Determination of the δ³⁴S of Total Sulfur in Solids: RSIL Lab Code 1800

The purpose of Reston Stable Isotope Laboratory Lab (RSIL) Code 1800 is to determine the δ(³⁴S/³²S), abbreviated as δ³⁴S, of total sulfur in a solid sample. A Carlo Erba NC 2500 elemental analyzer (EA) is used to convert total sulfur in a solid sample into SO₂ gas. The EA is connected to a continuous flow isotope-ratio mass spectrometer (CF-IRMS), which determines the relative difference in stable sulfur isotope-amount ratio (³⁴S/³²S) of the product SO₂ gas. The combustion is quantitative; no isotopic fractionation is involved. Samples are placed in tin capsules and loaded into a Costech Zero-Blank Autosampler on the EA. Under computer control, samples are dropped into a heated tube reaction tube that combines both the oxidation and the reduction reactions. The combustion takes place in a He atmosphere that contains an excess of oxygen gas at the oxidation zone at the top of the reaction tube. Combustion products are transported by a He 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 (GC). The gas is then introduced into the isotope-ratio mass spectrometer (IRMS) through a Thermo-Finnigan ConFlo II interface, which also is used to inject SO₂ reference gas and He for sample dilution. The IRMS is a Thermo-Finnigan DeltaPlus 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. The ion beams from SO₂ are as follows: m/z 64 = SO₂ = ³²S¹⁶O¹⁶O; and m/z 66 = SO₂ = ³⁴S¹⁶O¹⁶O primarily.