2.5. Method evaluation
A serial of experiments with regard to the limit of detection (LOD), limit of quantification (LOQ), linearity and reproducibility were performed to validate the proposed QuEChERS method. The linear regression analysis was performed from the matrix-matched standard calibration solutions by plotting the peak area ratios (y, targeted analytes/TPP) versus concentrations ofthe respective analytes (x). The LODs and LOQs were calculated as the concentration corresponding to the signals of 3 and 10 times the standard deviation of the baseline noise, respectively. The absolute recoveries of targeted analytes were calculated by comparing the peak areas from the spiked tobacco sample to those obtained from the working standard solution atthe same concentration (10 times of lowest concentration point of individual linear range). The reproducibility of the method was determined by precisions. The intra- and inter-day RSDs were calculated with the OCPs spiked at three different concentration levels (the spiked amounts of low, medium and high concentration levels are corresponding to 2, 10 and 100 times of lowest concentration point of individual linear range) in tobacco. Four parallel determinations over a day gave the intraday RSDs, and the inter-day RSDs were determined by analyzing sample solutions that had been independently prepared for three contiguous days. Furthermore, the apparent recoveries of targeted analytes in real samples were determined by comparing the calculated amounts of OCPs from the spiked tobacco samples (using matrix-matched calibration curves) with the total spiked amounts of OCPs [40].