## 2 Materials and Methods
### Validation of the Method  
The analytical method was validated by determination of the selectivity, linearity, precision, accuracy and stability of each analyte, according to the International Council for Harmonisation (ICH) Guidance for Industry [33].  
The standard stock solution containing a mixture of the reference compounds was prepared in triplicate in methanol and serially diluted (in two replicates) with the same solvent to six concentration levels (1%, 10%, 25%, 50%, 75% and 100% of the stock concentration) within the range of approximately 1.5-150 \(\mu\)g/mL for OA and 3.0-325.0 \(\mu\)g/mL for UA. Each sample was injected twice into the UHPLC system. Calibration tests were run at the beginning, midpoint, and the end of the analytical tests. Calibration graphs were constructed by plotting the mean peak area versus concentration. Linearity of the calibration curves was tested using two linear regression models (\(y=ax+b\); \(y=ax\)), and the \(F\)- and \(t\)-tests were applied to check statistical significance of the regression equations, slopes, and intercepts at the 99% confidence level. The residuals from linear regression models were tested ex post to check the validity of the assumptions of normality, independence, and homoscedasticity of the response variables. The possible matrix effects were evaluated by constructing the sensitivity plots for the standards dissolved in the real leaf samples exhibiting the most complicated matrices, i.e., the samples of _V. myrtillus_ and _G. procumbens_. Concentration ranges of the added standards were the same as those used for calibration. Statistical differences between the slopes of the matrix-matched linear regression equations and the calibration curves were tested by the [PERSON]'s HSD (honest significant difference) test at the 95% confidence level. The limit of detection (LOD) and limit of quantitation (LOQ) values were estimated from the standard solution diluted with methanol to provide serial solutions with their concentrations decreasing to the smallest detectable peaks. The LODs were accepted with the 3-signal-to-noise (S/N) ratio, while the LOQs were accepted if the relative standard deviation (RSD) values for peak area were less than 15% for both intraday and inter-day variability with S/N ratio greater than 10, according to [PERSON] [34].  
The measurement of intra- and inter-day variabilities was used to determine the precision of the method. The assessments were performed in triplicate using standard solution at two concentration levels and the real leaf sample of _A. uva usri_. The replicate solutions were injected twice and the RSD values were calculated for both retention time and concentration, and considered as a measure of precision. The repeatability (intra-day variability) was determined by analysing each sample within 24 h, while the reproducibility (inter-day variability) was measured on three non-consecutive days.  
The accuracy of the method was tested in the leaf extract of _G. procumbens_ by means of the standard addition/recovery procedure at three different concentration levels corresponding to the whole analytical range investigated. The samples were prepared in triplicate according to the procedure described above (Section 2.3) with the standards being added to the plant material prior to the extraction. The replicate samples were injected twice, and the accuracy was evaluated by calculating the mean recovery of the analytes from the spiked extracts versus the non-spiked samples.