EFFECT OF MOBILE-PHASE PH ON THE SEPARATION OF DRUGS USING HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

When developing a HPLC assay method for drugs, the pH of the mobile phase can be effectively used as a tool to accomplish complete separation. Using two weak acids (hydrocortisone sodium succinate and methylprednisolone sodium succinate) and a weak base (promethazine hydrochloride), the changes in the chromatograms (retention times and separation of peaks) have been determined at 7 different pH values of a mobile phase with other conditions remaining the same. It has been shown that the retention times of weak acids will decrease with increasing pH values while those of weak bases will increase with increasing pH. The optimum pH value for the separation of these drugs has been determined to be 5.