Evaluating a fast headspace method for measuring DIC and subsequent calculation of pCO2 in freshwater systems

A variety of different sampling and analysis methods are found in the literature for determining carbon dioxide (CO2) in freshwaters, methods that rarely have been evaluated or compared. Here we present an evaluation of an acidified headspace method (AHS) in which the dissolved inorganic carbon (DIC) is measured from an acidified sample and the partial pressure (pCO(2)) is calculated from DIC using pH and water temperature. We include information on practical sampling, accuracy, and precision of the DIC/pCO(2) determination and a storage test of samples. The pCO(2) determined from the AHS method is compared to that obtained from the more widely used direct headspace method (DHS) in which CO2 is equilibrated between the water and gas phases at ambient pH. The method was tested under both controlled laboratory conditions as well as wintertime field sampling. The accuracy of the DIC detection was on average 99% based on prepared standard solutions. The pCO(2) determination in lab, using the DHS method as a reference, showed no significant difference, although the discrepancy between the methods was larger in samples with <1000 mu atm. The precision of the pCO(2) determination was on average +/- 4.3%, which was slightly better than the DHS method (+/- 6.7%). In the field, the AHS method determined on average 10% higher pCO(2) than the DHS method, which was explained by the extreme winter conditions (below -20 degrees C) at sampling that affected the sampling procedure of the DHS method. Although samples were acidified to pH 2, respiration processes were still occurring (at a low rate), and we recommend that analyses are conducted within 3 days from sampling. The AHS method was found to be a robust method to determine DIC and pCO(2) in acidic to pH-neutral freshwater systems. The simple and quick sampling procedure makes the method suitable for time-limited sampling campaigns and sampling in cold climate.