Flow injection determination of carboxylate, phosphate, and sulfhydryl compounds using metal exchange complexation

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The determination of carboxylate, phosphorous, and sulfhydryl compounds has been studied using flow injection by measuring the decrease in absorbance of the Fe(III)–salicylate complex due to preferential ligand interaction with ferric ion. Targeted compounds include polycarboxylates such as sodium citrate, sodium oxalate, and EDTA, anionic phosphorous compounds such as sodium monofluoroorthophosphate, sodium trimetaphosphate, and sodium hexametaphosphate, and sulfhydryl compounds such as cysteine, glutathione, and captopril. Initial flow injection optimization has focused on citrate based on its ability to replace salicylate ion in the Fe(III)–salicylate complex causing a decrease in absorbance at 525nm proportional to the citrate concentration. Two flow injection analysis methods are developed. In the first method, offline reaction flow injection, sodium citrate dissolved in 100μmolL−1 Fe–salicylate is injected in a carrier solution of 100μmolL−1 Fe–salicylate. The decrease in peak area is linear over a range of 1.36–109μmolL−1 using a flow rate of 1mLmin−1 and an injection volume of 100μL. The effect of pH on the Fe–salicylate complex absorbance is studied from 1 to 3.5; pH 3 shows both a high and stable complex absorbance in the visible range which provided important potential selectivity over UV detection. The limit of detection is found to be less than 57nmolL−1 depending on the Fe(III)–salicylate concentration used. The second method is reverse flow injection using the sample as a flowing stream in which 3mmolL−1 Fe(III)–salicylate is injected and the decrease in the response with increased sample concentration was monitored. The commercially available pharmaceutical product (Citroma)® is used to assess the accuracy and precision of the two proposed methods as compared to a reference method.