Flowing afterglow mass spectrometry

Flowing afterglow mass spectrometry, FA-MS, is a sensitive and quantitative mass spectrometry analytical approach that offers a route to on-line, real-time deuterium abundance measurements in water vapour in breath and above aqueous liquids, including urine and serum. This method involves the production and flow of thermalised hydrated hydronium cluster ions in inert helium or argon carrier gas along a flow tube following the introduction of a humid air sample. These ions react in multiple collisions with water molecules, their isotopic compositions reach equilibrium and the relative magnitudes of their isotopomers are measured by a quadrupole mass spectrometer located downstream.

Trace Gas Analysis
One of the first papers reporting the use of the flowing afterglow was reported in Planet Space Sci in 1966 by Norton, Ferguson, Fehsenfeld, and Schmeltekopf. They studied ion-molecule reactions pertinent to the Martian atmosphere. This flowing afterglow technique replaced the then standard stationary afterglow. Flowing afterglow has many attractive aspects: well-understood laminar behavior, viscous gas flow, a large density of carrier gas which allows the study of thermalized reactions, and the capability to make new reactant ions in situ. The ambipolar plasma is sampled using a nosecone and detected using conventional quadrupole or tandem mass spectrometry, depending on the application. One of the drawbacks of the flowing afterglow technique is the possibility of generating multiple reactant ions. This problem is cicumvented by implementing the selected ion flow tube (SIFT).

The flowing afterglow technique can be used to identify and quantify the volatile organic compounds (VOCs) of a sample as long as the fundamental ion chemistry is known. The commonly used ions are H3O+, O2+*, and NO+. All ions have drawbacks and advantages. Strategies that have been employed to unequivacally identify the VOCs include using gas chromatography coupled with flowing afterglow and  using a complement of reagent ions. Detection limits are typically in the parts per billion range if there is limited sample or parts per trillion if there is an unlimited sample size.