1,4-Anthraquinone: A new useful pre-column reagent for the determination of N-acetylcysteine and captopril in pharmaceuticals by high performance liquid chromatography

1,4-Anthraquinone (ANQ) is proposed as a novel pre-column reagent for high performance liquid chromatography (HPLC) determination of N-acetylcysteine (NAC) and captopril (CAP) in pharmaceutical formulations. The derivatization reactions were carried out at room temperature: NAC at pH 8 for 1 min, while CAP at pH 7.5 for 20 min. Both reactions reached completeness at a reagent to thiol molar ratio of about 2.5. The synthesised derivatives were characterized by1H NMR and IR. The chromatographic separations were performed on a C18 Phenomenex Synergi Fusion 4 um (250 mm x mm I.D.) stainless steel column with detection at  λ = 300 nm. The mobile phase consisted of methanol/triethylammonium (TEA) phosphate buffer (pH 3; 0.05 mol/L) 75:25 (v/v) at a flow-rate of 0.4 mL/min for NAC and 88:12 (v/v), at a flow-rate of 0.6 mL/min for CAP. The validation parameters (linearity, sensitivity, accuracy, precision, specificity and stability) were highly satisfactory. Linear response was observed (determination coefficient ≥0.9996). Detection limits were about 8 and 18 ng/mL for NAC and CAP, respectively. Intra-day precision (relative standard deviation, R.S.D.) was ≤1.58%, for thiol to internal standard (IS) peak area ratio and ≤0.33%, for thiol and IS retention times (tR), without significant differences between intra- and inter-day data. Thiol recovery studies were satisfactory (99.50%) with R.S.D. ≤0.56%. The results highlight the high sensitivity of the method and the remarkable reactivity and selectivity of the reagent towards the thiol function. The developed method is suitable for the quality control of both thiols in commercial products. The method can be applied in any analytical laboratory not requiring a sophisticated instrumentation.