Optimized sample preparation for isotopic analyses of CO2 in air:: systematic study of precision and accuracy dependence on driving variables during CO2 purification process

A systematic analysis of efficiency, reproducibility and accuracy of cryogenic purification of CO2 from air samples for isotopic analyses is presented. The technical characteristics of the cryogenic line are given in detail. To study the cryogenic process, three different operating parameters are considered: flow rate of the gas entering the line, pressure of the gas in the line, and CO2-trap shape. Experimental results demonstrate that efficiency, reproducibility and accuracy strongly depend on the CO(2)trap shape. Moreover, a dependence of reproducibility and accuracy on the flow rate of the gas is found, but not on its pressure. High precision <= 0.02 parts per thousand for delta(13)C and <= 0.05 parts per thousand for delta(18)O) and good accuracy (< 0.09 parts per thousand for delta(13)C and < 0.14 parts per thousand for delta(18)O) is achieved after applying the N2O correction. Copyright (c) 2005 John Wiley & Sons, Ltd.