Discrimination and geo-spatial mapping of atmospheric VOC sources using full scan direct mass spectral data collected from a moving vehicle.

Volatile and semi-volatile organic compounds (S/VOCs) are ubiquitous in the environment, come from a wide variety of anthropogenic and biogenic sources, and are important determinants of environmental and human health due to their impacts on air quality. They can be continuously measured by direct mass spectrometry techniques without chromatographic separation by membrane introduction mass spectrometry (MIMS) and proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). We report the operation of these instruments in a moving vehicle, producing full scan mass spectral data to fingerprint ambient S/VOC mixtures with high temporal and spatial resolution. We describe two field campaigns in which chemometric techniques are applied to the full scan MIMS and PTR-ToF-MS data collected with a mobile mass spectrometry lab. Principal Component Analysis (PCA) has been successfully employed in a supervised analysis to discriminate VOC samples collected near known VOC sources including internal combustion engines, sawmill operations, composting facilities, and pulp mills. A Gaussian mixture model and a density-based spatial clustering of application with noise (DBSCAN) algorithm have been used to identify sample clusters within the full time series dataset collected and we present geospatial maps to visualize the distribution of VOC sources measured by PTR-ToF-MS.

Discrimination of constructed air samples using multivariate analysis of full scan membrane introduction mass spectrometry (MIMS) data.

RATIONALE: Volatile and semi-volatile organic compounds (S/VOCs) are important atmospheric pollutants affecting both human and environmental health. They are directly measured as an unresolved mixture using membrane introduction mass spectrometry (MIMS). We apply chemometric techniques to discriminate, classify, and apportion air samples from a variety of sources. METHODS: Full scan mass spectra of lab-constructed air samples were obtained using a polydimethylsiloxane membrane interface and an electron ionization ion trap mass spectrometer. Normalized full scan spectra were analyzed using principal component analysis (PCA), cluster analysis, and k-nearest neighbours (kNN) for sample discrimination and classification. Multivariate curve resolution (MCR) was used to extract pure component contributions. Similar techniques were applied to VOC mixtures sampled from different woodsmoke emissions and from the headspace above aqueous hydrocarbon solutions. RESULTS: PCA successfully discriminated 32 constructed VOC mixtures from nearly 300 air samples, with cluster analysis showing similar results. Further, kNN classification (k = 1) correctly classified all but one test set sample, and MCR successfully identified the pure compounds used to construct the VOC mixtures. Real-world samples resulting from the combustion of different wood species and those associated with water contaminated with different commercial hydrocarbon products were similarly discriminated by PCA. CONCLUSIONS: Chemometric techniques have been evaluated using full scan MIMS spectra with a series of VOC mixtures of known composition containing known compounds, and successfully applied to samples with known sources, but unknown molecular composition. These techniques have application to source identification and apportionment in real-world environmental samples impacted by atmospheric pollutants.

Characterization of a condensed-phase membrane introduction mass spectrometry (CP-MIMS) interface using a methanol acceptor phase coupled with electrospray ionization for the continuous on-line quantitation of polar, low-volatility analytes at trace levels in complex aqueous samples.

We report the development and application of a capillary hollow fibre membrane interface using methanol as an acceptor phase to deliver target analytes to an electrospray ionization source and a triple quadrupole mass spectrometer. Superior fluid handling systems lead to greater signal stability and membrane integrity for the continuous on-line monitoring of polar and charged analytes in complex aqueous samples with detection limits in the parts-per-trillion to parts-per-billion range. The system can be operated in either a continuous flow or a stopped acceptor flow mode - the latter giving rise to greater sensitivity. We report detection limits, enrichment factors and signal response times for selected analytes with polydimethylsiloxane and Nafion® polymer membrane interfaces. In addition, we demonstrate the use of this interface to detect pharmaceuticals and other contaminants in natural water and artificial urine. The improved sensitivity and analytical response times of our CP-MIMS system make it possible to continuously monitor dynamic chemical systems with temporal resolutions on the order of minutes. Presented is a comparison of the performance of CP-MIMS versus direct infusion electrospray ionization, demonstrating the potential advantages over direct infusion for trace analyte measurements in complex, high ionic strength samples. Furthermore, by continuously flowing a reaction mixture in a closed loop over the interface, we demonstrate the use of the system as an in situ reaction-monitoring platform for the chlorination of a model organic compound in aqueous solution.

The Use of MIMS-MS-MS in field locations as an on-line quantitative environmental monitoring technique for trace contaminants in air and water.

Membrane introduction mass spectrometry (MIMS) is emerging as an important technique for on-line, real-time environmental monitoring. Because MIMS interfaces are simple and robust, they are ideally suited for operation in MS instrumentation used for in-field applications. We report the use of an on-line permeation tube to continuously infuse an isotopically labeled internal standard for continuous quantitative determinations in atmospheric and aqueous samples without the need for off-line calibration. This approach also provides important information on the operational performance of the analytical system during multi-day deployments. We report measured signal stability during on-line deployments in air and water of 7% based on variation of the internal standard response and have used this technique to quantify BTEX (benzene, toluene, ethylbenzenes, and xylenes), pinenes, naphthalene and 2-methoxyphenol (guaiacol) in urban air plumes at parts-per-billion by volume levels. Presented are several recent applications of MIMS-MS-MS for on-line environmental monitoring in atmospheric and aqueous environmental samples demonstrating laboratory, remote and mobile deployments. We also present the use of a thermally assisted MIMS interface for the direct measurement of polyaromatic hydrocarbons, alkylphenols, and other SVOCs in the low ppb range in aqueous environmental samples and discuss improvements in both the sensitivity and response times for selected SVOCs. The work presented in this paper represents significant improvements in field deployable mass spectrometric techniques, which can be applied to direct on-site analytical measurements of VOC and SVOCs in environmental samples.