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

Last updated: 01-17-2021

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

Research article 07 Jan 2021
Research article | 07 Jan 2021
An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS: isomer-resolved measurements of indoor air
An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS: isomer-resolved measurements of indoor air
An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS: isomer-resolved measurements of indoor air An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS:... Megan S. Claflin et al.
Megan S. Claflin
Megan S. Claflin et al.
Megan S. Claflin
1Aerodyne Research Inc., Billerica, Massachusetts 01821, USA
2Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, Colorado 80309, USA
3Department of Chemistry, University of Colorado, Boulder, Colorado 80309, USA
anow at: Chemical Sciences Laboratory, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, 80305, USA
1Aerodyne Research Inc., Billerica, Massachusetts 01821, USA
2Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, Colorado 80309, USA
3Department of Chemistry, University of Colorado, Boulder, Colorado 80309, USA
anow at: Chemical Sciences Laboratory, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, 80305, USA
Correspondence: Megan S. Claflin (mclaflin@aerodyne.com)
Correspondence: Megan S. Claflin (mclaflin@aerodyne.com)
Received: 05 Jul 2020
Discussion started: 17 Jul 2020

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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.
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Claflin, M. S., Pagonis, D., Finewax, Z., Handschy, A. V., Day, D. A., Brown, W. L., Jayne, J. T., Worsnop, D. R., Jimenez, J. L., Ziemann, P. J., de Gouw, J., and Lerner, B. M.: An in situ gas chromatograph with automatic detector switching between PTR- and EI-TOF-MS: isomer-resolved measurements of indoor air, Atmos. Meas. Tech., 14, 133–152, https://doi.org/10.5194/amt-14-133-2021, 2021.
1 Introduction
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Historically, volatile organic compound (VOC) emissions from transportation were the most important air pollution source in urban environments (Gentner et al., 2017; Watson et al., 2001). However, with the success of emission-reduction strategies (Warneke et al., 2012; McDonald et al., 2013), other sources of anthropogenic VOCs are becoming significant in most developed nations, such as emissions from volatile chemical products (VCPs) (McDonald et al., 2018). VCPs consist of a large diversity of compounds, including oxygenated species like alcohols (e.g., glycols), esters, siloxanes, and carbonyls, along with hydrocarbons like alkanes, alkenes (e.g., monoterpenes), and aromatics (McDonald et al., 2018). This emission class stems from human activities such as the use of personal care products, paints, cleaning supplies, pesticide application, and the industrial use of solvents. Typically, VCPs are emitted in residential or commercial buildings, making their emissions highly variable both spatially and temporally, depending on the occupancy and activities occurring in the space (Weschler and Carslaw, 2018; Abbatt and Wang, 2020; Pagonis et al., 2019). To understand changing emission patterns, analytical instrumentation that can quantitatively detect these classes of VOCs with little ambiguity and high time resolution is needed, along with a range of studies to understand how emissions differ depending on the indoor environment and its use.
While indoor air quality has been studied for decades (Weschler and Shields, 1997; Wolkoff, 2013), recently the use of advanced gas-phase analysis techniques developed for atmospheric research, like in situ (real-time, direct air sampling) proton transfer reaction (PTR) and chemical ionization (CI) mass spectrometry (MS), have been applied for the characterization of indoor VOCs. These techniques have been used to characterize emissions in indoor environments such as a movie theater (Williams et al., 2016), art museum (Pagonis et al., 2019; Price et al., 2019), and university classroom (Liu et al., 2016, 2017; Tang et al., 2015, 2016) and to study how episodic events like cleaning and cooking impact indoor air quality (Wong et al., 2017; Kristensen et al., 2019; Lunderberg et al., 2019). While these studies conducted with PTR-MS and CIMS provide VOC emission signatures in a variety of environments, they often cannot provide molecular identification due to the detection of isobaric ions, which can be associated with multiple isomers, cluster ions, or fragmentation products that have the same molecular formula (Thompson et al., 2017). Without molecular identification, source apportionment and fate characterization remain difficult.
Improved molecular information can be gained by coupling gas chromatography (GC) with mass spectrometric detection (Warneke et al., 2003). Some studies have conducted offline GC measurements for indoor air research, which generally consist of sorbent tube or solid-phase microextraction (SPME) fiber collection with subsequent GC analysis (Gallagher et al., 2008; He et al., 2019; Sun et al., 2017; Liu et al., 2019). These studies focused on emissions from human skin and breath (Gallagher et al., 2008; He et al., 2019; Sun et al., 2017), with the exception of Liu et al. (2019), which utilized offline GC × GC analysis to study VOCs in a single-family home in northern California.
While these approaches provide some molecular identification and quantification, the low time resolution and time-consuming nature of offline methods, along with the potential for the introduction of artifacts due to sample handling between collection and analysis, are not ideal. In situ GC measurements of indoor environments are currently limited (Kristensen et al., 2019; Lunderberg et al., 2019; Rizk et al., 2018). During the single-family house study mentioned above (Liu et al., 2019), a semi-volatile thermal desorption aerosol gas chromatograph (SV-TAG) was deployed to make measurements during normal occupancy (Kristensen et al., 2019; Lunderberg et al., 2019). In the summer of 2018, an intensive indoor air study, HOMEChem, was conducted to study emissions and removal processes of gases and particles in a model home. This campaign included SV-TAG, an in situ four-channel GC with flame ionization detection (FID) and electron capture detection (ECD), and passive sampling for offline GC-MS samples (Farmer et al., 2019). The use of multiple types of chromatographic separation during this campaign illustrates the shift in focus for indoor air research toward more complete molecular analysis.
Building upon the research that has been conducted to study indoor environments, the ATHLETic center study of Indoor Chemistry (ATHLETIC) campaign was conducted during November of 2018 at the University of Colorado Dal Ward Athletic Center in Boulder, Colorado. The goal of ATHLETIC was to quantify the effects of human exercise, the use of chlorine-based cleaners, and other parameters on indoor air quality with instrumentation that provides high time resolution information and detailed characterization of both gases and particles. To address the need for high time resolution measurements and molecular identification of VOCs, we have developed an automated, field-deployable GC equipped with thermal desorption (TD) preconcentration and automated detector switching between two high-resolution time-of-flight mass spectrometers (HR-TOF-MS): a Vocus PTR-TOF-MS and an electron ionization (EI) TOF-MS for in situ measurements of VOCs. This system was deployed during the 2018 ATHLETIC campaign to characterize VOC emission profiles in the weight room facility. The instrument configuration and details of operation are discussed here, along with measurement results that were made possible through the analytical advances this technique offers. These results include the identification of a range of VOCs, including hydrocarbons, oxygenates, and halocarbons in the athletic center, along with details of their detection by both types of TOF-MS. We also report the quantification of anthropogenic monoterpenes and evidence of VOC emissions from humidity-dependent, heterogeneous reactions occurring on walls and surfaces in the indoor environment. The results presented here are a demonstration of this new GC-TOF-MS technique that produces three detailed and complementary data sets.
2 Methods
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2.1 Instrument description
The GC-TOF-MS system consists of three main components: (1) a thermal desorption preconcentrator (TDPC) for sample collection, (2) a gas chromatograph (GC) for sample separation, and (3) high resolution time-of-flight mass spectrometers (HR-TOF-MS) for sample detection. Each of these components is described in the following sections. While the in situ GC can be operated with either the Vocus PTR-TOF-MS or EI-TOF-MS as individual detectors, coupling the GC with both detectors creates a technique that produces three complementary data sets: (1) real-time Vocus PTR-TOF-MS, (2) GC-Vocus PTR-TOF-MS, and (3) GC-EI-TOF-MS. Hereafter, these three techniques will be referred to as RT-Vocus, GC-Vocus, and GC-EI-TOF, respectively. It should be noted that the instrument described in this work and deployed for the ATHLETIC campaign was a prototype system used to demonstrate this technique. The instrument has been undergoing continued development to improve sensitivities and chromatographic performance and to extend the volatility range of resolved compounds since this campaign.
2.2 Thermal desorption preconcentration
Ambient VOCs are typically present at low mixing ratios (sub-ppb), and thus to increase GC-MS sensitivity a preconcentration method is required. For this study, the samples were collected using a simplified version of a thermal desorption preconcentrator (TDPC) (Aerodyne Research, Inc.). Briefly, the TDPC employed for this study relied upon a single-stage adsorbent trap for preconcentration of analytes. The design is based upon that of Tanner et al. (2006), and uses a commercial cold-plate Peltier thermoelectric cooler (CP-110, TE Technology) to allow for precise ambient to sub-ambient temperature regulation. Results from this TDPC have been described previously (Anderson et al., 2019). The system was simplified by not using water trapping or oxidant scrubbing before sample collection due to the expected low humidity and oxidant mixing ratios in this study. The sample trap was a commercial glass sorbent tube (TO-15/TO-17 cold trap, Markes International) operated at 20 ∘C during sample collection to avoid potential water condensation. The chosen sample trap was a multi-bed adsorbent trap equipped with three stages of adsorbents (Tenax, Carbopack X, Carboxen 1003; Markes International, personal communication, 2020) to expand the volatility range of compounds that can be trapped and desorbed for analysis. The combination of adsorbents in the TO-15/TO-17 trap allows for the analysis of a wide range of VOCs (including oxygenates) in the C2–C32n-alkane volatility range. However, for the system deployed for this work, the instrument was optimized for VOCs in the C5–C12 volatility range. Details of operational parameters (e.g., temperatures, flows) are described in Sect. 2.8.
2.3 Gas chromatograph
To separate analytes before detection with TOF-MS, a compact GC from Aerodyne Research Inc. (hereafter referred to as ARI GC) was used. The ARI GC is designed to be an in situ, field-deployable system. It fits into a 55 cm × 55 cm × 30 cm rack, weighs 24 kg, consumes 300 W of power during typical operation, and contains all hardware for GC sample collection and control of TDPC and GC flows and temperatures, including a make-up flow needed for GC-Vocus measurements (described in Sect. 2.8). Here, the flow path contained three two-position chromatography valves with Nitronic 60 valve bodies (VICI Instruments): one 10-port and two 6-port valves (Fig. 1b) to direct flows during the GC cycle. The chromatography valves and transfer lines (Sulfinert-treated 304-SS, 1.6 mm OD, 0.76 mm ID, Restek) are housed in a heated enclosure held at 150 ∘C. The carrier gas (UHP helium; Matheson) was controlled by a mass flow controller (MKS Technology) with variable set-point capability in the range of 0.1–10 cm3 min−1. The GC column is housed in a custom interlocking aluminum spindle (12 cm × 3 cm) with surface-mounted flexible resistive heaters, as described by Lerner et al. (2017). For this study, the ARI GC was configured as a one-channel system (single-column separation), with a 30 m Rxi-624 analytical column (Restek, 0.25 mm ID, 1.4 µm film thickness) installed in the spindle. This column resolves non- to mid-polarity VOCs including hydrocarbons, oxygenates, and nitrogen- and sulfur-containing compounds, with the exception of high-polarity compounds like carboxylic acids. The volatility range that the GC can resolve is a function of both the chosen GC column and the TDPC adsorbent trap. With the combination of column and adsorbent trap used for this study, the ARI GC was optimized for C5–C12 hydrocarbons, along with oxygen-, nitrogen-, halogen-, and sulfur-containing VOCs.
Figure 1Instrument schematics of (a) dual-detector GC-TOF-MS instrument configuration with valving shown for GC detector selection (EI-TOF or Vocus) and Vocus inlet source (room or supply air or GC effluent). (b) GC flow path and valve positions to incorporate a single-stage thermal desorption preconcentrator (TDPC), single column separation, and dual TOF-MS detection.
2.4 HR-TOF-MS detection
2.4.1 EI-TOF-MS
The electron ionization mass spectrometer used in this study is a Tofwerk EI-TOF-MS (Tofwerk AG) that has been described previously (Obersteiner et al., 2016). While the EI-TOF has nominal mass resolution up to 5000 m∕Δm, here it was operated with a resolution of 3900 at m∕z 69 to optimize both mass resolution and instrument sensitivity. During acquisition, mass spectra were averaged on a 6 Hz time base to obtain enough data points across each chromatographic peak. The ionizer temperature was kept at 280 ∘C, with ionization energy set to 70 eV and an electron emission current of 0.3 mA. The interface between the GC and both EI-TOF and Vocus is described in Sect. 2.8.
2.4.2 Vocus PTR-TOF-MS
The proton transfer reaction mass spectrometer used in this study is a Tofwerk Vocus PTR-TOF-MS (Tofwerk AG) described by Krechmer et al. (2018). It has nominal resolution of 12000 m∕Δm and was operated with a resolution of 11 500 at m∕z 150. The Vocus was operated with a data acquisition rate of 1 Hz for RT-Vocus and 5 Hz for GC-Vocus measurements. The focusing ion-molecule reactor direct current (DC) and radial frequency (RF) voltages were set to 500 and 450 V, respectively, and it was operated at a pressure of 1.5 mbar, giving a reduced electrical field (E∕N) of 150 Td. Additional details of Vocus operation during this campaign are given in Finewax et al. (2020).
2.4.3 Instrument control, data acquisition, and analysis
ARI GC operation is fully automated via a Labview-based (National Instruments, Inc) stand-alone executable in a Windows 10 OS environment (Microsoft) on one of the TOF-MS computers (here, the EI-TOF computer was used). The ARI GC communicates with the control computer via USB 2.0, with two communication devices (data board, serial communications board) required for operation. Each mass spectrometer is equipped with its own acquisition software (Tofwerk AG), the EI-TOF-MS operating TofDAQ v.1.99 and the Vocus using Igor Pro-based (Wavemetrics) Acquility v.2.3.6, which acts as a command shell and GUI interface for TofDAQ.
The analysis of high-resolution mass spectrometric data from both the EI-TOF and the Vocus was performed using Tofware (v3.1.2; TofWerk AG and Aerodyne Research, Inc.), where both nominal (unit mass resolution, UMR) and accurate (high-resolution, HR) data were used for analysis. Once the data had undergone mass calibration and high-resolution ion peak fitting, the data was then imported into GC analysis software, TERN (Aerodyne Research, Inc.). TERN is a software package based in Igor Pro that automatically calculates chromatographic peak areas, for either UMR or HR data, by mathematically fitting peak functions to the data rather than peak integration (Isaacman-VanWertz et al., 2017). Instrument calibration and data normalization procedures employed for this study are described in Sect. 2.9.
2.4.4 Measurement site
ATHLETIC was a 3-week study conducted at the University of Colorado Dal Ward Athletic Center in November 2018 in Boulder, Colorado. During the campaign, instruments were housed within the athletes' weight room and sampled from both inside the weight room (hereafter “room air”) and the supply air from the heating, ventilation, and air conditioning (HVAC) system. During the measurement period, the instruments switched between sampling the room and supply air every 10 min via an automated valve system. The weight room is serviced by the main air handling unit (AHU) of the building that circulates ≈ 400–1400 m3 min−1, of which 200 m3 min−1 is supplied to the weight room. The fraction of outside air that was mixed with the main AHU flow varied from ≈ 10 % to 80 % during this study. The volume of the weight room is ≈ 1700 m3, which corresponds to an average residence time of air in the weight room of ≈ 8.5 min, and an outdoor air exchange rate of 0.7–5.6 air changes per hour (ACH). The Dal Ward Athletic Center is directly adjacent to the University of Colorado football stadium, Folsom Field. The athletic center is to the north of the football stadium and to the northeast of a field house where cooking and other activities occurred before and during two football games that took place during this study on 10 and 17 November.
The campaign included additional instrumentation that sampled gases and particles. Although ATHLETIC was a 3-week study, the GC-TOF-MS system operated for a subset of the campaign. Here, only results from the GC-TOF-MS system will be presented along with relative humidity (RH) and temperature data collected using a Picarro Gas Analyzer (G2401) and building space temperature sensors located on the walls of the main floor of the weight room (provided and operated by CU Facilities Management). Room RH was derived from the building temperature and local pressure along with the H2O mixing ratio measured by the Picarro instrument. A separate analysis of RT-Vocus data, focusing on species not discussed here, is published elsewhere (Finewax et al., 2020).
2.4.5 Sample inlet
The ARI GC houses three separate sample inlets, an ambient inlet and two calibration gas inlets (Fig. 1b). The GC ambient inlet sampled from the weight room via a 3.4 m PFA (0.16 cm OD) sampling line with a 30 cm3 min−1 flow rate. The two calibration gas inlets are for pressurized gases where each inlet has a critical orifice inline to regulate flow followed by a solenoid shut-off valve. The calibration inlets operate by overflowing the ambient inlet during the sampling period; this excess flow is ensured by setting the pressure on the gas cylinder regulator based upon the critical orifice diameter (typical size 75 µm) installed upstream of the solenoid valves. For this study, the calibration gases were (1) a custom-made multicomponent calibration mixture, a certified natural gas standard (Restek) diluted with UHP nitrogen and (2) a zero gas (ultra zero grade air, Airgas) for system zeros. For RT-Vocus sampling, room air was sampled at 10 L min−1 through a 1.3 m length PFA Teflon inlet with 0.47 cm inner diameter (ID) that was shared by all instruments. Supply air was sampled at the same flow rate through a 4.3 m length of PFA Teflon with the same ID. From those shared inlets, 1.6 L min−1 was pulled through a 1.5 m PFA (0.16 cm ID) sampling line, where 100 cm3 min−1 was sampled into the Vocus and the remainder to excess. Sample selection (room versus supply air) was done via automated valve switching, and a makeup flow was applied to the inlet not being sampled to ensure continued inlet passivation. The RT-Vocus room air inlet and GC ambient inlet were separate but co-located in the weight room. The GC did not sample from the supply air during this study.
2.5 Sample acquisition, separation, and detection
At the start of the GC cycle (22 min), the sample was collected onto the adsorbent trap held at 20 ± 1 ∘C for 6 min at 30 cm3 min−1. The adsorbent trap was then backflushed for 1 min with 2 cm3 min−1 of UHP helium (Matheson) to remove oxygen and water from the trap. Next, the carrier flow was increased to 5 cm3 min−1 and the sample was thermally desorbed onto the GC column by flash heating the adsorbent trap to 225 ∘C for 20 s at 10.5 ∘C s−1. During this sample transfer, the GC column was held at 40 ∘C. To gauge the desorption efficiency, we would run a sample and then an instrument blank, with no sample flow through the trap during the collection period, to measure the residual sample remaining in the trap. The result of the instrument blank was

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