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Skidmore College
Stable Isotope and Palæoclimate Analysis Laboratory

Analytical services: δ13C of dissolved inorganic carbon (DIC) in natural waters

The SPA lab does not routinely analyze δ13C of dissolved inorganic carbon (DIC) in natural waters; we have a very limited track record. However, we have the capability to analyze DIC isotopes using a Gas Bench II coupled to a MAT-253 IRMS. Water samples are injected into helium-flushed 12 ml Labco exetainer vials containing 0.5 ml of 85% phosphoric acid. After 18 hours of reaction time, DIC-derived CO2 evolves from the acidified water into the headspace and is passed on to a MAT-253 IRMS.

Calibrated in-house NaHCO3 standards that are interspersed between samples to correct for linearity effects and drift. A second calibrated laboratory standard is run "as a sample" to monitor quality control and long-term performance. Standards are prepared by dissolving NaHCO3 in water that has been stripped of DIC by sonicating under a weak vacuum for 1 hour. The in-house NaHCO3 powders have been previously calibrated against international Standard Reference Materials (NBS-19 and LSVEC) using a two-point linear normalization approach.  Normalized values are expressed relative to VPDB for δ13C.

DIC δ13C rates

Isotope Phase Quantity Measurement Precision Internal External
δ13C Liquid 20-50 µg ± 0.1 ‰ $8 $12

Sample types: natural waters

Sample requirements: Because we do not routinely analyze δ13C of DIC, please contact the SPA lab prior to submission. Typically, 1–5 ml of natural waters is required for analysis; most samples can be run using 1 ml, though samples very low in DIC will need the higher 5 ml amount. Water should be filtered through a binder-free 0.7 µm glass fiber syringe filter, placed in glass vials vials, filled until a positive meniscus forms,and then tightly capped so that no headspace or bubbles remains. Often waters are poisoned with HgCl2 to stop bacterial transformations. Alternatively, you may filter with with a 0.2 µm polysulfone syringe filters to remove bacteria and avoid the use of mercury (Doctor et al. 2008). Vials should be kept dark and refrigerated; provide shipping tracking information so we may be on hand to accept samples and keep them refrigerated.