Flowing afterglow mass spectrometry
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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.
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[edit] Trace Gas Analysis
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.
[edit] Instrumentation
In an FA-MS instrument, a weak microwave discharge is created in helium or argon carrier gas flowing through a narrow glass tube connected to a stainless steel flow tube. This forms a flowing afterglow plasma in the steel flow tube. The gas phase ion chemistry initiated by He+ or Ar+ ions reacting with trace amounts of H2O molecules results in the formation of H3O+ ions in the carrier gas. (Note that here H tacitly assumes the presence of both isotopes 1H and 2H). A sample of air/water vapour mixture to be analyzed is introduced at a known flow rate into the carrier gas and its composite water molecules react with the H3O+ ions to form the H3O+(H2O)0,1,2,3 cluster ions and their analogous 1H, 17O and 18O isotopic variants. The mixture of ions are sampled from the flowing swarm via a pinhole orifice located at the downstream end of the flow tube and they are mass analyzed by a differentially pumped quadrupole mass spectrometer (pressure less than 10-4 Torr) with a single channel multiplier ion counting detector.
A typical mass spectrum will show clusters of peaks at m/z 19,20,21; 37,38,39; 55,56,57; and 73,74,75. The deuterium content of a water vapour sample introduced into the helium carrier gas can be determined from such spectra if the 17O and 18O content of the ions is known. To properly understand how the analysis is achieved, we need to distinguish between the isotopic composition of the following three phases: the liquid water sample , the water vapour transferred from an aqueous sample headspace into the helium carrier gas and the H3O+(H2O)0,1,2,3 ions and their isotopomers that comprise the ion swarm created in the carrier gas.
[edit] References
Handbook of Stable Isotope Analytical Techniques (2004), Elsevier. ISBN 0-444-51114-8.
Norton, R. B., E. E. Ferguson, et al. (1966). "Ion-neutral reactions in the Martian ionosphere." Planetary Space Sci. 14(10): 969-78.
Adams, N. G.; Smith (1976). "The Selected Ion Flow Tube (SIFT); A Technique for Studying Ion-Neutral Reactions." Int J Mass Spec Ion Physics 21: 349-359.
