Prior to the preconcentration procedure, all the water samples were filtered through a 0.45 μm pore size membrane filter to remove suspended particulate matter and were stored at 4 ◦C
in the dark. To a 10 ml water sample, 1.0 ml of a solution containing 24 g L−1 Triton X-100 and 1.2 × 10−2 M PMBP and 1.0 ml of 0.1 M NaAc–HAc buffer solution (pH 5.5) were added. Then the above steps were followed.