Many previous methods were time-consuming partly becausenHNO3 levels were reduced by evaporation prior to analysis. We wished to reduce the time taken as well as the opportunity for contamination by eliminating this step so the analytical response to mercury standards prepared in different nitric acid concentrations was investigated. Figure 3 shows calibration curves for mercury standards prepared in 30%, 40% and 50% v/v nitric acid. As can be seen there is virtually no difference in the calibration curves. Absorbances were much lower at very high concentrations of HNO3 so 40% v/v acid was chosen as the target final concentration for routine analysis.