Analysis of the isobaric compounds propanol, acetic acid and methyl formate in humid air and breath by selected ion flow tube mass spectrometry, SIFT-MS

Abstract

Numerous analyses of exhaled breath using selected ion flow tube mass spectrometry, SIFT-MS, over the last few years have revealed the presence of volatile compounds with molecular weight 60 and the concern has been to identify which of the isobaric compounds from the set of 1-propanol, 2-propanol, acetic acid and methyl formate are present in human breath. The problem is compounded by the formation of hydrates of the characteristic primary product ions of the reactions of the H 3O + and NO + precursor ions with these compounds, this being particularly efficient for humid samples such as exhaled breath. Thus, the resulting product ion spectra are complex and choices have to be made as to which of the characteristic product ions and their hydrates can best be used for the quantitative analyses. To facilitate this choice for the particular problem of identifying and quantifying the four aforementioned isobaric compounds, a study has been made of the ion chemistry of H 3O + and NO + with the two propanol isomers, acetic acid and methyl formate for increasing sample humidity up to that of exhaled breath, which is about 6% by volume. The problems involved in the separate analysis of propanol have been met and solved by previous SIFT-MS studies and now the present study has revealed how acetic acid and methyl formate can be separately identified in a humid mixture using NO + precursor ions only. Following this work, the kinetics database entries for the SIFT-MS analyses of these compounds in breath have been constructed and the analysis of the exhaled breath of five healthy volunteers showed that, in addition to the propanol isomers, acetic acid was present at levels typically within the range from 30 to 60 parts-per-billion by volume and that methyl formate was not present above the limit of detection.