USGS Method I-1054

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 1951 is to determine the δ(³⁴S/³²S), abbreviated as δ³⁴S, of dissolved sulfate. Dissolved sulfate is collected in the field and precipitated with BaCl₂ at pH 3 to 4 as BaSO₄ in the laboratory. However, the dissolved organic sulfur (DOS) is oxidized to SO₂, and the carbonate is acidified to CO₂. Both are degassed from the water sample before the sulfate is precipitated. The precipitated BaSO₄ is filtered and dried before introduction into an elemental analyzer (EA) Carlo Erba NC 2500.

The EA is used to convert sulfur in a BaSO₄ solid sample into SO₂ gas, and the EA is connected to a continuous flow isotope-ratio mass spectrometer (CF-IRMS), which determines the differences in the isotope-amount ratios of stable sulfur isotopes (³⁴S/³²S) of the product SO₂ gas. The combustion is quantitative; no isotopic fractionation is involved. Samples are placed in a tin capsule and loaded into the Costech Zero Blank Autosampler of the EA. Under computer control, samples are dropped into a heated tube reaction tube that combines the oxidation and reduction reactions.

The combustion takes place in a helium atmosphere containing an excess of oxygen gas at the oxidation zone at the top of the reaction tube. Combustion products are transported by a helium carrier through the reduction zone at the bottom of the reaction tube to remove excess oxygen and through a separate drying tube to remove any water. The gas-phase products, mainly CO₂, N₂, and SO₂, are separated by a gas chromatograph. The gas is then introduced into the isotope-ratio mass spectrometer (IRMS) through a Finnigan MAT (now Thermo Scientific) ConFlo II interface, which also is used to inject SO₂ reference gas and helium for sample dilution. The IRMS is a Thermo Scientific Delta V Plus CF-IRMS. It has a universal triple collector with two wide cups and a narrow cup in the middle. It is capable of measuring mass/charge (m/z) 64 and 66 simultaneously.