An improved microextraction technique for measuring dissolved inorganic carbon (DIC), δ13CDIC and δ18OH2O from milliliter-size water samples

Abstract

Dissolved inorganic carbon (DIC), δ ¹³C_DIC and δ ¹⁸I_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 S 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 CO₂ is drawn through two −90°C traps to separate water vapor, and CO₂ 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 CO₂ gas in a fixed volume, and CO₂ is then analyzed for δ ¹³C_DIC and δ ¹⁸O_H2O using conventional mass spectrometry techniques. A correction factor of −1.10%o 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 l⁻¹; standard deviation δ ¹³C_DIC and δ ¹⁸O_H2O= ± 0.1‰), and accurate down to 1 mmol l⁻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.