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
≤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 μg/L As at pH ≤1
and 0.09 μg/L As(III) at pH ∼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.