Gradient HPLC/UV method for codeine phosphate (CAS 52-28-8) according to the USP monograph, on an ARION® Plus C18, 3.0 µm column (150 mm × 4.6 mm, ARI-5720-IK46). Mobile phase A: 2% CH3COOH in water + 50% (w/v) NaOH (pH = 4.5) and B: ACN, gradient elution at 1.88 mL/min and 37 °C, injection volume 10 µL, UV detection at 280 nm. Morphine, norcodeine, codeine, codeine N-oxide and codeine methyl ether are separated within about 26 minutes; the ARION® column reaches a tailing factor of 2.30 and a resolution of 1.60 between norcodeine and codeine (requirements: tailing 0.8–3.0, resolution NLT 1.5), against a tailing factor of 3.20 on the competitive column (Competitor T).