Rapid and precise δ13C measurement of dissolved inorganic carbon in natural waters using liquid chromatography coupled to an isotope-ratio mass spectrometer

Jay A. Brandes

Limnol. Oceanogr. Methods 7:730-739 (2009) | DOI: 10.4319/lom.2009.7.730

ABSTRACT: The measurement of the carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) in natural systems has provided a tool for examining a variety of processes, from net primary production to anthropogenic signatures of CO2 uptake in the oceans. Over the past decade, new δ13C-DIC methods have steadily decreased the sample analysis time and volumes for this measurement. The development of a new interface capable of inline acidification and extraction of CO2 from a liquid stream (LC-IsolinkĀ™; Thermo Electron) provides breakthroughs in sample size, ease of sample handling, and speed of analyses. Typical sample size injections of 25 µL provide precision and accuracy of 0.04‰, comparable to that obtained by off-line extractions/dual inlet analyses on much larger samples. Single-sample injection runs can be accomplished within 4 min, whereas sample preparation is limited to filtration to remove particulates. Thus combined runs of 200 samples over a 20-h period are possible. Highest-precision triplicate injection analyses of single samples require 11 min. Manually injected marine sample sizes of 2.5 µL provide nearly the same precision and accuracy as the larger loop sizes. For maximum precision, care must be taken to match sample and standard sizes to correct for linearity effects. Over wider concentration ranges typically found within estuarine systems, the overall precision of the method decreases to 0.06‰ with such correction.