contributes to the final signal, error occurs. The degree of interfer-ence was found to be pH dependent: at lower pH, more MMAV was reduced to MMAIII causing a larger error. It was observed that at pH
1.7, the error from MMA could be kept below 10%; at pH >1.8, the MMA signal could be further reduced but incomplete AsV–AsIII reduction caused AsV signal to decline, so recovery suffered. In the pH 1.7–1.8 range, MMA error could be managed at