Isotopes of Sulfur and Oxygen of Dissolved Sulfate

Requirements

  • Samples should be submitted in 250 mL wide mouth HDPE bottles (Fisher Scientific Cat # 14-379-007) or any 250 mL plastic bottle of your choice – Avoid tin foil lined caps. If you suspect low concentration of dissolved sulfate (< 100 mg/L), then submitting 500 mL of sample is required.
  • Filtering the samples in the field is not necessary as we will filter after receiving the shipment. This is included in the cost.
  • Bottles must be submitted with as little headspace as possible.
  • Samples do not need to be refrigerated, although, for long term storage, this is recommended.
  • Samples should be sent as soon as possible to our facility where they will be refrigerated upon arrival.

Methodology

The principle by which dissolved sulfate is converted to a useful medium for analysis of sulfur and oxygen isotopes is by the precipitation of barium sulfate. Water samples are gravity filtered using 0.7 μm GFF (glass fiber) filters. Filter is discarded. Using 0.5 N HCl, sample is acidified very slowly until pH reaches 3 to 4. A 0.25 M BaCl2 solution is prepared and 10 mL is added to the sample upon which a white precipitate is produced. The sample is allowed to precipitate barium sulfate overnight. The next day, the precipitate is collected via vacuum filtration using a 0.45 μm nitrocellulose filter (Fisher Scientific Cat # HAWP04700) and rinsed with 250 mL of deionized water to remove any residual barium chloride. The filter is removed, covered, and placed in a desiccator for several days and allowed to dry. Barium sulfate is then collected by scraping the powder into a 20 mL glass scintillation vial (Fisher Scientific Cat # 03-341-25E).

Sample analysis is done in two parts.

  1. δ34S analysis is performed by loading 300 to 400 μg of sample into tin capsules and analyzing these via a Carlo Erba NC2500 series elemental analyzer. Sample δ34S data is calibrated by analyzing, in conjunction with the samples, two barium sulfate standards in triplicate: IAEA-SO-6 and NBS-127 (NIST Ref Mat #8557). δ34S data is calibrated to the VCDT (Vienna Canyon Diablo Troilite) scale. Sample error can be reported as better than ±0.15‰.
  2. δ18O analysis is performed by loading 200 to 300 μg of sample into silver capsules and analyzing these via a Thermo Finnigan thermal conversion elemental analyzer (TCEA). Sample δ18O data is calibrated by analyzing, in conjunction with the samples, two barium sulfate standards in duplicate: IAEA-SO-5 and IAEA-SO-6, both from the International Atomic Energy Association. δ18O data is calibrated to the VSMOW (Vienna Standard Mean Ocean Water) scale. Sample error can be reported as better than ±0.21‰.

Turnaround Time

  • 8 weeks, possibly longer.