Determination of Monacolin K and Citrinin in the Presence of Other Active Ingredients Found in Selected Food Supplements by HPLC-DAD

A universal HPLC method with diode array detection was developed for the separation and determination of the lactone and acid forms of monacolin K in the presence of other active ingredients (vitamins B₁, B₆, B₁₂, and folic acid) found in selected dietary supplements. The method also enables the quantitative determination of citrinin in monacolin K. Chromatographic separation was performed on an ACE 5 C18-PFP column (250 × 4.6 mm) thermostated at 25 °C. The mobile phase consisted of 0.005 M phosphate buffer (pH 2.60) and acetonitrile under linear gradient elution conditions. Detection was carried out spectrophotometrically at 230 nm for monacolin K and 325 nm for citrinin. The total run time was 28 min. The method was validated and met the acceptance criteria for specificity, linearity, sensitivity, accuracy, and precision. Linearity was achieved over a broad concentration range: 12.48-37.44 μg·mL^-1 for MK and 3.48-5.22 μg·mL^-1 for CTN. The method is sufficiently sensitive, with LOD and LOQ values of 0.91-2.85 μg·mL^-1 and 2.18-3.48 μg·mL^-1 for MK and CTN, respectively. Good precision (RSD < 0.70%) and intermediate precision (RSD < 1.33%) were observed. The accuracy of the method, expressed as percentage recovery at three concentration levels, ranged from 98.73% to 100.64%. The analysis revealed that the monacolin K content in randomly selected dietary supplements did not comply with the manufacturer's declaration in any case.