Effect of water vapour pressure on monoterpene measurements using proton transfer reaction-mass spectrometry (PTR-MS)

Abstract

The effects of water vapour pressure (WVP) on the fragmentation of seven monoterpene and related C 10 volatile organic compounds (VOCs) in the drift tube of a proton transfer reaction-mass spectrometer (PTR-MS) were investigated. In addition, the combined effects of varying WVP and the ratios of electric field strength to number density of the buffer gas ( E/ N) were investigated in detail for three of these compounds, the monoterpenes α-pinene and sabinene plus the related C 10 VOC p-cymene. Under normal operating conditions ( E/ N = 124 Td), WVP affected the fragment patterns of all compounds with the exception of β-pinene and the three oxygenated C 10 VOCs. WVP had a significant effect on the fragment patterns of α-pinene and sabinene at the lower E/ N ratios (around 80 Td) but had little effect on fragmentation towards the higher E/ N ratios used here (∼142 Td). On the other hand, p-cymene fragmentation was most affected by WVP under normal operating conditions. PTR-MS sensitivity towards the three compounds was also considered under three conditions where reaction was assumed with (1) H 3O + only; (2) H 3O + and H 3O +H 2O; and (3) H 3O +, H 3O +H 2O and H 3O +(H 2O) 2. Our results indicate that α-pinene and sabinene react not only with H 3O + and H 3O +H 2O via direct proton transfer but also with H 3O +(H 2O) 2 via ligand switching. p-Cymene seems to react only with H 3O + via direct proton transfer and with H 3O +H 2O via ligand switching. It is speculated that the WVP effect on fragmentation results from the differing abundances of hydrated reagent ions which causes different frequencies of individual reactions to occur, thus, determining how ‘soft’ the overall reaction is. These results also indicate that under normal conditions, a correction should be made for WVP if the concentration of p-cymene in air samples is to be determined from the single ion signal of either protonated molecular ions or the most dominant fragment ions.