Minimization of phosphate interference in the direct determination of arsenic in urine by electrothermal atomic absorption spectrometry

A direct method for the determination of arsenic in urine by Zeeman atomic absorption spectrometry has been proposed by graphite furnace atomization with (NH4)3RhCl6 + citric acid as a mixed matrix modifier. Different matrix modifiers including palladium, nickel and rhodium, were used for the elimination of phosphate interference, which was most troublesome in the arsenic determination in urine. Rhodium was preferred to palladium and nickel; in its presence the determination of arsenic in urine can tolerate a char temperature of 1600 °C, a temperature high enough to drive off the urine matrix including calcium phosphate without losing arsenic. The citric acid in the furnace evolved active gases and carbon upon decomposition thus facilitating the creation of a favorable reducing environment for the formation of a Rh-As alloy or intermetallic compounds which stabilized arsenic to a high temperature. In the determination of 1 ng of arsenic, 1.0 mg ml−1 phosphate in the form of calcium phosphate can be tolerated. Analytical results obtained for standard urine sample compared (>92% recovery) well with the certified value. The lowest concentration of arsenic that could be determined was 20 ng ml−1 in the undiluted urine.