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Chemical ionization mass spectrometry (CIMS) using iodide as a reagent ion has been widely used to classify organic compounds in the atmosphere by their elemental formula. Unfortunately, calibration of these instruments is challenging due to a lack of commercially available standards for many compounds, which has led to the development of methods for estimating CIMS sensitivity. By coupling a thermal desorption aerosol gas chromatograph (TAG) simultaneously to a flame ionization detector (FID) and an iodide CIMS, we use the individual particle-phase analytes, quantified by the FID, to examine the sensitivity of the CIMS and its variability between isomers of the same elemental formula. Iodide CIMS sensitivities of isomers within a formula are found to generally vary by 1 order of magnitude with a maximum deviation of 2 orders of magnitude. Furthermore, we compare directly measured sensitivity to a method of estimating sensitivity based on declustering voltage (i.e., "voltage scanning"). This approach is found to carry high uncertainties for individual analytes (0.5 to 1 order of magnitude) but represents a central tendency that can be used to estimate the sum of analytes with reasonable error (similar to 30% differences between predicted and measured moles). Finally, gas chromatography (GC) retention time, which is associated with vapor pressure and chemical functionality of an analyte, is found to qualitatively correlate with iodide CIMS sensitivity, but the relationship is not close enough to be quantitatively useful and could be explored further in the future as a potential calibration approach. Alfred P. Sloan FoundationAlfred P. Sloan Foundation [P-2018-11129] Published version This research has been supported by the Alfred P. Sloan Foundation (grant no. P-2018-11129).

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.

VOCs related to oil and gas extraction operations in the United States were measured by H3O+ chemical ionization time-of-flight mass spectrometry (H3O+ ToF-CIMS/PTR-ToF-MS) from aircraft during the Shale Oil and Natural Gas Nexus (SONGNEX) campaign in March–April 2015. This work presents an overview of major VOC species measured in nine oil- and gas-producing regions, and a more detailed analysis of H3O+ ToF-CIMS measurements in the Permian Basin within Texas and New Mexico. Mass spectra are dominated by small photochemically produced oxygenates and compounds typically found in crude oil: aromatics, cyclic alkanes, and alkanes. Mixing ratios of aromatics were frequently as high as those measured downwind of large urban areas. In the Permian, the H3O+ ToF-CIMS measured a number of underexplored or previously unreported species, including aromatic and cycloalkane oxidation products, nitrogen heterocycles including pyrrole (C4H5N) and pyrroline (C4H7N), H2S, and a diamondoid (adamantane) or unusual monoterpene. We additionally assess the specificity of a number of ion masses resulting from H3O+ ion chemistry previously reported in the literature, including several new or alternate interpretations.

Volatile organic compounds were quantified during two aircraft-based field campaigns using highly-automated, whole air samplers with expedited post-flight analysis via a new custom-built, field-deployable gas chromatography – mass spectrometry instrument. During flight, air samples were pressurized with a stainless steel bellows compressor into electropolished stainless steel canisters. The air samples were analyzed using a novel gas chromatograph system designed specifically for field-use which eliminates the need for liquid nitrogen. Instead, a Stirling cooler is used for cryogenic sample pre-concentration at temperatures as low as −165 °C. The analysis system was fully automated on a 20-minute cycle to allow for unattended processing of an entire flight of 72 sample canisters within 30 hours, thereby reducing typical sample residence times in the canisters to less than three days. The new analytical system is capable of quantifying a wide suite of C2 to C10 organic compounds at part-per-trillion sensitivity. This manuscript describes the sampling and analysis systems, along with the data analysis procedures which includes a new peak-fitting software package for rapid chromatographic data reduction. Instrument sensitivities, uncertainties and system artifacts are presented for 35 trace gas species in canister samples. Comparisons of reported mixing ratios from each field campaign with measurements from other instruments are also presented.

Proton transfer reactions between hydronium ions (H3O+) and volatile organic compounds (VOCs) provide a fast and highly sensitive technique for VOC measurements, leading to extensive use of proton-transfer-reaction mass spectrometry (PTR-MS) in atmospheric research. Based on the same ionization approach, we describe the development of a high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) utilizing H3O+ as the reagent ion. The new H3O+ ToF-CIMS has sensitivities of 100–1000 cps ppb−1 (ion counts per second per part-per-billion mixing ratio of VOC) and detection limits of 20–600 ppt at 3σ for a 1 s integration time for simultaneous measurements of many VOC species of atmospheric relevance. The ToF analyzer with mass resolution (m∕Δm) of up to 6000 allows the separation of isobaric masses, as shown in previous studies using similar ToF-MS. While radio frequency (RF)-only quadrupole ion guides provide better overall ion transmission than ion lens system, low-mass cutoff of RF-only quadrupole causes H3O+ ions to be transmitted less efficiently than heavier masses, which leads to unusual humidity dependence of reagent ions and difficulty obtaining a humidity-independent parameter for normalization. The humidity dependence of the instrument was characterized for various VOC species and the behaviors for different species can be explained by compound-specific properties that affect the ion chemistry (e.g., proton affinity and dipole moment). The new H3O+ ToF-CIMS was successfully deployed on the NOAA WP-3D research aircraft for the SONGNEX campaign in spring of 2015. The measured mixing ratios of several aromatics from the H3O+ ToF-CIMS agreed within ±10 % with independent gas chromatography measurements from whole air samples. Initial results from the SONGNEX measurements demonstrate that the H3O+ ToF-CIMS data set will be valuable for the identification and characterization of emissions from various sources, investigation of secondary formation of many photochemical organic products and therefore the chemical evolution of gas-phase organic carbon in the atmosphere.