Application of Response Surface Methodology for Optimisation of Simultaneous UHPLC-PDA Determination of Oleanolic and Ursolic Acids and Standardisation of Ericaceae Medicinal Plants

A fast and sensitive ultra-high performance liquid chromatography-photodiode array (UHPLC-PDA) method for simultaneous quantification of oleanolic acid (OA) and ursolic acid (UA) in plant materials was developed. A central composite design combined with a response surface methodology was utilized to establish optimal separation conditions. The final separation was accomplished on a Zorbax Eclipse XDB-C18 column (1.8 μm, 100 mm × 3 mm I.D., Agilent, Santa Clara, CA, USA) using a mixture 90:10 (v/v) of methanol and 1% (w/v) aqueous orthophosporic acid as a mobile phase at a flow rate of 0.44 mL/min and temperature of 18 °C. The analysis was completed in 6.2 min with satisfactory resolution of 1.5 between the target analytes. The developed method proved to be precise (relative standard deviations below 3.2%), accurate (recoveries in the range of 95.27%–98.60%), and sensitive (limits of detection (LODs) in the range of 0.047–0.051 mg/mL). The method was then successfully applied to evaluate OA and UA content in real samples of selected Ericaceae plant materials (leaves of Arctostaphylos uva ursi, Vaccinium myrtillus, Vaccinium vitis idaea, Gaultheria procumbens). The content of OA and UA in investigated samples varied in the range of 0.74–4.47 mg/g dry weight (dw) and 1.30–18.61 mg/g dw, respectively.