Improving the accuracy and precision of an arsenic field test kit: increased reaction time and digital image analysis

Two procedures to improve the performance of the Hach EZ test kit for quantifying inorganic arsenic concentrations in drinking water have been investigated. In the first, a digital image of the colored spot formed on the test strip, obtained with a flat-bed scanner was analyzed, by the computer program Colors, for the R, G, and B values. Calibrations were constructed by plotting the B values as a function of concentration. Agreement between the experimentally determined B-values and those of the printed chart was only obtained by either increasing the reaction time (to 40 min) or increasing the reaction temperature. The precision as a function of concentration was quantified. A comparison with previously estimated values for visual comparison of the colours, showed that the improved precision of the digital analysis would produce fewer false positive and fewer false negative results at the important threshold values of 10 and 50 μg L−1. By running the test for 24 h, improved performance at the low concentration (around 10 μg L−1) end of the response scale was obtained.