Direct measurement of atmospheric formaldehyde using gas chromatography-pulsed discharge ionisation detection

A gas chromatographic system has been developed for the direct analysis of atmospheric formaldehyde and other oxygenated hydrocarbons. This method utilises the trapping of analytes in a loop cooled with liquid nitrogen, separation by gas chromatography and then subsequent detection using a pulsed discharge helium ionisation detector (pdHID). The detection limit of this instrument is estimated to be 32 parts per trillion by volume (pptv) for 0.2 l of gas sampled at a flow rate of 30 ml min–1 (S/N at 4:1). A number of other compounds have been monitored simultaneously using this method, including higher molecular weight aldehydes and acetone. Continuous measurement of formaldehyde can be performed with a time resolution of 15 min, with longer analysis times required for inclusion of other species. Calibrations were performed using a permeation tube instrument for both formaldehyde and acetone. A linear response has been observed for formaldehyde sample volumes of 60–200 ml of a 272 ppbv sample (20–68 ng). For acetone a linear response was observed from 175–360 ml of a 30 ppbv standard (12–26 ng). A flame ionisation detector (FID) was also utilised during system development to confirm separation of formaldehyde from atmospheric hydrocarbons.