Effects of mineral acids on absorption signals of metal analytes in water by atomic absorption spectroscopy

Abstract

A major requirement in flame spectroscopic methods is that the sample be introduced into the excitation source in form of an aqueous solution. Unfortunately, many materials of interest such as soils, animal tissues, plants, petroleum products and minerals are not directly soluble in water and extensive preliminary treatment is often required to obtain a solution of the analyte in a form ready for atomization. On the other hand, some samples contain both organic and inorganic matrices, which require digestion prior to instrumental analysis. This requires careful consideration of digestion reagents. In this project, the effects of mineral acids on absorption signals of metal analytes in water by atomic absorption spectroscopy were studied. This was done with respect to variations in metal concentration and acid concentration with the aim of determining the most suitable acid for digestion purposes and the most effective acid concentration for trace metal analysis by Atomic Absorption Spectroscopy (AAS). Different concentrations of metal analytes of manganese, iron, copper, zinc and lead were digested with different concentrations of nitric, hydrochloric and perchloric acids and analyzed using the Atomic Absorption Spectrophotometer. In this analysis, water samples from River Njoro (a fresh water system) and Lake Nakuru (a highly alkaline water system) were used to confirm the applicability of the results obtained. In the standards, lake Nakuru and River Njoro water samples, it was realized that perchloric and nitric acids had equal but higher peak enhancing effects as compared to hydrochloric acid on the absorption signals of all metal analytes except for iron in Lake Nakuru water sample and copper in both Lake Nakuru and River Njoro water samples. The most effective concentration for digestion purposes was at 10 % v/v for all acids. It was therefore concluded that acids have varied peak enhancing effects on absorption signals of metal analytes in water. This variation is dependent on the concentration and nature of the acid with the more oxidizing acids having greater peak enhancing effects than the less oxidizing acids. For effective determination of Mn, Fe, Pb and Zn metal analytes in River Njoro and Lake Nakuru water samples more reliable results would be obtained when 10 % v/v nitric or perchloric acids are used for digestion purposes. While for effective determination of Cu in Lake Nakuru and River Njoro water samples, more reliable results would be obtained when 10 % perchloric, nitric or hydrochloric acids are used for analysis.