MINIMIZATION OF PHOSPHATE INTERFERENCE IN THE DIRECT DETERMINATION OFARSENIC IN URINE BY ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY

A direct method for the determination of arsenic in urine by Zeeman at omic absorption spectrometry has been proposed by graphite furnace ato mization with (NH4)(3)RhCl6 + citric acid as a mixed matrix modifier. Different matrix modifiers including palladium, nickel and rhodium, we re used for the elimination of phosphate interference, which was most troublesome in the arsenic determination in urine. Rhodium was preferr ed to palladium and nickel; in its presence the determination of arsen ic in urine can tolerate a char temperature of 1600 degrees C, a tempe rature high enough to drive off the urine matrix including calcium pho sphate without losing arsenic. The citric acid in the furnace evolved active gases and carbon upon decomposition thus facilitating the creat ion of a favorable reducing environment for the formation of a Rh-As a lloy or intermetallic compounds which stabilized arsenic to a high tem perature. In the determination of 1 ng of arsenic, 1.0 mg ml(-1) phosp hate in the form of calcium phosphate can be tolerated. Analytical res ults obtained for standard urine sample compared (>92% recovery) well with the certified value. The lowest concentration of arsenic that cou ld be determined was 20 ng ml(-1) in the undiluted urine.