Instrumentation
In the selected ion flow tube mass spectrometer, SIFT-MS, ions are generated in a microwave plasma ion source, usually from a mixture of laboratory air and water vapor. From the formed plasma, a single ionic species is selected using a quadrupole mass filter to act as "precursor ions" (also frequently referred to as primary or reagent ions in SIFT-MS and other processes involving chemical ionization). In SIFT-MS analyses, H3O+, NO+ and O2+ are used as precursor ions, and these have been chosen because they are known not to react significantly with the major components of air (nitrogen, oxygen etc.), but can react with many of the very low level (trace) gases.
The selected precursor ions are injected into a flowing carrier gas (usually helium at a pressure of 1 Torr) via a Venturi orifice (~1 mm diameter) where they travel along the reaction flow tube by convection. Concurrently, the neutral analyte molecules of a sample vapor enter the flow tube, via a heated sampling tube, where they meet the precursor ions and may undergo chemical ionization, depending on their chemical properties, such as their proton affinity or ionization energy.
The newly formed "product ions" flow into the mass spectrometer chamber, which contains a second quadrupole mass filter, and an electron multiplier detector, which are used to separate the ions by their mass-to-charge ratios, m/z, and measure the count rates of the ions in the desired m/z range.
Read more about this topic: Selected-ion Flow-tube Mass Spectrometry