Concurrent use of H 3O +, NO +, and O 2+ precursor ions for the detection and quantification of diverse trace gases in the presence of air and breath by selected ion-flow tube mass spectrometry

Abstract

Selected ion-flow tube mass spectrometry, SIFT-MS, relies on chemical ionization of trace gases in air and breath samples using precursor ions that can be rapidly changed to allow the analysis of transient or limited-volume samples. The precursor ion species of choice are H 3O +, NO + and O 2 + because they do not react with the major components of air. In this article, we present the results of a study designed to investigate if consistent quantification of chemically different compounds can be realized using these three precursor ion species in the presence of humid air and breath. The neutral compounds included in the study are ammonia, dimethylamine, acetone, benzene, isoprene, ethanol, and 1-propanol. These were chosen primarily because the reactions of these compounds with the three precursor ions are representative of the diverse ion chemistry met in SIFT-MS analyses and, in addition, because of their biological and environmental significance, which renders them of particular interest. The results of this study show that consistent quantification can be achieved for all these neutral compounds when the complete ion chemistry involved in the analyses is properly accounted for. It is particularly important to account for the involvement in the ion chemistry of hydrated hydronium ions when using H 3O + precursor ions and for the presence of hydrated product ions produced when very humid samples are being analyzed. This study also indicates that all three precursor ion species are not always suitable for the analysis of particular compounds but that two of the three can always be used. The classes of compound that are best analyzed by each precursor ion species are also indicated. These results indicate the power of SIFT-MS in minimizing ambiguity and improving the accuracy of on-line, direct analysis of the trace gases in humid air and breath.