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, and W. D. Rugh College of Oceanic and Atmospheric Sciences, Oregon State University Corvallis, OR 97331-5503, USA Limnology and Oceanography; Methods (2005), V3 (8), pp349–360, doi: 10.4319/lom.2005.3.349 A new method is presented for automated measurement of the δ13C of the dissolved inorganic carbon in natural waters, using a Gas Bench-II online with a DELTA XL IRMS. This technique requires a volume (30-500 mL), analyses are rapid (~80 samples/d), and little / 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 precision of this technique is estimated to be better than ± 0.15‰ (1 sample std. dev.), which is similar to the precision of methods in use. This precision can be improved upon (averaging ± 0.04‰, but generally better than ± 0.07‰) 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 Gas Bench-II.