Equal volume of methanol and water was mixed to give 1:1 methanol:water solution. Using this solution, 1 mg/mL of BPA stock solution was prepared in a dark amber volumetric flask by mixing 50 mg BPA standard with 50 mL 1:1 methanol:water solution and stored in a refrigerator calibrated at 4 °C away from sunlight. Using the stock solution, dilute calibration standard concentrations of 0.04, 0.20, 0.40, 0.80, 1.00, 2.00, 3.00, 4.00, 5.00 and 6.00 ng/mL were prepared using 1:1 methanol:water solution as the diluent via serial dilution.