Physicochemical studies of indocyanine green (ICG): absorbance/concentration relationship, pH tolerance and assay precision in various solvents

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Abstract
Summary Indocyanine green (ICG) obeyed the Beer-Lambert law within the concentration range 1.25 μg/ml–10.0μg/ml in distilled water, methanol, dimethylformamide (DMF), 1.2-propanediol and aqueous buffers (pH 9.0), but only up to 7.5 μg/ml in human bile and 0.5% human albumin, and only to 5.0 μg/ml in human duodenal fluid. ICG was rapidly (<1 h) decomposed to a colorless derivative at pH11, but remained relatively stable for 48 h at pH 8–10. ICG is an indicator and a weak acid with a pKa of 3.27. In bile stabilized with 25% methanol, the precision of the method (CV) is 5% and the accuracy is 106–127%.