## 3 Results and Discussion
### Method Application to Real Samples  
The developed method was applied to the simultaneous determination of OA and UA in the leaves of selected Ericaceae species used in traditional medicine (Table 7). The leaves of _A. uva nvisi_, _V. vitis iidaea_, and _V. myrtillus_ are well known herbal remedies commercially available in European countries, including Poland, recommended mainly for the treatment of urinary tract infections. _G. procumbens_ is a plant native to North America and introduced to Europe as a source of leaves and essential oil used as anti-inflammatory agents of wide applicability. In all of the tested plant materials OA and UA constituted the dominating part of the triterpene acid fraction with other triterpene peaks negligible or not existing at 215 nm, which is typical for the analysed taxons [46; 47] and illustrated on the respective chromatograms (Figure 5). The amounts of OA and UA in the investigated plant tissues varied from 0.74 \(\pm\) 0.01 mg/g dw to 4.47 \(\pm\) 0.02 mg/g dw and from 1.30 \(\pm\) 0.03 mg/g dw to 18.61 \(\pm\) 0.23 mg/g dw, respectively. With the levels generally over three times higher than those of OA, the UA was the primary constituent in all samples (\(p\) < 0.001) except of _V. myrtillus_. The highest concentrations of both analytes were found in the leaves of _A. uva nvisi_. The detailed studies of triterpenes in the leaves of _V. vitis iidaea_ revealed that the content of both acids can vary significantly depending not only on the place of plant origin (i.e., local geoclimatic conditions), but also on the time of harvest and leaf development stage [48]. For UA it ranged from 1.59 mg/g dw to 3.16 mg/g dw, while for OA it ranged from 0.81 mg/g dw to 1.44 mg/g dw, which is consistent with the data obtained in our study. In the case of leaves of _A. uva nvisi_ and _V. myrtillus_ our results are higher than the previous ones, but the relative ratio remains similar. For comparison, [PERSON] et al. [47] found in the leaf extract from _A. uva nvisi_ 12.4 mg/g dw of UA and 2.7 mg/g dw of OA. The leaves of _V. myrtillus_, depending on the collection sites, contained 0.85-0.87 mg/g dw of OA and 0.75-0.78 mg/g dw of UA [49]. These variations could be attributed not only to the differences in the harvest, preparation, and storage conditions between the commercial samples analysed in the present work and the plant material from natural habitats assayed earlier, but also presumably to the different extraction condition applied in the studies [48]. As standardisation procedures are generally meant for routine analyses of large amount of samples, the economy factor is of uttermost importance. During our experiments we noticed an increased need for inline filter change, which was likely caused by waxy substances extracted with chloroform. Therefore, we proposed an alternative extraction procedure that contains one additional step, i.e., pre-extraction of the plant material with _n_-hexane. This allowed for the removal of waxes and substantially (over two times) elongated the usability of the filters, reducing the costs of the analyses. However, due to the partial solubility of the investigated analytes in _n_-hexane, the results obtained in this way are underestimated. A correction factors were thus introduced allowing for quick \begin{table}
\begin{tabular}{c c c} \hline \hline
**Species** & **OA (mg/g) 1** & **UA (mg/g) 1** \\ \hline _Arctostaphylos uva nvisi_ & 4.47 \(\pm\) 0.022 D & 18.61 \(\pm\) 0.233 D \\ _Gaultheria procumbens_ & 1.58 \(\pm\) 0.049 B & 5.76 \(\pm\) 0.043 C \\ _Vaccinium myrtillus_ & 1.78 \(\pm\) 0.003 C & 1.30 \(\pm\) 0.034 A \\ _Vaccinium vitis iidaea_ & 0.74 \(\pm\) 0.008 A & 2.50 \(\pm\) 0.021 B \\ \hline \hline \end{tabular}  
* Values calculated per dry weight basis and presented as means (_n_ = 3) \(\pm\) standard deviation. Different superscripts (capitals, A–D) in each column indicate significant differences in the means (_p_ < 0.05).  
\end{table} 
Table 7: Content of OA and UA in investigated leaf samples. recalculation of the factual OA and UA content.