Analyzing Organic Constituents on Atmospheric Particulate Matters by Comprehensive Two-Dimensional Gas Chromatography-Mass Spectrometry (GCxGC-MS)

The presence of organic compounds on the particular matter (PM) or aerosols can be arisen from the absorption of gaseous organic compounds on the existing aerosols, or from organic precursors to form secondary organic aerosols (SOA) through photochemistry. As a result, knowing the chemical composition of aerosols can shed light to the understanding of particle formation and source characteristics. The objective of this study is to characterize organic constituents on aerosols relevant to their emission sources and the key compounds revealing the evolution of aerosols with the use of a novel analytical technique.

The conventional GC-MS technique has been used to analyze the organic composition of aerosols. However, the high organic complexity of aerosol samples often renders GC-MS technique unsatisfactory in terms of compound separation and identification. As a result, time-of-flight mass spectrometry (TOF-MS) coupled with comprehensive two dimensional GC (GC×GC-TOF) was preferred. A flow type of modulator instead of a thermal type was used in GC×GC as a prelude to field applications without the need of cryogen. The high data sampling rate of 50 Hz for the TOF-MS is pivotal to produce very detailed and reproducible GC×GC results. The tolerance of high carrier gas flow rates of up to 5 mL/min played the key role to achieve high sample throughput and thus better sensitivity.

The solution extracted with ultrasound assisted technique was purged by nitrogen to reduce volume before GC injection. The GC×GC results have been obtained by analyzing both PM₁₀ and PM_2.5 samples collected on quartz filter papers. By spiking with a known amount of long-chain alkanes as the markers of molecular size, we found that the majority of the organic analytes were in the range of 12 - 30 carbon numbers falling in the category of semi-volatile organic compounds (SVOCs). If excluding alkanes, 45 and 56 compounds of alcohol, aldehyde, ketone, and ester varieties were able to be tentatively identified for the PM₁₀ and PM_2.5 samples, respectively, which are mostly primary organic aerosols (POA). Intriguingly, trace amounts of plasticizers and phosphorus flame retardants were also found. Compounds such as these are unique to the specific sources, called markers, demonstrating the wide spread of these hazardous compounds in the environment.