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A National User Facility for the Scientific Community

Proton Transfer Reaction Mass Spec(PTRMS)

Contact: Alexander,Mikaela L

EMSL's field-deployable Proton Transfer Reaction Mass Spectrometer (PTR-MS) is capable of real-time, online quantification of volatile organic compounds in air. The instrument provides an extremely fast response time of about one second, and detection limits for a one-second integration time range from 50 pptv to about 300 pptv, depending on the nature of the analyte.

Proton Transfer Reaction Mass Spectrometer (PTR-MS)
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Proton Transfer Reaction Mass Spectrometer (PTR-MS)
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A time series of toluene measurements (every 15 sec. with dwell time of 2 sec.) as the Battelle G-1 aircraft flew west across Seattle towards the town of Bremerton. The black trace shows the 0.1 to 3 um particle concentration while the red trace gives the 3 - 20 nm particle concentration. The 3-20 nm particle size range is indicative of newly formed particles. The spike in this size range at 2:48 PM coincides with a spike in toluene concentration, suggesting evidence for the role of anthropogenic hydrocarbons in new particle formation.

Organic species are measured using chemical ionization mass spectrometry. H3O+ is used as the reagent ion and results in a soft ionization of most organic species with negligible fragmentation. The air sample is continuously drawn into a reaction chamber where it encounters the reagent ion. Organic species having a proton affinity greater than that of H2O will react with H3O+ in a proton transfer reaction:

R + H3O+ → RH+ + H2O

The major constituents of air do not react with H3O+. A small portion of the flow through the reaction chamber is sampled by a quadrupole mass spectrometer where the RH+ ions are mass filtered and detected by an ion multiplier. The amount of analyte (R) in the sample air is determined by a simple formula relating the H3O+ count rate, the RH+ count rate, the rate constant for the ion-molecule reaction, and a fixed reaction time. The rate constants for many of these reactions are known, but can also be calculated from theory.

The PTR-MS is ideally suited for performing fast real-time measurements of particular species that are known to be present in a mixture or that have unique masses in order to study a rapidly evolving temporal process. The instrument is built into an aerospace grade rack and has been successfully flown on a research aircraft to collect measurements of urban air pollution. The figure below illustrates its measurement capability. These data were collected when the PTR-MS was installed on the Battelle G-1 research aircraft during an air quality field experiment in Puget Sound during the summer of 2001. The PTR-MS is an enabling technology that will allow in situ studies of the fate and transformation of organic matter in the atmosphere not previously possible with older technologies.

In addition, recent collaborations using this instrument include the following:

  • Collecting atmospheric measurements of volatile organic compounds in Mexico City with researchers from the Massachusetts Institute of Technology
  • Studying pyrolysis of jet engine oil with Boeing
  • Developing a proton transfer reaction ion trap mass spectrometer with researchers at San Francisco State University.