The urine samples were analyzed by using a slightly modified
quantitative method based on the Sandell-Kolthoff reaction. Approximately
0.25 mL of the urine sample was digested in 1.0 mL of
ammonium persulfate at 91–95C for 60 min to remove interfering
substances. An aliquot of 0.05 mL of the digested urine sample was then
pipetted into the wells of a 96-well microplate, followed by 0.1 mL of
arsenic acid and 0.05 mL of ceric ammonium sulfate solution. The
reaction was then determined spectrophometrically with a microplate
reader at 405 nm after 25 min incubation at 30C. Each of the plates had
a unique standard curve (ranging from 0 to 320 mg/L) and internal
controls (ranging from 80 to 600 mg/L)