The method
The apparatus consists of three parts: The ion source where ions are produced by a hollow cathode discharge using water vapor as the molecular source of ions; the drift tube where proton transfer reactions to the trace constituents in the air occur; and finally the ion detector which provides sensitive detection of mass specified ion scans of the molecules of interest.
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The hollow cathode ion source converts water vapor in the plasma discharge into H3O+ ions via well known ion - molecule reactions. Ions extracted from the source enter a short "source drift region", also filled with water vapor, in which the ions are collisionally equilibrated with water vapor. The drift velocity of the ions is maintained at a sufficiently high value, by the applied electric field, that clustering of the hydronium ions with water molecules to form higher hydrates is efficiently suppressed. Reactant ion purity is thus easily controlled and a drift field of 120 Td is sufficient to ensure that more than 99 % of all reactant ions are present as the unsolvated hydronium ion, H3O+ and to a small fraction as its monohydrate. H3O+ ions transfer protons to nearly all VOCs with rates equal to the respective gas kinetic collision frequencies. Reactant ions are injected into the drift tube. The drift tube is maintained at a buffer gas pressure of typically 2 mbar. The air to be analyzed operates as buffer gas. After entering the drift region proton transfer reactions occur between H3O+ and any molecules Ri whose proton affinity exceeds that of water and the product ions [RiH+] are monitored in the downstream quadrupole or time of flight mass spectrometer.
Under typical operating conditions, only a small fraction of the primary ions react with VOCs in the buffer gas, so that the density of the Ri molecules is obtained from the relation
where ki is the respective reaction rate constant for the proton transfer from H3O+ to Ri and t is the transient time for the H3O+ ions traversing the drift tube.
The combination of an intense source of H3O+ primary ions, giving typically up to 30.000.000 counts/s of primary ions, a cross-section for proton transfer which ensures unit conversion efficiency of primary ions to secondary ions on every collision, a quadrupole mass analyzer which selects the ion peaks (in case of time of flight based systems a full mass spectrum with an approximate resolution of m/Δm = 5000 can be obtained within a split second) specific to the trace compounds present in the gas, and an electron multiplier detector with single particle detection efficiency are important elements of the PTR-MS analyzer.
Collectively these characteristics result in single-digit pptv detection sensitivity for VOCs in air.
Related document:
Download a PTR-MS technology factsheet (0.3 MB .pdf)
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