An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS: isomer-resolved measurements of indoor air

Abstract

Abstract. We have developed a field-deployable gas chromatograph (GC) with thermal desorption preconcentration (TDPC), which is demonstrated here with automatic detector switching between two high-resolution time-of-flight mass spectrometers (TOF-MSs) for in situ measurements of volatile organic compounds (VOCs). This system provides many analytical advances, including acquisition of fast time–response data in tandem with molecular speciation and two types of mass spectral information for each resolved GC peak: molecular ion identification from Vocus proton transfer reaction (PTR) TOF-MS and fragmentation pattern from electron ionization (EI) TOF-MS detection. This system was deployed during the 2018 ATHLETIC campaign at the University of Colorado Dal Ward Athletic Center in Boulder, Colorado, where it was used to characterize VOC emissions in the indoor environment. The addition of the TDPC-GC increased the Vocus sensitivity by a factor of 50 due to preconcentration over a 6 min GC sample time versus direct air sampling with the Vocus, which was operated with a time resolution of 1 Hz. The GC-TOF methods demonstrated average limits of detection of 1.6 ppt across a range of monoterpenes and aromatics. Here, we describe the method to use the two-detector system to conclusively identify a range of VOCs including hydrocarbons, oxygenates, and halocarbons, along with detailed results including the quantification of anthropogenic monoterpenes, where limonene accounted for 47 %–80 % of the indoor monoterpene composition. We also report the detection of dimethylsilanediol (DMSD), an organosiloxane degradation product, which was observed with dynamic temporal behavior distinct from volatile organosiloxanes (e.g., decamethylcyclopentasiloxane, D5 siloxane). Our results suggest DMSD is produced from humidity-dependent heterogeneous reactions occurring on surfaces in the indoor environment, rather than formed through gas-phase oxidation of volatile siloxanes.

Ethanol-based disinfectant sprays drive rapid changes in the chemical composition of indoor air in residential buildings

“…However, the PTR-TOF-MS cannot separate isomers and fragment ions of different origins; thus, the potential indoor formation of formaldehyde and acetaldehyde cannot be evaluated. Real-time separation of isomers requires a PTR-TOF-MS configured with a fast gas chromatograph ( Claflin et al, 2021 ).…”

Trends and applications in plant volatile sampling and analysis

“…As PTR-MS has limited capability to distinguish between isomers, a portion of the VOCs pulled from the cuvettes can be trapped in adsorbent tubes for subsequent GC-MS analysis. Latest developments now allow to operate individual instruments in PTR-ToF-MS or in GC-PTR-ToF-MS (or electron impact-ToF-MS) mode, which makes the separate trapping of VOCs for GC-MS analysis unnecessary (Claflin et al, 2021). The VOC-SCREEN system also correlates VOC emissions with the physiological state of the plant by simultaneous measurements of CO 2 assimilation and transpiration rates.…”

Constraining the response factors of an extractive electrospray ionization mass spectrometer for near-molecular aerosol speciation

“…If H 3 O + depletion is negligible, as is the case in this study, the abundance of [M+ H] + ion scales linearly with the concentration of M with a slope equal to the product of the species-dependent reaction rate constant k MH , drift time t, and the H 3 O + reagent ion abundance. The proton-transfer reaction is exothermic, and the ionization-induced fragmentation occurs to varying degrees (∼ 0 % to ∼ 100 %) as the result of dehydration, H 2 elimination, alkyl group loss, or HNO 3 loss (Claflin et al, 2021;Duncianu et al, 2017;Leglise et al, 2019;Pagonis et al, 2019;Yuan et al, 2017), which can be represented by the term F MH + . The observed intensity of any ion is also a function of the ion abundance, I MH + , and ion transmission efficiency, T MH + .…”

“…For instance, a comparison of gas-phase measurements by nitrate ion atmospheric pressure chemical ionization, iodide ion chemical ionization, and the Vocus-PTR shows that they differ in the reported relative abundances of OH-TMB oxidation products (Wang et al, 2020). In addition, artefacts such as ion fragmentation are known to occur for PTR-based techniques (Yuan et al, 2017), which could be substantial (> 70 %) for limonene oxidation products (Claflin et al, 2021;Pagonis et al, 2019); the formation of [M+ H 2 O] + adducts in the EESI-TOF, which has been reported previously to occur to a minor extent (Lopez-Hilfiker et al, 2019), also contributes to the discrepancies between the EESI-TOF and Vocus-PTR. These uncertainties complicate our data interpretation but do not change the overall trend in the sensitivity estimation, as shown below in Fig.…”