ASSESSMENT OF HOLDING TIMES FOR ANALYSIS OF DISSOLVED INORGANIC CARBON (DIC)

Surface streams and ground waters that originate from or pass through carbonate rocks contain dissolved inorganic carbon (DIC). In coal mine drainage waters, where sulfuric acid drives carbonate dissolution, high DIC concentrations may be present. If the DIC is primarily present as CO₂, it may be lost from the sampling vials after sample collection but prior to analysis. To determine if this is an important issue, 60 vials of water were obtained from a coal mine discharge known to contain high-CO₂ waters (~5 mM DIC; ~4.5 mM CO₂). The samples were collected in headspace-free 40-mL glass vials with PTFE-silicone septa lids (Thermo Scientific, Rockwood TN) intended for DIC sample collection. Laboratory analysis of the samples was completed using the inorganic carbon method on a Shimadzu TOC-VCHP Carbon Analyzer (Kyoto, Japan). All calibration curves had R² values > 0.999. The DIC was measured in the vials on days 1, 2, 3, 4, 8, 12, 20, 28 and 46; Day 1 analyses were completed within 2 hours of sample collection. The sample vials were stored together in a dark refrigerator.

The DIC measured in the laboratory on Day 1 was 4.80 +/- 0.12 mM C (n=12). Over the next 46 days, the DIC concentration decreased to 2.63 +/- 0.87 mM C (n=6). Based on the average concentrations, this is a loss of 45%. A loss in precision between replicate vials was also observed. On Day 1, the relative standard deviation (RSD) in DIC concentration from the 12 vials was <0.5%. The RSD generally increased over time and the Day 46 DIC concentrations had an RSD of 2.8%. This suggests that while mass loss occurs, it is not consistent between vials.

The analytical method calls for multiple aliquots to be taken from each sample vial, measured individually, and averaged. Each aliquot creates headspace in the vial possibly enhancing gas loss, however, the trends observed in the time study were consistent even when only the first aliquot from each vial was considered. This study suggests that water samples high in CO₂ should be analyzed well before the recommended maximum holding time of 28 days or mass loss may bias the measured concentrations low.