A gas-phase chemiluminescence-based analyzer for waterborne arsenic

We show a practical sequential injection/zone fluidics-based analyzer that measures waterborne arsenic. The approach is capable of differentiating between inorganic As(III) and As(V). The principle is based on generating AsH3 from the sample in a confined chamber by borohydride reduction at controlled pH, sparging the chamber to drive the AsH3 to a small reflective cell located atop a photomultiplier tube, allowing it to react with ozone generated from ambient air, and measuring the intense chemiluminescence that results. Arsine generation and removal from solution results in isolation from the sample matrix, avoiding the pitfalls encountered in some solution-based analysis techniques. The differential determination of As(III) and As(V) is based on the different pH dependence of the reducibility of these species to AsH3. At pH < or =1, both As(III) and As(V) are quantitatively converted to arsine in the presence of NaBH4. At a pH of 4-5, only As(III) is converted to arsine. In the present form, the limit of detection (S/N = 3) is 0.05 microg/L As at pH < or =1 and 0.09 microg/L As(III) at pH approximately 4-5 for a 3-mL sample. The analyzer is intrinsically automated and requires 4 min per determination. It is also possible to determine As(III) first at pH 4.5 and then determine the remaining As in a sequential manner; this requires 6 min. There are no significant practical interferences. A new borohydride solution formulation permits month-long reagent stability.