Assay of Two Antibacterial/Anticoccidial Drugs in Combination with Vitamin K3 for Oral Solutions: Stability Studies and Method Development Using HPLC-DAD: Appraisal of the Method’s Eco-Friendliness and Functionality

A novel, green, stability-illustrating HPLC-DAD method was validated for the simultaneous analysis of menadione (MND), dimetridazole (DMT), and sulfadimethoxine sodium (SLF) in a veterinary powder for the first time. These compounds are commonly combined in veterinary premixes and powders to enhance animal growth, prevent bacterial infections, and improve feed efficiency. Separation was achieved isocratically on a C18 column using a mobile phase of 0.05M KH2PO4: acetonitrile (80:20, v/v) at a flow rate of 2.0 mL/min, with detection at 260 nm. The represented HPLC-DAD method was rapid, yielding retention times under 5.2 min, and exhibited excellent linearity over the tested ranges (10.0–30.0, 20.0–60.0, and 20.0–60.0 µg/mL for MND, DMT, and SLF, respectively). Forced degradation studies, conducted according to the International Council for Harmonisation (ICH) guidelines, confirmed the method’s specificity in distinguishing the active pharmaceutical ingredients from their degradation products. The highest degradation was observed for MND (photolytic, 26.52%), DMT (alkaline, 21.12%), and SLF (oxidative, 27.16%). The method’s environmental sustainability was evaluated using the Analytical GREEnness (AGREE) metric (score: 0.75) and the Green Analytical Procedure Index (GAPI), while its practicality was supported by a high Blue Applicability Grade Index (BAGI) score of 80.0. This stability-indicating method represents the first robust, green, and reliable analytical approach for this triple veterinary formulation.