Errors in measurements of CO 2 with the use of drying agents

Abstract

To assess the errors in measurements of CO 2 concentration that arise from the widespread use of drying agents in clinical and physiological studies, continuous measurements of CO 2 concentration were made with an infra-red analyser before and after standard gas mixtures were passed through a tube containing one of several drying agents: calcium chloride; calcium sulphate (Drierite); alumina; silica gel; and magnesium perchlorate. The response time of the infra-red analyser was independent of the concentration of CO 2 used (0–1%), but it was related to flow rate, dead space and the amount and type of drying agent used. At a flow rate of 1 litre/min, alumina (145 g anhydrous weight) trapped virtually all the CO 2 (0.8% in air) passing over it; silica gel (145 g anhydrous weight, mesh 6–20) adsorbed approximately 2 mmol CO 2 (38% of total) over a period of 15 min; calcium sulphate (Drierite) absorbed a variable amount, depending on the batch; and calcium chloride adsorbed virtually no CO 2. Increasing hydration of the drying agents reduced their capacity to adsorb CO 2. Nitrogen was found to elute CO 2 from the drying agents at about the same rate as it had been adsorbed. It is concluded that awareness of these adsorption/elution phenomena by drying agents should prevent errors from being made in the calibration of CO 2 analysers, in the analysis of CO 2, and in the measurement of specific activity or isotopic enrichment of CO 2.