Speciation-Capable Field Instrument for the Measurement of Arsenite and Arsenate in Water

Hydride generation to form arsine and in-line preconcentration of the arsine into an alkaline KMnO4 receiver followed by molybdenum blue (MB) colorimetric determination of the arsenate formed is proposed for the highly sensitive and separate measurement of total inorganic As and As(III). Reduction of As to AsH3 is carried out by NaBH4; when the reduction is carried out at pH 1, all the inorganic As is reduced to AsH3, and when carried out at pH 7, only As(III) is reduced. Reductions at the two different pH levels are carried out in two different arsine generators simultaneously using constant addition of NaBH4 with solenoid pumps. The AsH3 is collected by individual porous membrane diffusion scrubbers filled with a stationary solution of KMnO4, and the contents of the two scrubbers sequentially enter a flow analysis stream. MB is formed by merging with a ammonium molybdate−ascorbic acid reagent, passing through a heated reactor, and is then measured by a LED/photodiode-based absorbance detector. Robustness was confirmed for total As using three types of certified natural water samples. Speciation analysis data from well water samples analyzed by this method agree well with HPLC−ICPMS measurements in a different laboratory. The system has been successfully applied to field measurements of As(III) and As(V), where levels were significantly below 1 μg/L. For a 20-mL sample, the limits of detection (LODs) for this inexpensive instrument are 0.3 μg/L for both As(III) and total As. When an 80-mL sample is analyzed, LODs are 0.07 μg/L As(III) and 0.09 μg/L total As. The general approach should be applicable to many other analyte species of interest that can be isolated from the matrix by the formation of a suitable volatile compound that can be recaptured.