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

M. E. Torres, A. C. Mix, W. D. Rugh

Limnol. Oceanogr. Methods 3:349-360 (2005) | DOI: 10.4319/lom.2005.3.349

ABSTRACT: A new method is presented for automated measurement of the d13C 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 ìg 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.