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Analytical services: Carbonate δ18O and δ13C isotopes

The SPA lab has two automated analyzers for biogenic and inorganic mineral carbonate δ18O and δ13C isotopes: a Kiel IV Carbonate Device and a GasBench II, both of which are coupled to the MAT-253 LGIR mass spectrometer.  The Kiel device is slower and more expensive to run but has marginally better precision at small samples sizes (<50 µg of pure carbonate). The Kiel device is limited to solid carbonate material. The Gasbench has faster throughout, uses cheaper consumables and generally is more likely to work at any given time. The Gasbench can analyze the isotopic composition of DIC as well as solid carbonates.

Kiel IV Carbonate Device

Solid carbonate samples are analyzed for δ18O and δ13C isotopes by acid digestion using an individual vial acid drop ThemoScientific Kiel IV carbonate device interfaced to ThermoScientific MAT-253 dual-inlet isotope ratio mass spectrometer (IRMS). Air is removed from the reaction vial using vacuum pumps. Samples are reacted at 70°C in orthophosphoric acid (specific gravity ~ 1.90 g/cm3) to generate carbon dioxide and water. Water is cryogenically removed from CO₂ and non-condensable gases are pumped away, prior to introduction of the purified CO₂ into the IRMS.

During a run sequence, three each of NBS-19 and LSVEC international standards are used to normalize isotope results using a two-point linear normalization technique. Two NBS-19 limestone samples are run "as a sample" to monitor quality control and long-term performance. Normalized delta values are expressed relative to VPDB for δ13C and δ18O.

Gas Bench II

Solid carbonate δ13C and δ18O ratios in carbonate minerals can be determined by the analysis of CO₂ generated by the reaction of carbonate with orthophosphoric acid. Air is removed from the reaction vial by flushing with pure helium. Depending on the carbonate material, the reaction is kept at 72°C and allowed to proceed for between 1 and 48 hours. Calcite often requires only 1 hour, aragonite requires 24 hours, while dolomite and apatite require 48 hours. The CO₂ gas evolved ends up in the headspace of the reaction vial such that the δ13C of the CO₂ gas is equivalent to that of the carbonate. The headspace CO₂ is analyzed using a MAT-253 IRMS. The SPA lab uses smaller 4.5 ml vials (see Breitenbach and Bernasconi 2011) to successfully analyze 20 µg of carbonate with the same precision as 100 ug in the standard 12 ml Labco vial.

During a run sequence, three each of NBS-19 and LSVEC international standards are used to normalize isotope results using a two-point linear normalization technique. Two NBS-19 limestone samples are run "as a sample" to monitor quality control and long-term performance. Normalized delta values are expressed relative to VPDB for δ13C and δ18O.

Automated δ18O and δ13C carbonate device rates

Dual Inlet: Kiel IV Carbonate Device

Isotope	Phase	Quantity	Measurement Precision	Internal	External
δ18O, δ13C	Solid	20–50 µg	± 0.08 ‰	$10	$16

Sample types: Biogenic and inorganic minerals (e.g., calcium carbonate, fossil calcite shells).

Conflo: GasBench II

Isotope	Phase	Quantity	Measurement Precision	Internal	External
δ18O, δ13C	Solid	20–200 µg	± 0.1 ‰	$6	$10

Sample types: Biogenic and inorganic minerals (e.g., calcium carbonate, fossil calcite shells) and many carbonate containing materials\rock\sediment.

Solid carbonate sample preparation guidelines

Sample size: For pure calcium carbonate measurements, sample sizes of 30–70 µg return optimal isotopic precision on the Kiel IV. For the Gasbench II, sample sizes of 30–200 µg are optimal; we use smaller 4.5 ml vials for Gasbench II samples below 80 µg. Additional material is always welcome as samples occasionally fail to analyze; replicates will be analyzed at no additional cost if additional material is provided. Samples as small as 10 µg have been reliably measured on the Kiel; precision usually degrades to +/- 0.3 per mil for 10 µg samples. For bulk sediment/soil samples, either include additional material so we may estimate percent inorganic carbon and ensure an adequate amount of carbonate or provide your own estimate; bulk material samples should contain the equivalent of at least 10 µg of carbonate.

Powdered samples: Static charge on very small (<80 µg ) fine-grained powdered samples can be a problem because the static charges can make it difficult to remove powdered samples from plastic containers. To get around this problem, we use a mailer system developed by the KPESIL lab at the University of Kansas for safely mailing powdered samples as small as 10 µg. If you would like to use the mailer, we will ship it to you with instructions. If you are planning on using our lab for powdered carbonate analysis, contact us while you are performing sampling (e.g. micromilling) of your material so that you may place the sample powder directly into the mailer as it is collected to save time and effort. Larger samples are easily submitted already packed in Labco exetainer vials (either 4.5 ml or 12 ml) or in any well-sealed sample container (glass preferred).

Non-powdered samples: We can analyze most whole samples (e.g. carbonate rock, fossil shells, etc.) following standard techniques. Ship the material as appropriate. However, there is an additional charge of $2 for grinding and/or subsampling from whole materials.