Investigation of the negative sulphate peaks obtained in the analysis of high-concentration chloride solutions by ion chromatography

The object of this work was to detect reliably 0.5 mg l–1 of sulphate in 10 g l–1 of chloride (the maximum amount that could be analysed) in order that 2–3 mg l–1 of sulphate could be detected in oil reservoir formation waters, which contain, typically, 40–60 g l–1 of chloride. When a solution containing 10 g l–1 of chloride was subjected to ion chromatography a small positive peak followed by a small negative peak occurred where the sulphate peak should have been. This made the detection and quantification of less than 2 mg l–1 of sulphate in 10 g l–1 of chloride uncertain. Investigation showed that the phenomenon was caused by sulphate contamination at the 0.01–0.02 mg l–1 level in the eluent. The use of a high-capacity anion-exchange pre-column to clean up the eluent eliminated the problem and allowed the desired detection limit to be achieved.