3.2 Method validation
The described SPE-HPLC method was validated with respect to specificity, linearity, accuracy, precision, LOD and LOQ. The method specificity was assured by checking the peak purity of the analytes in the HPLC chromatogram; no interferences co-eluted with the analytes were detected.
The investigated compounds were quantified by integration of peak areas at 210 nm using an internal standard method. The calibration curve of each compound was performed with at least six appropriate concentrations in triplicate. Least-squares linear regression was used to determine the calibration parameters for each of the three standards. A summary of the calibration studies for the three analytes is presented in Table 2. The linearity of all calibration curves was determined by calculating the correlation coefficients (r 2 ), which varied from 0.998 8 for epicatechin to 0.999 8 for gallic acid.
The dilute solution of the standard mixture solution was further diluted to a series of concentrations with 30% methanol for the gain of the LOD and LOQ. As shown in Table 1, LODs and LOQs were less than 0.466 and 0.800 g•mL1 in DAD at 210 nm, respectively.
The overall RSD precision value (intra-and inter-day) for the retention times and peak-areas were better than 0.12% and 2.23%, respectively.
The accuracy was expressed as percentage recovery of the three different concentrations. The mean recoveries of the analytes were calculated at three concentration levels as reported in Table 2. The overall mean recovery ranged from 87.9% to 103%, showing the developed method is reproducible with good accuracy.