Sustainable Analysis of Diclofenac Salts: A Chemometric Approach to Mixed-Mode Liquid Chromatography With Charged Aerosol Detection

Active pharmaceutical ingredients (APIs) are often used in salt form because of enhanced bioavailability. This study aims to propose a new environmentally friendly method for the analysis of raw diclofenac substance, achieving simultaneous analysis of diclofenac and its counterions (Na⁺ and K⁺), utilizing mixed-mode liquid chromatography (MMLC) and charged aerosol detector (CAD). To optimize the critical method characteristic—the mobile phase composition—a 3² full factorial design of experiments and multiobjective decision making using Derringer's desirability function were employed. Two optimized methods with acceptable run times and satisfactory peak separation were developed. The methods compared the use of acetonitrile (ACN) and acetone (ACE) in terms of method sustainability. The mobile phase composition in the first method (MMLC–ACN) was 40% ACN and 60% ammonium acetate buffer (48.00 mM, pH 4.82), whereas in the second, improved method (MMLC–ACE), it was 50% ACE and 50% ammonium acetate buffer (40.00 mM, pH 4.62). The eco-friendliness of the developed methods was assessed using the GAPI, the Analytical GREEnness (AGREE) score, and the Greenness Index. The method with ACE as the mobile phase modifier demonstrated a better environmental profile, achieving an AGREE score of 0.69, compared to the ACN-based method, which scored 0.60. Method performance characteristics of the MMLC–ACE method used for the quantitative analysis of diclofenac salt raw materials were evaluated according to ICH Q2(R2) guidelines: precision—repeatability (RSD from 1.07% to 2.41% and recovery >97%), selectivity between critical peak pair (α_Na/K > 1) and obtained linear response within concentration range of 50%–150% (r > 0.99).