Extension of an Ultra-High Performance Liquid Chromatography–MS/MS Method for the Determination of C3 Epimers of 25-Hydroxyl Derivatives of Vitamin D in Human Plasma

Current vitamin D quantification methods do not account for 25-hydroxyl epimers, which can falsely increase concentrations and mask actual deficiencies. Previously, we developed an ultra-high performance liquid chromatography-tandem mass spectrometry method to measure 25(OH)D3, 3-epi-25(OH)D3 and 25(OH)D2; here, we extended this method to include 3-epi-25(OH)D2. Analytes were separated using a Shimadzu UPLC with a Kinetex F5 column (100 × 2.1 mm, 2.6 μm). The mobile phase contained 0.1% formic acid in methanol and water (70:30, v/v). The internal standard, deuterated 25(OH)D3 and analytes were extracted with hexane. Detection was performed by a mass spectrometer equipped with a triple quadrupole after prior electrospray ionization. It demonstrated sufficient precision and spike recovery within and between days, with a coefficient of variation ≤15% and an error of determination ≤18%. The method exhibited linearity in the 2-100-ng/mL concentration range. The limits of quantification and limits of detection were 2 and 1 ng/mL, respectively. Extraction recoveries ranged from 70.05% to 97.13%. The matrix effect, carryover and dilution integrity were evaluated and met the FDA acceptance criteria. The stability of all metabolites in plasma was confirmed after 3 h of storage at room temperature and after three cycles of freezing at -80°C and thawing. Applying the method to clinical samples showed a high 25-hydroxyl epimer derived from vitamin D.