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 δ 13 C 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 CO 2 (referenced to an array of international standards) and analyses are monitored against a stock solution of reagent NaHCO 3 , which remains isotopically stable in powder form. NaHCO 3 can be analyzed with traditional acid‐digestion methods using dual‐inlet mass spectrometry, as well as in dissolved form on the GasBench‐II. Standard NaHCO 3 stock solutions made to approximate dissolved inorganic carbon concentrations in water samples are isotopically stable over a period of a several weeks.