Carbonate Isotopes

Requirements

  • Samples should be ground to a powder and homogenized before sending.
  • If samples are moist or wet, they should be dried at 46 °C in a drying oven for several days.
  • Samples should be sent in 2 mL low retention or siliconized microcentrifuge tubes (Fisher Scientific Cat # 02-681-321 or NC9492494).
  • The percentage and carbonate type for each sample must be indicated in any correspondence sent with the shipment, e.g., Calcite 100% or Dolomite 50%.
  • Received samples deemed too coarse (non-homogenized) will be ground via mortar and pestle at the client’s expense on an hourly rate (see Pricing, Billing, and Payment).

Methodology – Two Options

1. Continuous Flow

Samples and standards are loaded into 12 mL round bottom borosilicate glass Labco exetainer vials (Cat # 938W) and capped. Vials are purged for ten minutes with ultra-high purity helium. Using 1 mL disposable syringes with 25G needle attached, five to ten drops of 100% prepared phosphoric acid (ρ=1.9023 g/cm3) are injected into the exetainer and the samples are allowed to equilibrate for 24 hours at 30 °C. The analysis is performed using a needle syringe injected via a CTC Analytics Combi-Pal autosampler. Sample CO2 gas is passed through a Poraplot Q GC column (25 m x 0.32 mm ID, 32 °C, 2.0 mL/min) inside a Thermo Fisher GasBench II front-end peripheral, then to a Thermo Fisher Delta V Advantage isotope ratio mass spectrometer for analysis of δ13C and δ18O. Isotope data is corrected for any linearity effects displayed by a standard analyzed ten times at variable weight. Precision for δ13C is ±0.08‰ and for δ18O is ±0.23‰.

Turnaround Time:

    • ≤ 40 samples: 2-3 weeks to process.
    • > 40 samples: 6-8 weeks to process.

2. Dual Inlet

Samples and standards are loaded into Kiel vials obtained from VM Glass Company (# VMS-943). A total of 38 samples, in addition to six internationally recognized calibration standards and two internally calibrated quality control standards. Vials are placed in an autosampler tray and placed in a Thermo Fisher Kiel III peripheral set at 70 °C. Samples are processed automatically, one at a time, whereby three drops of 100% prepared phosphoric acid are added to the evacuated vial. Reaction times depend on the type of carbonate mineral submitted, i.e., Calcite=12 minutes, Dolomite=15 minutes, Siderite=20 minutes. Sample CO2 gas is then transferred to a ThermoFisher MAT252 dual inlet isotope ratio mass spectrometer for analysis of δ13C and δ18O. Although raw isotope data is calibrated to the internally used CO2 calibration tank, a second calibration is performed using the plotted curve generated by data from the international standards. Precision for δ13C is ±0.03‰ and for δ18O is ±0.07‰.

Turnaround Time:

    • ≤ 40 samples: 2-3 weeks to process.
    • > 40 samples: 6-8 weeks to process.