A Technique for Rapid Gas Chromatography Analysis Applied to Ambient Organic Aerosol Measurements from the Thermal Desorption Aerosol Gas Chromatograph (TAG)

Abstract

While automated techniques exist for the integration of individual gas chromatograph peaks, manual inspection of integration quality and peak choice is still required due to drifting retention times and changing peak shapes near detection limits. The feasibility of a simplified method to obtain multiple bulk species classes from complex gas chromatography data is investigated here with data from the thermal desorption aerosol gas chromatograph (TAG). Chromatograms were divided into many "chromatography bins" containing total eluting mass spectra (both from resolved species and unresolved complex mixture [UCM]), instead of only integrating resolved peaks as is performed in the traditional chromatography analysis method. Positive matrix factorization (PMF) was applied to the mass spectra of the chromatography bins to determine major factors contributing to the observed chemical composition. PMF factors are not highly sensitive to the specific PMF error estimation method applied. Increasing the number of chromatography bins that each chromatogram was divided into improved PMF results until reaching 400 bins. Increasing the number of bins above 400 does not significantly improve the PMF results. This is likely due to 400 bin separation providing bin widths (4.6 s) that match the narrowest peak widths (4.8 s) of compounds found in the TAG chromatograms. The bin-based method took only a small fraction of the time to complete compared to peak-integrated method, significantly saving operator time and effort. Finally, high-factor solutions (e.g., 20 factors) of bin-based PMF can separate many individual compounds, homologues compound series, and UCM from chromatography data.

A technique for rapid source apportionment applied to ambient organic aerosol measurements from a thermal desorption aerosol gas chromatograph (TAG)

“…Gas chromatograms are divided by retention time into evenly spaced bins, within which the mass spectra are summed. A previous chromatogram binning method was introduced for the purpose of chromatogram structure deconvolution (e.g., major compound classes) (Zhang et al, 2014). Here we extend the method development for the specific purpose of determining aerosol samples' sources.…”

“…In 2013, the SemiVolatile (SV-)TAG was developed to extend TAG's capability to include quantitative characterization of semi-volatile organic compounds (Zhao et al, 2013), and in 2014 Isaacman et al (2014) introduced a technique for online chemical derivatization on the SV-TAG system to improve quantification of oxygenated molecules (Isaacman et al, 2014). Recently, a combined TAG-AMS instrument which can simultaneously measure the bulk and speciated composition of organic aerosols has been presented (Williams et al, 2014). The advantage of providing speciated timelines for organic chemicals is significant; however the time required for chromatographic peak identification, integration, and confirmation of integration quality for hundreds of compounds limits the wider application of using a chromatographic approach for aerosol analysis.…”

“…The advantage of providing speciated timelines for organic chemicals is significant; however the time required for chromatographic peak identification, integration, and confirmation of integration quality for hundreds of compounds limits the wider application of using a chromatographic approach for aerosol analysis. Additionally, a significant fraction of GC-MS data are typically present as an unresolved complex mixture (UCM) when analyzing ambient aerosol samples (Williams et al, 2006(Williams et al, , 2010(Williams et al, , 2014, and peak integration methods typically ignore the material that is not resolved. Therefore, rapid techniques for comprehensively analyzing the complete chromatographically separated mass spectral data, including UCM signal, may broaden the application of the various TAG measurement methods.…”

“…While inorganic ions, elemental carbon (EC)/organic carbon (OC), OC functional groups, and trace metals from offline filters analyses of atmospheric aerosol have been used for source apportionment (Chueinta et al, 2000;Ito et al, 2004;Lee et al, 1999;Ramadan et al, 2000;Ahlm et al, 2013), recently high-time-resolution Aerodyne aerosol mass spectrometer (AMS) mass spectra and the Aerosol Chemical Speciation Monitor (ACSM) have been extensively used to determine the major components of ambient OA Ng et al, 2011). Online and offline measurements of molecular level marker molecules have also been used to apportion the major chemical components and source attributions of these organic aerosols (Schauer et al, 1996;Jaeckels et al, 2007;Zhang et al, 2014;Williams et al, 2010Williams et al, , 2014.…”

“…One such method was recently introduced, in which each chromatogram is evenly divided into bins and PMF is performed on the covariance of signal vs.retention time to deconvolute the chromatogram into homologous compound series, individual compounds, and multiple UCM components (Zhang et al, 2014). In order to pursue source apportionment, we initially attempted to input these deconvolution chromatogram factors to a second PMF analysis.…”