Routine analysis of atmospheric methane (CH4) is achieved using a modified commercial Gas Chromatograph (Clarus 500, Perkin Elmer), which includes gas chromatography followed by detection of CH4 using a Flame Ionisation Detector (FID). Air is drawn from the top of the sampling tower (25 masl) through ¼” tubing (Synflex 1300) at a flow rate of 200 cm3 min-1 using a diaphragm pump (N86-ATE, KNF) upstreamof the sample loop. To prevent particulate matter from entering the analytical instrumentation the sample line is fitted with a 40-micron filer at the top of the tower and a 2-micron filter at the base of the tower (TF-series, Swagelok). The air is then passed through a counter-flow Nafion drier (MD-series, Perma Pure) to remove the bulk of the moisture. Following this the air is passed through an ethanol chiller trap (-90°C) before flushing a 10 cm3 sample loop held at a constant temperature of 80° C inside a controlled zone. The return ‘dry’ air flow from the sample loop is used as the counter flow gas for the Nafion drier. Prior to sample loop injection the sample pressure is allowed to equilibrate for 0.5 mins.
The air in the sample loop is injected at 0.5 mins and immediately enters the pre-column containing Unibeads 1S (Grace Discovery Sciences, 80/100 mesh, stainless steel, 3/16” OD x 200 cm length) which is operated at a constant temperature of 80° C. This stage separates the CH4 from carbon dioxide (CO2) and non-methane hydrocarbons (NMHCs). After 5.5 mins the sample is transferred to an analytical column consisting of Molecular Sieve 5A (60/80 Mesh, stainless steel, 3/16” OD x 400 cm length) also held at 80° C and the pre-column is back-flushed. Following further separation the sample passes to the FID where the CH4 is detected following it’s elution from the analytical column at 8 mins. The carrier gas used in this system is nitrogen N4.8 grade (99.998 % purity, BOC) at a flow rate of 60 ml min-1 during chromatographic separation and 80 ml min-1 during the back-flushing of the pre-column. This is further purified using a heated getter (Supelco, model no. 29546-U) and OMI purifier tube (Supelco, model no. 23909) to reduce impurities below 10 ppb. The FID is operated at 200°C with hydrogen N4.5 grade as the fuel gas (99.995 % purity, BOC) and BTCA 178 air as the air supply (BOC). The fuel gas (H2) is further purified using a hydrocarbon trap (Supelco, model no. 24449) before entering the FID.
A run time of 10 min allows five air samples to be analysed every hour with a workingstandard analysed after every sixth run. This results in five fully calibrated measurements of CH4 each hour. Detector response is recorded and analysed using Perkin Elmer GC control software (TCNav 6.3.2). Normalised peak heights (peak height / average of bracketing working standards) measured from samples are referenced to the instruments response function determined using calibrated primary reference gases on the NOAA2004. The range of concentration in the primary reference gases is 1754 ppb to 2496 ppb (5 cylinders). Instrument repeatability is assessed by calculating the deviation in concentration of the working standard from the bracketing two working standards as the concentration of this is known as it is determined at the same time as the non-linear response function. The estimated repeatability using this approach is routinely better than ± 3 ppb. The accuracy of the system is assessed through the analysis of a target gas every 6 hours. The target gas is a cylinder of gas with accurately assigned concentrations of the target species determined at a central calibration laboratory (in this instance MPI-Jena). This target gas is introduced to the system as a sample and following analysis the deviation of CH4 concentration from the assigned concentrations are calculated. The estimated accuracy achieved using the target tank is better than ± 3 ppb.
Routine data work-up occurs on a monthly basis using bespoke procedures written for Igor Pro.