Stock standard solution of Cu (100 μgmL−1) from National Research Center for Certified Reference Material (NRCCRMS, China) was used as stock solutions for calibration. The calibration standards were prepared by diluting the stock standard solution in 0.5% nitric acid. The calibration range was from 0.005 to 1 mg L−1 and it was modified according to the expected Cu concentration range of the samples.