Errors in measurements of CO2 with the use of drying agents

To assess the errors in measurements of CO2 concentration that arise from the widespread use of drying agents in clinical and physiological studies, continuous measurements of CO2 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 CO2 used (0-1%), but it was related to flow rate, dead space and the amount and drying agent used. At a low rate of 1 litre/min, alumina (145 g anhydrous weight) trapped virtually all the CO2 (0.8% in air) passing over it; silica gel (145 g anhydrous weight, mesh 6-20) adsorbed approximately 2 mmol CO2 (38% of total) over a period of 15 min; calcium sulphate (Drierite) absorbed a variable amount, depending on the bath; and calcium chloride adsorbed virtually no CO2. Increasing hydration of the drying agents reduced their capacity to adsorb CO2. Nitrogen was found to elute CO2 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 CO2 analysers, in the analysis of CO2, and in the measurement of specific activity or isotopic enrichment of CO2.