Determination of the pK(a) of 12-S-hydroxyheptadecatrienoic acid by studies of its vial surface adsorption using high-performance liquid chromatography and liquid scintillation technique

When dissolving 12-S-hydroxyheptadecatrienoic acid (HHT) in aqueous buffer systems, a partition equilibrium is achieved between the liquid buffer phas e and the solid vial phase. This adsorption equilibrium is dependent on the vial material, the ratio of solvent volume to moistened vial surface area (V/A), the pH of the aqueous solution, the buffer additives and the contact time. Partition ratios (E) were established following determination of the recoveries (<(omega)over bar>) of HHT in the liquid phases. The pK(a) of H HT could be calculated to be 4.4 +/- 0.2. The HHT concentration in the liqu id phase rose considerably when increasing the pH value (17% at pH 1.9, up to 97% of applied HHT at pH 8.0) and increasing the VIA-ratio, as well as w hen using glass instead of polypropylene (PP) or adding acetonitrile as an organic solvent and HHT-affine substances like albumins or globulin. The HH T concentration was found to decrease during contact to PP surfaces as a re sult of HHT adsorption and diffusion into the hydrophobic vial material. Th is effect could be minimized by adding a larger percentage of acetonitrile (50-60%). (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.