Evaluation of analytical buffers as alternatives to phosphate-buffered saline in diffusion studies with plant extracts using LC-MS detection

Diffusion studies are crucial for cosmetic and pharmaceutical formulation development to assess skin permeation of small molecules, often accessible via ultraviolet-visible (UV/Vis) absorption. For complex plant extracts, ultra high performance liquid chromatography/mass spectrometry (UHPLC/MS) has emerged as a predominant analytical tool. However, the commonly used phosphate-buffered saline (PBS) as physiological receptor fluid can affect LC-MS analysis by altering chromatographic performance and ionization efficiency. Alternative protocols for testing release and skin permeation of phytochemicals remain to be established. We therefore evaluated different buffer media at pH 7.4 (PBS according to European Pharmacopoeia, ammonium acetate 0.1 M, ammonium formate 0.15 M) for their suitability as acceptor fluids in release and permeation studies with cellulose acetate and porcine membranes. Korean red ginseng (KRG) extract was incorporated as model extract in three different formulation types (nanoemulsion, hydroalcoholic gel, amphiphilic cream). Storage stability was assessed via rheology and pH where required. Representative ginsenoside saponins Rb1 and Rg1 (logP 0.23 and 1.12) were quantified via UHPLC/MS. Interestingly, the results and differences between buffer media differed depending on membrane type. Drug release and permeation through the synthetic membrane varied among buffers for all vehicles (23.2-70.8 µg/cm2), whereas permeation through pig skin was comparable for all tested buffers (0.2-10.4 µg/cm2). Using alternative buffers, a comparison of vehicle types showed hydroalcoholic gels superior to the two multiphase systems regarding drug release and permeation. These results suggest that all tested alternative buffer media are suitable for comparative diffusion studies of plant-based actives studies especially when using biological membranes.