Traditionally GC-MS has been the method of choice for analyzing dilute mixtures of volatile compounds: a procedure which can be quite time-consuming. When mixtures of chemically different compounds are present, sometimes different chromatographic columns must be used to achieve analysis and in some cases a pre-concentration step is also required, greatly adding to the complexity of the measurement and the sample time.
Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) was introduced in 19961 and has greatly simplified the process for analysis as it bypasses many of the steps necessary in GC/MS. SIFT-MS is a direct analysis technique that is based on known ion-molecule reactions of mass-selected reagent ions with analytes in the sample mixture. Traditionally reagent ions in SIFT-MS were generated from moist air and were confined to the positive ions H3O+, NO+ and O2+ before mass selection and passage into a flow tube where the reagent ion – analyte reactions take place. Recently the available reagent ions has been extended to include the negative ions O–, OH–, O2–, NO2– and NO3–. These new ions are generated from a mixture of air and water vapor as for the positive reagent ions.
The benefits of negative reagent ions are very evident to those who need to monitor carbon dioxide, acid gases like hydrogen chloride, HCl, and other analytes such as Vikane, SO2F2, that are unreactive with the positive reagent ions.
Most samples analyzed using SIFT-MS are primarily comprised of ambient air, which contains residual CO2 at about 400 ppm by volume. Slow three-body reactions of CO2 with the negative reagent ions O– (CO3–), OH–(CO3H–), and O2–(CO4–) yield product signals in ambient and human breath samples. See Fig. 1.
Figure 1. Full scan mass spectra for the reagent ions (a) O-, (b) OH-, (c) O2-, and (d) NO2- are shown under three configurations: blue, with a closed sample inlet (only nitrogen carrier gas is in the flow tube); red, when ambient air is introduced; and green when human breath is sampled.
HCl is non-reactive with the positive reagent ions, but reacts rapidly with O–, OH–, O2– and NO2–. The reaction chemistry for NO2– is:
NO2– + HCl Cl– + HNO2 k = 1.4×10– cm3 s-1
A comparison between a known mixture of HCl in nitrogen and the concentrations as monitored by SIFT-MS is shown in Fig. 2.
Figure 2. Comparison of SIFT-MS measurements (using the NO2– reagent ion) with actual concentration prepared by staic dilution of a certified mixture of HCl in nitrogen.
The sensitivity of the SIFT-MS technique is defined by the rate constant of the reagent ion- analyte reaction and usually a linear correlation with concentration is obtained from pptv to about mid- ppmv. Further details may be found in ref. 2.
1. Spanel, P.; Smith, D. Selected ion flow tube: a technique for quantitative trace gas analysis of air and breath. Med. Biol. Eng. Comput. 1996, 34, 407-419.
2. Hera, D.; Langford, V.S.; McEwan, M.J.; McKellar, T.I.; Milligan, D.B. Negative reagent ions for real time detection using SIFT-MS. Environments 2017, 4, 16;doi10.3390.
Posted by Professor Murray McEwan, CTO at Syft Technologies and Professor Emeritus at the University of Canterbury