Proton Transfer Reaction - Mass Spectrometry
What is PTR-MS?
PTR-MS is an abbreviation for Proton Transfer Reaction – Mass Spectrometry, an online mass spectrometry technology originally developed by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria.
PTR-MS allows the simultaneous real-time monitoring of volatile organic compounds (VOC) like acetone, acetaldehyde, methanol, ethanol, benzene, toluene, xylene and others present in ambient air. PTR-MS is known for its online detection limit of < 5 pptv (depending on the PTR-MS model).
IONICON Analytik is continuously improving this technology and the only company to produce high quality PTR-MS instruments for scientists all over the world.
What are the advantages of PTR-MS?
Efficient and low energetic ionization
Proton transfer reaction is a chemical process that offers a very efficient ionization for most VOCs at reaction rates of approximately 2x10-9cm3/s. Since proton transfer is a soft method, the fragmentation rates can be kept very low compared to e.g. electron impact ionization or charge transfer.
Online quantification without calibration
The generation of the primary H3O+ ions and the chemical ionization of the VOCs are individually controlled and spatially separated processes. This leads to constant and well defined conditions in the drift region and to a high purity of the primary ion (H3O+) signal (impurities consist mainly of O2+) making the calculation of absolute concentrations possible without the use of gas standards.
Very low online detection limit
H3O+ ions do not react with any of the major components present in clean air due to their low proton affinities. This allows on one hand the analyzed air to be used directly as a buffer gas and makes on the other hand PTR-MS very sensitive to trace gases in sample air.
The real-time advantage
Unlike offline technologies, PTR-MS provides real-time data due to its very fast response time of less than 100ms. Averaging over minutes and long sampling intervals are not necessary with a IONICON PTR-MS.
No sample preparation
Another big advantage of PTR-MS is that the samples do not need to be prepared before the measurement (e.g. pre-concentration procedures), thus headspace samples can be introduced directly into the reaction chamber (drift tube).
Robust and easy to use
Our instruments are completely software controlled and are connected via a LAN-cable to a laptop computer. A single power button switches on the whole system. No gas supply is necessary and the instrument can be operated in nearly every environment. Light weight (55-170 Kg), space-saving and portable (Compact PTR-MS) or on wheels (Standard and High-Sensitivity PTR-MS), our PTR-MS systems are often used for field campaigns or variable location measurements. Only 15 minutes after pressing the power button the Compact PTR-MS is ready to measure. Little maintenance completes the whole package.
Click here for some testimonials and references of our customers.
Which compounds can be detected with PTR-MS?
IONICON uses a soft ionization 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 do therefore not interfere with the measurement. This is one of the main reasons for our market-leading low online detection limit for trace compounds in the range of a few pptv.
Most of the common VOCs have higher proton affinities than H2O and thus can be monitored and quantified down to single-digit pptv-levels.
Click here to view a schematic slide of proton affinities and detectable compounds.
A IONICON PTR-MS system consists of...
...three main parts:
Ion source: Production of H3O+ ions at high purity level (up to 99%) from water vapor via hollow cathode discharge.
PTR Drift tube: The sampled air containing VOCs undergoes (mostly) non-dissociative proton transfer from H3O+ ions, which are injected into the drift tube via a venturi-type inlet (pressure in the drift tube ~2.2mbar).
Analyzing system:
Quadrupole based systems: A quadrupole mass filter in conjunction with a secondary electron multiplier allows mass separation and detection of the ions.
Time of flight based systems: A high resolution Time-of-Flight (TOF) mass spectrometer separates the ions according to their mass to charge (m/z) ratio. The resolution is sufficient to distinguish between isobaric molecules and makes an unambiguous identification possible.
Click here to see an animation of how PTR-MS works!
Learn more about the PTR-MS technology and access theoretical background information here.
Related document:
Download a PTR-MS technology factsheet (0.2 MB .pdf)
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