3.4. Chemical Composition of Peppermint Leaf Dry Extract
ICH
The presence of flavonoids and phenolic acids in peppermint leaf dry extract was confirmed by the UHPLC-ESI-MS method described previously by Bodalska et al. [The content of the dominant compounds was determined by HPLC-DAD. This method was developed and optimized previously by Fecka and Turek [The HPLC-DAD method was validated according to ICH guidelines for linearity, detection and quantification limits, intra- and inter-day precision. Calibration curves for the quantified compounds were determined from 6 measurement points, and double injections were performed for each concentration. The correlation coefficients of the calibration curves (The content of individual polyphenols (mg/g dry extract) was determined using the external standard method based on the area of the corresponding peaks. Polyphenol content was then converted to micromolar concentrations. The contents of flavonoids, phenolic acids, and polyphenols and their percentages in the polyphenol fraction (non-volatile compounds) were also calculated.