify components of a monoterpene mixture in less than 45 s by the MXT-1 column and to separate them in less 180 s by the MXT-Volatiles column. As an illustrative example, the headspace of three conifer needle samples was analysed by both reagent ions with both columns showing that mainly α-pinene, β-pinene and 3-carene were present.