Construction of a low-cost automated chromatographic system for the measurement of ambient methane

A fully automated gas chromatographic system was constructed to perform in situ measurement of atmospheric methane. To reduce cost, this system avoided the use of a conventional gas chromatography (GC), instead a direct resistive heated molecular sieve packed column with precise temperature control was constructed to permit rapid temperature ramping and cooling and, therefore, shorter turn-over time. No heat-cut or back-flush of high boiling residue was necessary, which often requires a pre-column and additional flow of carrier gas. The precision (n = 47) of the system was better than 0.20% at near background level (1.8 ppmv), and the linearity of the system was better than 0.9990. The system was deployed in the field to perform on-site unattended measurement of ambient methane. The fast cycled measurement facilitated detailed observation of diurnal concentration variation caused by temperature inversion, which could induce nearly two-fold of concentration enhancement.