Selected ion flow tube mass spectrometry, SIFT-MS, is a non-separative method for direct quantitative analyses of volatile compounds, VOCs, in air and humid breath based on chemical ionization. Selected reagent ions, either H3O+, NO+ or O2 + (non-reactive with major components of air), ionize analyte molecules during a defined time in a flow tube by ion-molecule reactions thus producing analyte ions that are characteristic of the neutral analyte VOCs. Concentrations can be calculated in real-time from the ion count rates. Direct on-line analysis of single or multiple breath exhalations or off-line analysis of breath samples collected into bags can be performed. Several volatile breath metabolites have been quantified by SIFT-MS, including ammonia, acetone, hydrogen cyanide, alcohols, pentane, acetic acid, methane, and sulphur compounds. Their potential as biomarkers is discussed.

• Keywords
• Biomarkers, Breath VOCs, Metabolites, SIFT-MS,
• Publication type
• Journal Article MeSH
• Review MeSH

Selected ion flow tube mass spectrometry, (SIFT-MS), involves the partial conversion of mass-selected precursor ions to product ions in their reactions with the trace gases in an air sample that is introduced into helium carrier gas in a flow tube. The precursor and product ions are then detected and counted by a downstream quadrupole mass spectrometer. Quantification of particular trace gases is thus achieved from the ratio of the total count rate of the product ions to that for the precursor ions. However, it is important to appreciate that in this ion chemistry the light precursor ions (usually H3O+ ions) are invariably converted to heavier product ions. Hence, the product ions diffuse to the flow tube walls more slowly and thus they are more efficiently transported to the downstream mass spectrometer sampling orifice. This phenomenon we refer to as diffusion enhancement. Further, it is a well-known fact that discrimination can occur against ions of large mass-to-charge ratio, (m/z), in quadrupole mass spectrometers. If not accounted for, diffusion enhancement usually results in erroneously high trace gas concentrations and mass discrimination results in erroneously low concentrations. In this experimental investigation, we show how both these counteracting effects can be accounted for to increase the accuracy of SIFT-MS quantification. This is achieved by relating the currents of ions of various m/z that arrive at the downstream mass spectrometer sampling orifice disc to their count rates at the ion detector after mass analysis. Thus, both diffusion enhancement and mass discrimination are parameterized as a function of m/z and these are combined to provide an overall discrimination factor for the particular analytical instrument.

• Publication type
• Journal Article MeSH