Briefly, whole samples (about 10 mL) were measured into a quartz beaker, then 10 mL of HNO3 was added, and the mixture was gently heated on a hot plate until the brown fumes given off by the reaction turned white. The beaker was brought down from the hot plate to cool to room temperature. The concentrated mixture (containing the sample) was rinsed with 10 mL of deionized water and filtered (using Whatman filter paper) into a 25-mL standard volumetric flask. The Beer’s law principle (A 1⁄4 abc) was used to plot the absorbance data against concentration, where A is the absorbance, a is the absorption coefficient (a constant that is character- istic of the absorbing species at a specific wavelength), b is the length of the light path intercepted by the absorption species in the absorption cell, and c is the concentration of the absorbing species.