Breath analysis using Proton Transfer Reaction Mass Spectrometry (PTR-MS)
Background:
Proton Transfer Reaction Mass Spectrometry (PTR-MS) is a relatively new MS technique. It was developed recently at the Institut für Ionenphysik of the University of Innsbruck together with IONICON Analytik GmbH, Innsbruck (www.ptrms.com). PTR-MS basically employs H3O+ as the ionising agent in a chemical ionisation. The advantage of using H3O+ as the primary ion lies in its non-dissociative proton transfer reaction with most volatile organic compounds (VOCs), whereas it does not react with any natural components of air. Compared to conventional MS, the non-dissociative character of the proton transfer reaction leads to less complex spectra and no previous separation of the sample compounds is needed. PTR-MS is a very fast and powerful tool for obtaining real-time data with an detection limit of a few pptv. PTR-MS offers itself for many applications: environmental research, food research and medical applications. In our group, the Medical Physics Group at the Institute of Radiation Protection, we use the PTR-MS for medical applications in the frame of radiation protection. Specifically we are working on following projects:
Topics of research:
- Breath analysis on patients suffering from diabetes mellitus - Volatile organic compounds in breath as specific markers of diabetes (in German)
- Breath analysis with patients beeing anaesthestised - A possible tool for real-time monitoring of the concentration of anaesthetics in blood (in German)
Description of the PTR-MS:
PTR-MS uses proton transfer reactions to chemically ionize volatile organic compounds (VOCs) present in gaseous media e.g. in air. With this technique, a variety of organic species in complex matrices can be monitored on-line with detections limits as low as a few pptv.
The apparatus consists of three parts (see figure): 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 characteristic of the molecules of interest of mass selected ions.
Method of measurement
The hollow cathode ion source converts water vapor in the plasma discharge into H3O+ ions via 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, so 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 l20 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 (X) are injected into the drift tube. The drift tube is maintained at a buffer gas pressure of typically 2 mbar. The air to be analysed operates as buffer gas. After entering the drift region proton transfer reactions occur between H3O+ and any molecules X whose proton affinity exceeds that of water and the product ions [XH+] which subsequently are monitored in the downstream quadrupole mass spectrometer.
H3O+ + X → XH+ +H2O
Compared to other mass spectrometry techniques, the most important advantages of PTR-MS are:
- soft ionisation procedure
- real-time measurement at pptv levels
- easy handling of the relatively small device
Contact persons:
Matthias Greiter, phone: +49 89 3187-3068
Dr. Uwe Oeh phone: +49 89 3187-4247