Determination of trace amounts of sulfur in high-purity iron by infrared absorption after combustion: removal of sulfur blank

In order to determine trace amounts of sulfur in a high-purity iron with the infrared absorption method after combustion, it is necessary to decrease and stabilize sulfur blank on an analytical procedure. Sulfur blank means sulfur species as contaminants which are contained in a reagent, in a ceramic ware and absorbed on surface of an analytical sample and so on employed during an analytical procedure. As a signal of sulfur blank overlaps on a signal of sulfur in an analytical sample and causes error of determination of sulfur in the sample, it needs to be removed before analysis. Sulfur blank is decreased and stabilized by following operations: (a) an accelerator for combustion was heated for 28 min at 1003 K in ambient atmosphere; (b) a ceramic crucible was heated during more than 20 min at 1623 K in ambient atmosphere: (c) oxygen was purified through a gas purifier; (d) an analytical sample was cleaned in an acidic solution. The operation (a) was examined in detail. When a mixture of 0.5 g tungsten with 1.5 g tin as accelerator was heated for over 28 min at 1003 K in ambient atmosphere, sulfur blank value was decreased from 2.75 μg down to 0.94 μg. Consequently, the heating time was fixed for 28 min. As sulfur blank was increased with increasing of mass of an accelerator, its mass was fixed on 2 g. When relationship between sample weight (X axis) and amount of sulfur (Y axis) determined was plotted, the relation became linear. Sulfur content in a sample could be found from a slope of the straight line. By this method, sulfur contents in 3 samples were determined to be 0.71, 1.91 and 1.05 μg/g, respectively. Moreover, sulfur blank on an analytical procedure could be given by the intercept on the Y axis of the straight line. Sulfur blank values were in the range of 1.03-1.11 μg. Sulfur blank values on this manner were very stable for each sample throughout the analytical procedure. When it was defined that detection limit of sulfur was 3 times of standard deviation of sulfur blank and 1 g of a sample was used for analysis, it corresponded to 0.15 μg/g of sulfur.