Selected ion flow tube mass spectrometry, SIFT-MS, has been widely used in industry and research since its introduction in the mid-1990s. Previously described quantitation methods have been advanced to include a gas standard for a more robust and repeatable analytical performance. The details of this approach to calculate the concentrations from ion-molecule reaction kinetics based on reaction times and instrument calibration functions determined from known concentrations in the standard mix are discussed. Important practical issues such as the overlap of product ions are outlined, and best-practice approaches are presented to enable them to be addressed during method development. This review provides a fundamental basis for a plethora of studies in broad application areas that are possible with SIFT-MS instruments.

• Keywords
• SIFT-MS, VOCs, ion molecule reactions, negative reagent ions, nitrogen carrier gas, quantitation, selected ion flow tube mass spectrometry, volatile organic compounds,
• Publication type
• Journal Article 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