## 1 Introduction  
Oleanolic acid (3$-3-hydroxyolean-12-en-28-oic acid, OA) and ursolic acid (3$-3-hydroxyurea-12-en-28-oic acid, UA) are two isomeric pentacyclic triterpenes (Figure 1) that are ubiquitous constituents of various medicinal and dietary plants [1]. Numerous studies have demonstrated their versatile biological properties with anti-inflammatory activity being the most important from phytotherapeutic point of view. As the underlying mechanism of this action, the suppression of immune cells and cytokines (TNF-\(\upalpha\), IL-12, IL-6, interferon-\(\upgamma\)) production either via inhibition of pro-inflammatory enzymes (elastase, cyclooxygenase, lipoxygenases, nitric oxide synthase) and NF-\(\upkappa\)B-regulated genes expression or direct cytokine receptor interactions has been proposed [1; 2]. Recently, the potent antimicrobial activity of both compounds has also been demonstrated towards various bacterial strains including _Escherichia coli_--a common pathogen of urinary tract infection [3]. OA and UA were found to modulate bacterial resistance to some antibiotics and produce in combination with them synergistic antibacterial effect [4]. Other biological properties attributed to OA and UA include anti-proliferative, anti-cancer, anti-mutagenic, anti-leukemic, anti-atherosclerotic, anti-hypertensive, hepatoprotective, and antioxidant activities [1; 5; 6]. The presence of OA and UA in plant materials is, therefore, regarded as beneficial for human health. Ericaceae is a large family of flowering plants comprising over 4000 species of great morphological and ecological diversity, and with almost worldwide distribution [7]. Many of its members have found use in numerous countries as traditional herbal medicinal products applied routinely in over-the-counter (OTC) drugs and dietary supplements in the treatment of bacterial infections or inflammatory ailments, especially urinary tract disorders. Triterpenes are important active constituents of Ericaceae species, and OA and UA often comprise the major part of the lipophilic fractions in these plants. Considering the proven biological activity of both acids, which closely corresponds with the range of the medical application of Ericaceae plant materials, the determination of their content might be of particular interest as far as the standardisation of these plant materials is concerned [8]. Therefore, there is a persistent interest in the development of fast, selective, and sensitive analytical methods allowing separation and reliable simultaneous quantification of the two analytes. There are several different approaches reported recently for quantification of OA and UA in plant materials including thin-layer chromatography [9], gas chromatography [10], capillary electrophoresis [11], high-performance liquid chromatography (HPLC), supercritical fluid chromatography [12], micellar electrokinetic chromatography [13], and nuclear magnetic resonance [14]. HPLC is, by far, the most frequently used technique in standardisation studies, mainly because of the high flexibility and relatively good ratio between the analytical performance and maintenance costs. However, when it comes to separation of OA and UA, the conventional RP-HPLC analysis is often problematic and some major issues arise during the assays. The close structural similarity of both compounds, manifested in similar log \(P\) (logarithm of the octanol/water partition coefficient) values (6.468 and 6.427, respectively) [15], makes them particularly difficult to separate which results in co-elution or only partial separation of the critical peaks. Although some significant effects of organic solvents and pH of the mobile phase on selectivity have been reported [16], the stationary phase efficiency is believed to be the most important factor determining the resolution of the overlapping bands of native acids.