For the measurement of As, Cd, Pb, and Tl in urine or whole blood, judicious choices of internal standard elements for matrix correction and the development of a refined isobaric arsenic correction are necessary to produce accurate ICP-MS results. Ga and Rh are chosen as internal standards for As and Cd respectively. Bi is better for the correction of Pb and Tl than Re. An empirically derived equation relating the measurement of 16O 35Cl to the 40Ar 35Cl contribution to the arsenic signal at mass 75 is refined by measuring the responses at mass 51 and 75 for urines with added hydrochloric acid. Overall, ICP-MS results for blood and urine are within 6% of Zeeman GFAAS results for patient samples. For surveys, the overall average of ICP-MS results is within 3% of target.