Proton-transfer-reaction mass spectrometry PTR-MS

© Fraunhofer IVV

Proton-transfer-reaction mass spectrometer (PTR-MS) equipped with a quadrupole MS (PTR-QMS; hs-PTR-MS model)

© IONICON Analytik GmbH

Proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOFMS; PTR-TOF 8000 model)

On-line measurements of volatile organic compounds (VOCs) for sensory analyses

Method:

Proton-transfer-reaction mass spectrometry (PTR-MS) is a soft chemical ionisation technique that offers low fragmentation of target analyte molecules such that invariably the molecular ions can be detected, thereby simplifying the identification of product ions in the resulting mass spectra. The fast response time of only a few milliseconds offers on-line detection of VOCs, thus dynamic changes in the concentrations of target compounds of different chemical classes can be monitored in real-time. Unlike conventional PTR-MS, which incorporates a quadrupole mass spectrometer with nominal mass resolution (PTR-QMS), the recent coupling of a time-of-flight mass spectrometer to the PTR-MS ion source and reaction chamber (PTR-TOFMS) offers an unsurpassed mass resolution for an on-line VOC detection system: The PTR-TOF 8000 system (IONICON Analytik GmbH) operated in the Department of Sensory Analytics’ on-line laboratory has a mass resolving power of up to 8000 m/Δm, meaning that many isobaric substances are distinguishable. As an example, two protonated molecules at a nominal mass of 100 Th can be individually detected down to a mass separation of 0.013 Th. By comparison, PTR-QMS offers only unitary mass resolution (1 m/Δm). The higher mass resolving power of PTR-TOFMS improves the ability to identify compounds in complex matrices such as food. An additional advantage of PTR-TOFMS is its high time resolution: a single flight time event in the TOF-system typically takes 30 µs, thus a mass spectrum can be recorded in only a fraction of a second. PTR-QMS, in comparison, requires a detection time of at least 10 ms per ion, resulting in a longer duration for recording an entire mass spectrum. Nevertheless, PTR-QMS is well-suited for many specific applications, for example for the rapid detection of individual compounds or for continuous measurements over an extended period (e.g. storage experiments).

The Department of Sensory Analysis’ on-line VOC analysis laboratory is equipped with:

  • A high sensitivity PTR-QMS instrument
  • A high sensitivity, high resolution PTR-TOFMS instrument (PTR-TOF 8000)
  • A gas calibration unit (GCU) for calibration of the PTR-MS instruments
  • A buffered end-tidal (BET) sampler for breath gas analysis (see left photo below)
  • A breath collection unit (BCU) for sampling end-tidal breath gas and ambient air
  • An incubator with custom-built PTR-MS ports for active sampling of samples held at elevated temperature
  • A refrigerator with custom-built PTR-MS ports for active sampling of samples held at low temperature (e.g. for storage assessments)