Analysis of Dissolved Organic Carbon (DOC) in Water by TOC-IRMS
THE SIF IS NOT CURRENTLY ACCEPTING NEW PROJECTS FOR DOC ANALYSIS. This page will be updated when this status changes
The SIF provides 13C isotope analysis of dissolved organic carbon using a TOC analyzer interfaced to an IRMS.
Dissolved inorganic carbon (DIC) is no longer analyzed using this system; please refer to DIC in Water.
Analysis
DOC is analyzed for 13C using an O.I. Analytical Model 1030 TOC Analyzer (Xylem Analytics, College Station, TX) interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK) utilizing a GD-100 Gas Trap Interface (Graden Instruments). Samples are acidified and purged with helium off-line to remove all dissolved inorganic carbon (DIC). Depending on sample concentration, a 1- to 9-mL aliquot of sample is transferred into a heated digestion vessel and reacted with sodium persulfate to convert DOC into a pulse of CO2. The CO2 is carried in a helium flow to the isotope ratio mass spectrometer where the 13C/12C ratios are measured.
During analysis, samples are interspersed with several replicates of at least three different laboratory reference materials. These reference materials have been previously calibrated against international reference materials, including: IAEA-600, USGS-40, USGS-41, and Elemental Microanalysis (EM) reference materials. A sample’s provisional isotope ratio is measured relative to a reference gas peak analyzed with each sample. These provisional values are finalized by correcting the values for the entire batch based on the known values of the included laboratory reference materials.
The final 13C delta values, delivered to the customer, are expressed relative to international standard VPDB (Vienna Pee Dee Belemnite). For information on delta notation and the international standards, please refer to a stable isotope reference such as Sharp, Z. (2005) Principles of Stable Isotope Geochemistry (Prentice Hall).
Limit of Quantification and Analytical Precision
0.4‰ for 13C of DOC at DOC concentrations greater than 0.5ppm