Dissolved Inorganic Carbon δ13C Analysis

Sample Material: Seawater or freshwater

Sample Collection: DIC samples are notoriously difficult to collect and preserve. If you are sampling from a Niskin bottle you want to be one of the first people in the line-up to sample (only those collecting CFC’s and oxygen go before you). Collect the water sample in a glass vial (not plastic). Minimize the amount of sample disturbance and air bubbles. Fill the vial so that there is no headspace. Add saturated HgCl2 (10µl HgCl2 per 50 ml of water) and then seal the vial. Ideally, use a solid cap (i.e., not a cap with a septa). If you need to use a septa cap (crimp type or screw on) you want to use ones made of butyl rubber. Do NOT use Teflon-backed septa as they are permeable and the DIC (as CO2) will diffuse out. Refrigerate the samples until analysis.

Table 1.

DIC concentration (mM) Volume (ml) for one analysis
15-20 (anoxic porewater) 0.10 to 0.15
10 0.25
2-3 (typical seawater) 0.70
1 ~2
0.5 ~4

Sample volume: Depends on DIC concentration. Concentration data must be provided when samples are submitted. We do NOT measure DIC concentrations.

DIC Concentration: >0.5 mM (for samples with <0.5 mM DIC refer to Trace Gas Method)

Precision: ±0.15 permil for δ13C

Method:
The appropriate amount of sample (or standard) is pipetted into a Labco exetainer vial (7ml size) and then closed with rubber septa caps. The vials are then placed in a water-cooled (13°C) rack and allowed to temperature equilibrate for 15 minutes. The vials are then purged with He for 5 minutes each (Fig. 1). When purging of all samples is complete ~0.1 ml of 85% orthophosphoric acid is added by syringe to each vial (more may be used for larger volumes of water) and the sample is gently agitated for a few seconds to mix the water and acid (Fig. 2). The samples then equilibrate for 10 hours after which they are analyzed by continuous-flow mass spectrometry using the GasBench-DeltaV system. The run is standardized using a combination of sodium bicarbonate (in solution) and calcium carbonate (as a solid) standards.

Figure 2.

Figure 1.

Suggested Reading:

Torres, M.E., Mix, A.C., and Rugh, W.D. (2005). Precise δ13C analysis of dissolved inorganic carbon in natural waters using automated headspace sampling and continuous-flow mass spectrometry. Limnology and Oceanography: Methods, 3, 349-360.