Automatic two-stage thermal desorption gas chromatography for low-volatility organic vapour determination

Abstract

Automatic two-stage thermal desorption gas chromatography has been found useful in determining the vapour from two involatile liquids. Methyl salicylate (boiling temperature 223.3 °C) and 2,2′-dichlorodiethyl ether (boiling temperature 178.5° C) can be collected on silanised glass-wool and the amount determined over the range 1 ng–1 µg for methyl salicylate and 10 ng–100 µg for dichlorodiethyl ether. The log-log calibration graphs are linear over this range. Considerable memory effects are observed for both the compounds, which are attributed to characteristics of the equipment. Tests on the primary alcohols up to pentanol show that the memory effect is noticeable for propanol, butanol and pentanol (boiling temperatures above 97 °C). Modification of the transfer system to bypass the buffer volume minimises the memory effect, but the limiting factor is thought to be the maximum temperature (150 °C) to which the injection block can be raised.

Calibration standards were obtained by passing air or nitrogen over the liquids in an apparatus contained in a thermostated water-bath and measuring the amount of compound evaporated over 1 month, from the increase in mass of an absorption tube. The rate of mass loss of methyl salicylate from the liquid was 2.97 µg min^–1 and that of dichlorodiethyl ether was 28.0 µg min^–1 into a 100 ml min^–1 air flow at 15 °C. Low concentrations of the vapour were obtained by subsequent gas dilution. Calibration was achieved by passing samples of the vapour laden gas through adsorption tubes packed with silanised glass and subsequently desorbing the vapour on to the gas chromatograph.