Miniaturized and direct spectrophotometric multi-sample analysis of trace metals in natural waters

Abstract

Trends in the analysis of trace metals in natural waters are mainly based on the development of sample treatment methods to isolate and pre-concentrate the metal from the matrix in a simpler extract for further instrumental analysis. However, direct analysis is often possible using more accessible techniques such as spectrophotometry. In this case a proper ligand is required to form a complex that absorbs radiation in the UV-Vis spectrum. In this sense, the hydrazone derivative, di-2-pyridylketone benzoylhydrazone (dPKBH) forms complexes with Cu and V that absorb light at 370 nm and 395 nm respectively. Although spectrophotometric methods are considered as time and reagents consuming, this work is focused on its miniaturization, by reducing the volume of sample, as well as time and cost of analysis. In both methods, a micro-amount of sample is placed into a microplate reader with capacity for 96 samples, which can be analyzed in times ranging from 5 to 10 minutes. The proposed methods have been optimized using a Box-Behnken design of experiments. For Cu determination, concentration of phosphate buffer solution at pH 8.33, masking agents (ammonium fluoride, and sodium citrate) and dPKBH were optimized. For V analysis, sample pH=4.5 was obtained using acetic acid/sodium acetate buffer, and masking agents were ammonium fluoride and 1,2-cyclohexanediaminetetraacetic acid. Under optimum conditions both methods were applied to the analysis of certified reference materials TMDA-62 (lake water), LGC-6016 (estuarine water) and LGC-6019 (river water). In all cases results proved the accuracy of the method.