Detachable Trap Preconcentrator with a Gas Chromatograph–Mass Spectrometer for the Analysis of Trace Halogenated Greenhouse Gases

A detachable trap preconcentrator coupled with a gas chromatograph-mass spectrometer (GC-MS) was developed for measuring trace amounts of anthropogenic halogenated greenhouse gases such as hydrofluorocarbons (HFCs) and nitrogen trifluoride (NF₃). Hayesep D cooled to -135 °C was used as an adsorbent for preconcentrating the target analytes. A differential trapping method was applied to remove major interfering substances such as CO₂, N₂, and O₂ in order to ensure sufficient sampling volume of secondary injection trap. This was accomplished without using any CO₂-removal agent such as molecular sieve adsorbents. Consequently, the temperature of the primary transfer trap was set to -75 °C for selective desorption of a significant amount of CO₂ that could be vented out. In the meantime, the major components of air, e.g., N₂ and O₂, were vented out before transferring the analytes to the secondary injection trap, in order to protect the gas plumbing from pressure shock induced by rapid temperature ramping over 100 °C/min in the secondary trap. When the traps were heated, linear motion was operated to detach them from the copper baseplate on the freezer, thereby restricting heat transfer to the freezer and maintaining the freezer close to the background temperature of -135 °C. This trap design is a key improvement to address the insufficient cooling capacity of the employed freezer, allowing sensitive detection of trace halogenated greenhouse gases in GC-MS. NF₃ and various HFCs at ambient levels were quantitatively and qualitatively measured with a precision of 0.35% at rates below 45 min/cycle. In particular, the limit of detection for NF₃ was evaluated to be 0.2 pmol/mol, with linear responses at ambient concentration.