Review of Gas-Chromatographic Measurement Methodologies for Atmospheric Halogenated Greenhouse Gases.

Gas chromatography (GC) is crucial for measuring atmospheric halogenated greenhouse gases (hGHGs), usually coupled with electron capture detector (ECD, with higher sensitivity) or mass spectrometry (MS, with higher selectivity). This review compares GC-ECD and GC-MS for analyzing atmospheric hGHGs in terms of analytical methodology, performance, and instrumentation. For hGHGs such as SF₆, chlorofluorocarbons, and N₂O, ECD can be employed in the single column, forecut-backflush (FCBF), and preconcentration methods. The order of appearance of SF₆ and N₂O is an important consideration for selecting the separation column to avoid chromatographic interference from the long-tailed N₂O and O₂ on SF₆. Single column and FCBF GC-ECD methods suffer from nonlinear responsivity, but the preconcentration method can compensate for nonlinearity. The last method also offers a low drift, which eliminates the need for multipoint calibration and enables perfect linearity at atmospheric SF₆ levels. GC-MS demonstrates strong separability and identification capabilities, and over 60 hGHGs can be qualitatively analyzed by leveraging the separation power of MS and established MS databases. However, GC-MS requires a preconcentrator operating at -165 °C utilizing specialized adsorbents. Two notable preconcentrator-GC-MS systems, Medusa-GC-MS and detachable trap preconcentrator (DTP) GC-MS, differ in trap design, temperature scheme, and separation column type. Medusa-GC-MS employs a three-phased temperature operation before MS. DTP-GC-MS separates the preconcentration cycle into highly and less volatile compounds, using a different temperature scheme from that of Medusa-GC-MS. The preconcentrator-GC-MS system is widely employed for measuring perfluorocarbons, hydrofluorocarbons, and other hGHGs. This method necessitates multiple adsorption traps to discriminate the most abundant air components.