A thermal desorption unit coupled to a gas chromatograph fitted with two flame ionisation detectors (TDGCFID). Air was drawn down a 2 m ¼ inch Silonite (Entech) coated heated stainless steel tube at 20-30 slpm using a metal bellows compressor (Senior Aerospace MB-158) and back-pressure regulated to maintain a 25 sccm sampling flow through a custom built pre-concentration unit. Calibration gas was provided from a 10 L Experis coated aluminium cylinder (Air Liquide) working standard of N2 spiked with 0.4 ppb NMHC, linked to an NPL30 primary calibration standard. VOC free blanks were created from compressed air, through a bed of 1% Pt on Alumina beads at 375℃. Air, calibration and blanks were de-humidified with a -40℃ water trap and 500 mL samples were pre-concentrated on a multi-bed sorbent trap at around -120℃ , flow controlled by a downstream Alicat MC mass flow controller. After removal of CO2 and other permanent gases at -80℃, the preconcentration trap was desorbed and refocussed before final desorption in a H2 carrier gas flow onto a WAX GC column (Agilent VF-WAX-MS 60 m, 0.25 mm I.D. 150 µm film). Analytes not retained on the WAX column (C2 - C6 aliphatics) were diverted by Deans Switch and separated on a PLOT column (Agilent Al2O3/Na2SO4 50 m, 0.32 mm I.D. 5 µm film). Oxygenates, >C6 aliphatics and aromatics were not diverted and analytes eluting from each column were detected by flame ionisation detectors.