A 0.2g sample is first combusted in a resistance furnace at 550 °C in a pure oxygen environment to remove sulphide sulphur, although this is dependent on mineralogy (see 4F – Sulphide method description and below from ASTM). A catalyst is added to the sample and the temperature of the resistance furnace is increased to 1450 °C. During combustion, sulphur-bearing elements are reduced, releasing sulphur, which binds with oxygen to form SO2. Sulphur is measured as a SO2 in the infrared cell. ELTRA instruments are used for analysis.
According to ASTM E1915 in regards to sulphate sulphur, “In the absence of sulphide forms of sulphur, total sulphur may be used to estimate the sulphate sulphur concentration. The pyrolysis residual sulphur may be the best estimate of sulphate sulphur, in the presence of barite, alunite, jarosites, since these sulphate forms are not dissolved by sodium carbonate and in the presence of orpiment and realgar, since these sulphide minerals are soluble in sodium carbonate. The sodium carbonate sulphur loss may be the best estimate of sulphate sulphur in the presence of metal sulphide minerals other than iron, which may not be lost by pyrolysis and the absence of barite, alunite, jarosites, orpiment and realgar. ”