Sample Preparation for Dissolved Inorganic Carbon (DIC) in Water by GasBench-IRMS

 

Submitting Water Samples for DIC analysis

The best way to maintain the isotopic integrity of your DIC sample is to perform the Exetainer Gas Evolution Technique in the field.

The optimal DIC concentration range is: 1 µg C/ml to 200 µg C/ml, with a maximum of 500 µg C/ml (or 0.1 mM to 15 mM, with a maximum of 40 mM).

Exetainer Gas Evolution Technique for DIC

You can prepare water samples in Exetainers with phosphoric acid as described below (see also Zhong-Ping et al. 2008).

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 to at least 6-10 millitorr using a vacuum pump fitted with a 24 gauge needle to pierce the septum cap. Helium flush-fill the vials at 500 mL/min for at least 20 seconds. Using a syringe with a 24-gauge needle, add up to 4 mL water, depending on expected DIC concentration.

We do not accept the submission of prepared DIC samples in smaller Exetainer vials or alternative vials.

Label each vial with a unique Sample ID# as your data will be reported using this ID. An estimate of the DIC concentration in your samples will help us prepare the appropriate range of laboratory reference materials.

Submitting Unprepared Water Samples for DIC Preparation and Analysis by the SIF

If you cannot perform the gas evolution technique, to lower the risk of diffusion and loss of CO2 from your water sample, please collect samples following these guidelines.

Submit unprepared water samples in 12 ml Exetainer vials.

Submit unprepared water samples in 10-ml to 40-ml glass vials with open-top (i.e., pierceable) caps and butyl rubber septa. Please note that PTFE-silicone septa are not gas impermeable; your dissolved gas sample can be lost through these septa. If your vials come with PTFE-silicone septa (e.g., I-Chem vials), you must replace the PTFE-silicone septa with butyl rubber septa (see below for recommended supplies).

Please contact us in advance for approval if you plan to submit unprepared DIC samples in containers that do not have septum-lined open-top caps.

The water sample should fill the vial completely (i.e., have a positive meniscus) with no headspace or air bubbles. Any air bubbles or leaks are a source of gas exchange, which may significantly alter your sample. Refrigerate samples (do not freeze) prior to shipping and ship in insulated containers as soon as possible after collection.

Please contact us if you have questions.

Preserving Samples

SIF does not accept DIC samples that have been preserved using sodium azide. Filtration (<0.45 um) is only recommended if done carefully, without entraining air bubbles or allowing prolonged exposure to atmosphere. If you cannot perform the gas evolution technique in the field, please contact us to discuss options regarding DIC sample preservation.

Supplies

*Note: Labco has a wider vial style that ends in ###W/W.  We DO NOT accept these vials at the SIF because the vials are too wide for our autosampler racks. 

 

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.

Zhong-Ping, L., Ming-Xin, T., Li-Wu, L., Zuo-Dong, W., Li, D., Ming-Feng, Z. 2007. Determination of isotope composition of dissolved inorganic carbon by gas chromatography- conventional isotope-ratio mass spectrometry. Chin. J. Anal. Chem. 35: 1455-1458.

Taipale, S.J., Sonninen, E. 2009.  The influence of preservation method and time on the δ¹³C value of dissolved inorganic carbon in water samples.  Rapid Communications in Mass Spectrometry 23: 2507-2510.

Waldron, S., Scott, E.M., Vihermaa, L.E., and Newton, J. 2014. Quantifying precision and accuracy of measurements of dissolved inorganic carbon stable isotopic composition using continuous-flow isotope-ratio mass spectrometry. Rapid Communications in Mass Spectrometry 28: 1117-1126.