Principles of PTR-MS Technology

(You can find info about the different PTR-MS instruments here.)

(Download a fact-sheet on PTR-MS detectability here.)

PTR-MS is an abbreviation for proton-transfer-reaction mass spectrometry.The technology was developed by scientists at the Institute of Ion Physics at the Leopold-Franzens University in Innsbruck, Austria in the mid-1990s.

Advantages of PTR-MS

Very low detection threshold (pptv-range)

The PTR-MS technique is a soft ionisation method based on proton transfer from H₃O⁺ ions to all compounds with a higher proton affinity than water. Common constituents of air such as N₂, O₂,Ar, CO₂ etc. have lower proton affinities than H₂O and are therefore not detected. This leads to a unique low detection threshold for trace compounds in the range of a few pptv.

Soft ionisation method - proton transfer

The very soft ionisation technique reduces mass fragmentation, which enhances the interpretability of the resulting mass spectra. Mathematical models enable the possibility to quantify the detected masses of compounds with reasonable accuracy. Calibration of the instrument can improve the accuracy to better than ±10%.

No sample preparation

Another benefit using H₃O⁺ ions is that the results of PTR-MS measurements are not modified or falsified by high concentrations of water vapour in samples to be analysed.

On-linereal-time measurement

Compared to other analytical methods, PTR-MS systems enable the ability to measure on-line and in real-time due to the instruments' response time of less than 100 ms. There is no need for specific sample preparations before injection into the PTR-MS inlet. Because of well-known parameters in the reactionchamber it is possible to quantify concentrations without calibration,obtaining a certain accuracy.

The Method

The apparatus consists of three parts: The ion source, where ions are produced by a hollow cathodedischarge using water vapour 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-selected ionscharacteristic of the molecules of interest. The hollow cathode ion source converts water vapour in the plasma discharge into H₃O⁺ ions via well-known ion-molecule reactions. Ions extracted from the source enter a short "source drift region", which is also filled with water vapour, in which the ions are collision-equilibrated with the water vapour. The drift velocity of the ions is maintained at a sufficiently high value by an applied electric field such that clustering of the hydronium ions with watermolecules to form higher hydrates is efficiently suppressed. The reactant ionpurity 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 hydroniumion, 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, which is maintained at a buffer gas pressure of typically 2mbar. The air to be analysed operates as the buffer gas. After entering the drift region proton transfer reactions occur between H₃O⁺ and any molecules R_i whose proton affinity exceeds that of water; the product ions (R_iH⁺) are monitored in the downstream quadrupole mass spectrometer. Under typical operating conditionsonly a small fraction of the primary ions react with VOCs in the buffer gas, so that the density [R_i] of the R_i molecules is obtained from the relation

[R_iH⁺] = [H₃O⁺]0 (1-e^-k[Ri]t)≈ [H₃O⁺]₀[R_i]kt

where k_i is the respective reaction rate constant for the proton transfer from H₃O⁺ to R_i and t is the transient time for the H₃O⁺ ions traversing the drift tube. The combination of an intense source ofH₃O⁺ primary ions, giving typically 10.000.000 counts/s of primary ions, across-section for proton transfer that ensures unit conversion efficiency of primary ions to secondary ions on every collision, a mass analyser that selects the ion peaks specific to the trace compounds present in the gas, and anelectron multiplier detector operated in pulse counting mode with single particle detection efficiency are important elements of the PTR-MS analyser.Collectively these characteristics result in pptv detection sensitivity for VOCsin air.