Analysis of DIC in Water by Trace Gas-IRMS

The SIF provides ¹³C isotope analysis of dissolved inorganic C using a Sercon Trace Gas system interfaced to an IRMS.

Analysis of DIC in Water

Water samples (1-4 mL) are injected into evacuated 12 mL septum capped vials (Exetainers, Labco, High Wycombe, UK) containing 1 mL 85% phosphoric acid, which forces the equilibrium between CO₂ and H₂CO₃ to gaseous CO₂. Samples and standards are co-equilibrated before analysis. The CO₂ is purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min). The gas is sampled either:  a) for high concentration samples, by a six-port rotary valve (Valco, Houston TX) with a 250 µL loop programmed to switch at the maximum CO₂ concentration in the helium carrier or b) for low concentration samples, the entire CO₂ content is frozen in a trapping loop then released to the GC column. The CO₂ is passed to the IRMS through a Poroplot Q GC column (15m x 0.53mm ID, 25°C, 3 mL/min). A reference CO₂ peak is used to calculate provisional delta values of the sample CO₂ peak. Final δ¹³C values are obtained after adjusting the provisional values such that correct δ¹³C values for laboratory standards are obtained. Two laboratory standards are analyzed with every 10 samples. The laboratory standards are lithium carbonate (calibrated against NIST 8545) dissolved in de-ionized water.

Pricing

Sample Preparation

You can prepare water samples in Exetainers with phosphoric acid as described below or submit samples (1 mL or more) in vials of your choice and we will prepare them here (see pricing differences above). To prepare samples, first add 1 mL 85% phosphoric acid to each 12 mL Exetainer vial (Labco Ltd, High Wycombe, UK). Cap the vial and evacuate with a vacuum pump fitted with a needle to pierce the septum cap. Add 4 mL water (or as little as 1 mL if needed) using a syringe with a needle. Label each vial with a unique ID# as your data will reference your samples using this ID. Some estimate of the DIC concentration in your samples will help us prepare standards in the appropriate range and to decide whether cryofocussing of the CO₂ is needed.

Shipping

Carefully package groups of exetainers in ziplock bags, small boxes, or their original boxes. Then package these sets in a larger sturdy box with styrofoam peanuts or bubble wrap. Make sure the samples are very secure, loose vials can break during shipping. Coolers will not be returned.

*NEW* Please complete an Analysis Order Form and Sample List for your samples. E-mail the completed forms to sif@ucdavis.edu and include a printed copy with your samples. The SIF uses this form to track your samples and to contact you regarding receipt of samples, data, and invoicing. For mixed analysis requests, we recommend completing a separate form for each type of analysis.

Contact information
UC Davis Stable Isotope Facility
Department of Plant Sciences
One Shields Avenue, Mail Stop 1
Davis, CA 95616
USA

Phone:(530) 754-7517, Fax: (530) 752-4361
E-mail: sif@ucdavis.edu

Supplies

*Note: Labco has a new vial style that ends in ###W/W. We DO NOT recommend these vials when sending samples to the SIF because the vials are too wide for our autosampler rack. Supplier: Labco UK + International or Labco USA + Canada

References

Atekwana, E.A., Krishnamurthy, R.V. 1998. Seasonal variations of dissolved inorganic carbon and δ¹³C of surface waters: application of a modified gas evolution technique. Journal of Hydrology 205: 265-278

Tips

1) Avoid over-tightening exetainer caps. Vials will leak when caps are over-tightened, causing the septa to pucker and pull away from the vial. Turn caps until snug, but not so far that the septa start to pucker.

2) Do not apply silicone grease, adhesives, or sealant to the tops of the vials. These materials clog the autosampler needle. The process of removing these sealants increases the risk of losing your gas sample due to dislodged septum.

Updated June 18, 2009