Selected ion flow tube mass spectrometry is a sensitive and quantitative mass spectrometry technique for trace gas analyses using chemical ionisation of sample trace gases by selected positive ions during a well-defined time period along a flow tube. Absolute concentrations of trace compounds in humid air or breath are calculated in real time from the reagent and product ion signal ratios without the need for calibration using standards. SIFT-MS is primarily used for on-line quantification of trace gases in human breath for clinical diagnosis and therapeutic monitoring. SIFT-MS has also great potential as a tool for non- invasive physiological monitoring. Other valuable applications are now emerging in agriculture and animal husbandry, environmental research and food technology.

[edit] Instrumentation

The selected ion flow tube, SIFT, technique is a fast flow tube / ion swarm method to react positive or negative ions with atoms and molecules under truly thermalised conditions over a wide range of temperature. It has been extensively used to study ion­molecule kinetics. Its application to ionospheric and interstellar ion chemistry over a 20-year period has been crucial to the advancement and understanding of these interesting topics.

In the SIFT-MS instrument the ions are created in an microwave plasma ion source which is external to the flow tube. The ions are then extracted from the ion source, selected according to their mass-to-charge ratio using a quadrupole mass filter and injected into a flowing carrier gas (usually helium at a pressure of 0.5 to 1 Torr) via a small orifice ( ~1 mm diameter).

The carrier gas is inhibited from entering the quadrupole mass filter chamber by injecting it into the flow tube through a Venturi-type inlet at near-supersonic velocity in a direction away from the orifice. In this way a swarm of a single ion species thermalised at the same temperature as the carrier gas are convected along the flow tube (50 to 400 mm long), sampled by a downstream pinhole orifice, mass analysed and counted by a differentially-pumped quadrupole mass spectrometer system.

[edit] References

• Encyclopedia of Spectroscopy and Spectrometry, (1999), Elsevier. ISBN 0-12-226680-3
• Rapid Communications in Mass Spectrometry 18 (2004) 1869-1873.
• Handbook of Stable Isotope Analytical Techniques (2004), Elsevier. ISBN 0-444-51114-8.