Precise δ13C analysis of dissolved inorganic carbon in natural waters using automated headspace sampling and continuous‐flow mass spectrometry.

Abstract

A new method is presented for automated measurement of the δ13C of the dissolved inorganic carbon in natural waters, using a Finnigan GasBench‐II headspace sampler online with a Finnigan DELTAplusXL gas‐source isotope‐ratio mass spectrometer. This technique requires a small volume (30–500 mL), analyses are rapid (~80 samples/d), and little or no manual preparation is necessary. The flexibility offered by using sample loops of various sizes permits analyses of aliquots containing 2 to 50 µg C. Based on multiple replicate measurements over a 7‐week period, the overall precision of this technique is conservatively estimated to be better than ± 0.15‰ (1 sample standard deviation), which is similar to the precision of methods in current use. This precision can be improved upon (averaging ± 0.04‰, but generally better than ± 0.07‰) by chilling the sample during equilibration at a controlled temperature of 12.0 ± 0.5°C. Standardization is provided by tank CO2 (referenced to an array of international standards) and analyses are monitored against a stock solution of reagent NaHCO3, which remains isotopically stable in powder form. NaHCO3 can be analyzed with traditional acid‐digestion methods using dual‐inlet mass spectrometry, as well as in dissolved form on the GasBench‐II. Standard NaHCO3 stock solutions made to approximate dissolved inorganic carbon concentrations in water samples are isotopically stable over a period of a several weeks.