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| The '''selected ion flow tube''', '''SIFT''', technique is a fast flow tube method for the study of the reactions of [[ion|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-?¢ÀÜ?°?É‚Ä??¢‚Ǩ?°?É‚Äû?ɬ??¢ÀÜ?°?ɂİ?Ǭ¨?¢‚Ä?¬†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]].
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| ===Instrumentation===
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| '''The SIFT apparatus'''
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| 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).
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| 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.
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