Investigating matrix interference in the pharmacopeial limit test for aluminum in citric acid: a re-examination, for revision of the method

In many pharmacopoeias, the limit test used for determining the level of aluminum in citric acid labeled for use in the manufacture of dialysate, is based on solvent extraction using 8-hydroxyquinoline and measurement of fluorescence. However, the fluorescence intensity (F.I.) readout from the extract of citric acid samples has been found to be highly dubious, showing low value, and even lower than that of a blank solution. The aim of this work therefore was to examine what effects the matrix has on the test. The comparison of the two standard curves of aluminum solutions in water, against those prepared in citric acid solutions revealed that they differed greatly in terms of slope and y-intercept. In addition, the F.I. values on the plot of the citric acid solution were much lower than that prepared in the water. In another experiment, a decrease in the F.I. of aluminum solution was clearly seen when the co-existing concentration of citric acid was increased. The results inferred that citric acid interfered with the test due to its acidity and metal-chelating capabilities. Based on this evidence, the pharmacopeial limit test for aluminum in citric acid should be revised; otherwise, it could yield results that underestimate aluminum levels and lead to inaccurate conclusion