SIFT-MS, Selected Ion Flow Tube Mass Spectrometry

Selected Ion Flow Tube Mass Spectrometry, SIFT-MS, is a sensitive and quantitative mass spectrometry technique for trace gas analyses using chemical ionization of the sample trace gases by selected positive ions during a well-defined time period along a Selected Ion 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 used for on-line quantification of trace gases in human breath for physiological monitoring, clinical diagnosis and therapeutic monitoring. Other applications are in health and safety, agriculture, animal husbandry, environmental research and food technology.

SIFT - Selected Ion Flow Tube

The selected ion flow tube, SIFT, technique is a fast flow tube / ion swarm method for the study of the reactions of ions (positive or negative) with atoms and molecules.

The selected ion flow tube, SIFT, technique is a fast flow tube method for the study of the reactions of ions (positive or negative) with atoms and molecules under truly thermalised conditions over a wide range of temperature. It has been extensively used to study ion-molecule kinetics. Its application to atmospheric and interstellar ion chemistry over a 20-year period has been crucial to the advancement and understanding of these interesting topics. Recently it has been developed as a very sensitive analytical technique SIFT-MS Selected Ion Flow Tube Mass Spectrometry for the quantification of trace gases in air and in human breath down to the ppb level in real time using Chemical Ionization.

Some details about this technique and its use in scientific resarch are given here: SIFT science

An excellent compliation of data including SIFT measurements by V. Anicich was published in 2003 by JPL, NASA:

An Index of the Literature for Bimolecular Gas Phase Cation-Molecule Reaction Kinetics, JPL Publication 03-19. Pasadena: NASA

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/7981/1/03-2964.pdf

Instrumentation

The SIFT apparatus

In the SIFT apparatus the ions are created in an 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 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 ( ~1 m long), sampled by a downstream pinhole orifice, mass analysed and counted by a differentially-pumped quadrupole mass spectrometer system.

Interesting articles:

The power of SIFT-MS for breath analysis

Research links:

• Centre for Science & Technology in Medicine, Keele University
• J. Heyrovský Institute of Physical Chemistry
• York University
• Chemistry Department, University of Canterbury
• The Department of Analytical Science and Informatics (DASI)

Industrial links:

• Trans Spectra Limited
• Instrument Science Ltd.

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