Resolving Isomers in Indoor Air Using GC Combined with Detector Switching

An In Situ Gas Chromatograph with Automatic Detector Switching Between Vocus PTR-TOF-MS and EI-TOF-MS: Isomer Resolved Measurements of Indoor Air

Claflin, M. S. et al.
Atmospheric Measurement Techniques In review 2020 
DOI:10.5194/amt-2020-271

Indoor air contains a complex mixture of volatile chemical products (VCPs), but has not been thoroughly studied. The identities of many of these species are unknown, and which species are present may change considerably depending on outdoor air quality, human activities in the building, which materials and paints are present, and building ventilation. This challenging chemical environment requires novel and ingenious use of analytical instrumentation.

In this paper, the authors combined a specialized thermal-desorption VCP collector, gas chromatography, an EI-TOF, and a Vocus 2R PTR-TOF to understand VCPs present in indoor air in high chemical detail. The high resolving power of the two TOFWERK mass spectrometers provided a clear advantage for molecular identification. The authors operated the instrument on an automated revolving cycle of three configurations: (1) direct Vocus PTR-TOF analysis of ambient air, (2) GC-Vocus PTR-TOF, and (3) GC-EI-TOF. The combination of these configurations provided real-time analysis of air, interpretation of the Vocus PTR-TOF spectrum enabled by GC pre-separation, and highly specific molecular identification enabled by GC-EI-TOF. When acting as a detector for the GC, both mass spectrometers were operated with 5-6Hz full-spectrum acquisition. When measuring ambient air, the Vocus PTR-TOF was operated with 1Hz time resolution.

The GC-EI/PTR-TOF instrument was installed in the weight room of an in-use athletic center and automatically collected GC samples on a 22 minute time-basis. More than 50 VCPs were quantified during the study, including speciated monoterpenes and halocarbons. They found that the presence of people and intrusion of outdoor air significantly affected the air chemistry in the room. For example, monoterpene concentrations significantly increased when people and air from outdoor sources entered the room. Analysis of monoterpene speciation allowed distinction between human-related and outdoor monoterpene sources. Several siloxanes, which are traditionally a challenging analytical target, were also quantified, revealing anthropogenic sources and involvement in condensed-phase reactions.

The authors found the following aspects of TOFWERK instrumentation to be particularly important to this study:

  • detection of analytes with two complementary ionization techniques
  • real-time ambient air measurement
  • full-spectrum acquisition allowing matching of EI spectra to NIST database
  • flexible and sophisticated data analysis software

Realtime Vocus detection of C10H17+ with occasional speciation with GC-Vocus analysis. The realtime Vocus trace contains only room air measurements for simplicity. Pie charts show the GC-Vocus speciation of the realtime Vocus C10H17+signal into six resolved monoterpene species during each of the highlighted (grey) sampling periods. The pie chart fractions represent the contribution (in concentration, ppb) of each monoterpene isomer to the total monoterpene concentration measured by GC-Vocus. Some specific events are numbered and highlighted in light blue with the following distinctions: 1) exercise session, 2) in flow of outdoor air, 3) football game, and 4) post-game activities.
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