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We measure routinely δ²H and δ¹⁸O of water samples δ¹³C and δ¹⁸O of carbonates using a GasBench® peripheral, and the δ¹³C and δ¹⁵N values of organic matter using an elemental analyzer

Water

Deuterium/hydrogen ratios of water samples (δ²H-H₂O):

Equilibration of an aliquot of water (typically 200 uL) with a mixture of 2% H₂ in helium.
After placement of the samples at the bottom of 12 mL Labco Exetainers, the vials are capped with screw caps and rubber septa, and flushed with a 2% H₂ in Helium gas mixture.
The samples are equilibrated at 25°C for at least 40 min (more for saltwater samples) in the presence of a catalyst (rod covered in Pt powder, Cu pellets, and activated charcoal) in the vials. The equilibrated gas is injected into the IRMS and the δ²H is measured.

δ¹⁸O of water Samples (δ¹⁸O-H₂O):

Equilibration of an aliquot of water to be analyzed (typically 200 uL) with a mixture of 0.3% CO₂ in helium.
After placement of the water samples at the bottom of 12 mL Labco vials, the latter are capped with a screw cap and rubber septum, and flushed with 0.3% in helium gas mixture.
After equilibration for 24h at 25°C, the equilibrated gas is injected into the IRMS and δ¹⁸O is measured. All vials are equilibrated for an identical time for more consistent results.

GasBench (back) and vial heating block with PAL autosampler (front). This peripheral is used for water and carbonates analyses.

Carbonates

δ¹⁸O and δ¹³C of carbonates:

Conducted by dissolution of an aliquot of carbonate powder (see Table 1 for optimal weight of carbonate minerals) with phosphoric acid. The phosphoric acid (3-4 drops) is added manually after the vials (typically 12 mL ) have been flushed with ultra-pure helium gas.
CO₂ generated is equilibrated with He gas at 65 °C for at least 2h.
Resulting gas mix analyzed on IRMS after being “cleaned” through GasBench column and water traps.

Organic Matter

δ¹³C and δ¹⁵N of bulk organic matter

Measured by combustion of organic compound and analysis of the resulting gas.
Samples are weighed and packed in tin cups later inserted directly into the combustion oven of our Costech Elemental Analyzer. In the presence of O₂, the samples are burned instantly at 1600°C and the sample is completely combusted. After passing through a reduction column (to reduce remaining O₂ and NO_x), the resulting gases (N₂ and CO₂) are injected into the IRMS for measurment of δ¹⁵N and δ¹³C.
The Costech EA is equipped with a Thermal Conduction Detector (TCD) and can measure the weights % of organic N and C.
The accuracy and precision of the measurement largely depends on the weight of organic N and C in each sample. Organic N should range between 0.06 mg and 0.15 mg. Organic C should range between 0.25 and 0.70 mg. These weights are ideal, but can be worked around to some extent. Note that samples which organic C/N molar ratio is >20 will have to be analyzed (and charged) twice, once for N_org, and once for C_org. If uncertain, contact us at magen@umces.edu, or by phone at 410-326-7459.

Costech ECS 4010 Elemental Analyzer used for the analysis of solid organic matter samples

Gases

δ¹³C of Methane

Measured by Cavity Ring Down Spectrometry (CRDS) using a Picarro G2201-i Analyzer
Samples are contained in glass bottles with butyl rubber septa for a gas tight seal. The gas is retrieved from the bottles' headspace using a needle, and mixed with methane-free air into a 140 mL syringe. The mix is then injected directly into the Analyzer.
Knowing the concentration of methane is a prerequisite for this method. The Picarro Analyzer can measure samples which methane concentrations range between 10 ppm and 1,500 ppm. The volume of gas needed is 100 mL for samples at 10 ppm. If you don't know the concentrations of your samples, we offer the possibility to analyze methane concentrations on a separate Gas Chromatograph equipped with a FID.

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Water

Deuterium/hydrogen ratios of water samples (δ2H-H2O):

δ18O of water Samples (δ18O-H2O):