An improved microextraction technique for measuring dissolved inorganic carbon (DIC), δ 13C DIC and δ 18O H2O from milliliter-size water samples

Abstract

Dissolved inorganic carbon (DIC), δ 13C DIC and δ 18O H2O are traditionally measured using three different analytical techniques. Herein is described a precise, accurate and relatively simple technique for analyzing all three parameters from a single ⩽ 2-ml water sample. Water samples are injected through a septum into a temperature-controlled, water-jacketed, evacuated vessel containing 0.3 ml of orthophosphoric acid and a magnetic spinbar. The extraction line and vessel are coupled directly to the inlet of the mass spectrometer. Evolved CO 2 is drawn through two −90°C traps to separate water vapor, and CO 2 is condensed into a liquid-nitrogen trap. Yields of 98–100% are achieved by using a capillary tube between the reaction vessel and traps, dynamic “cold” pumping with liquid nitrogen, acidification to < 1 pH unit, and vigorous stirring. DIC is measured as a function of the voltage produced by mass 44 of the CO 2 gas in a fixed volume, and CO 2 is then analyzed for δ 13C DIC and δ 18O H2O using conventional mass spectrometry techniques. A correction factor of − 1.10‰ is applied to δ 18O-aqueous in order to compare it with δ 18O-vapor measured by the conventional Epstein-Mayeda method. The method is fast (45 min. per sample), reproducible (standard deviation DIC=±0.1 mmol 1 −1; standard deviation δ 13C DICand δ 18O H2O= ±0.1‰), and accurate down to 1 mmol 1 −1. It represents a considerable refinement over existing methods, and is especially valuable for studies in which sample size is a limiting factor. It can also replace the more cumbersome conventional methods where sample size is no object.