A paper-based Raman spot test for sulfate determination

A paper-based substrate with a delimited analytical area was employed as a spot test platform for sulfate determination. Barium chloride was deposited onto the paper surface to promote the in situ formation of barium sulfate precipitate, enabling quantitative Raman analysis. The experimental parameters were optimized, resulting in a spot diameter of 2 mm on linen paper and the use of 7.0 µL of a 0.05 mol L^-1 barium chloride solution, which provided a linear analytical response up to 400 mg L^-1, with a coefficient of determination of 0.983 and a detection limit of 1.25 mg L^-1 by the deposition of 7.0 µL of sulfate solution. The analytical sensitivity increased with the number of successive sulfate solution depositions, reaching signal saturation after seven depositions. The presence of carbonate ions reduced the method sensitivity, requiring sample acidification prior to analysis. Sulfate determination in produced water samples resulted in relative errors ranging from 5.3 to 19%, which are considered acceptable given the complexity of the matrix and the simplicity of the proposed spot test approach.