Naturally occurring kaolinite as a sorbent for solid-phase extraction of calcitriol in soft capsules

Abstract An environmentally friendly, simple and reliable method to quantify calcitriol in soft capsules was developed. Calcitriol with strong physiological activity has a low content (1.55 μg·g−1) in calcitriol soft capsules. Due to the poor stability of calcitriol, a large amount of medium chain triglycerides (>99.99%) need to be used as excipients to protect it. Herein, to achieve accurate quantification of the active ingredients in the calcitriol soft capsules, removing the interference of fat-soluble matrix is the main challenge. This experiment developed a cost-effective solid-phase extraction (SPE) clean-up method based on natural kaolinite (Kaol), which is followed by liquid chromatography coupled with Photodiode array detector (HPLC-DAD) analysis. In the self-made Kaol-SPE, calcitriol could remain on the Kaol sorbent (Al2O3: 39.5%, SiO2: 46.5% and H2O: 14.0%) by polar interactions, n-π interaction and lewis acid base interaction, whereas the hydrophobic medium chain triglycerides could be sufficiently washed off by n-hexane. Thus, the selective extraction of calcitriol and clean-up of fat-soluble matrix were achieved. Kaol was characterized by Fourier transform infrared spectrometry, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and nitrogen adsorption-desorption. Adsorption isotherms were determined to infer the adsorption mechanism and adsorption capacity of calcitriol on Kaol. The fitting results showed that it was followed Langmuir model and the maximum adsorption capacity was 5.495 mg·g−1 (fast adsorption rate, 1 min). Various parameters such as SPE cartridges, Sorbent dosage (20–120 mg), washing solvent, eluting solvent and its volume (0.2–4.0 mL) were investigated. Under optimized conditions, good linear range was 0.15–2.99 μg∙g−1. Recoveries is satisfactory (98.15–101.64%), and the relative standard deviations (RSDs) are only 1.22%. Minor matrix effect was 96.72%, which was excellent. Moreover, low quantitation limit (LOQ) of calcitriol is 0.015 μg·g−1. This method was further validated to determine the content of calcitriol in different batches of commercial calcitriol capsules. The method can directly process diluted samples without liquid-liquid extraction, and has the feature of automation. Thus, this work provides a simple, convenient and economical platform for the determination of calcitriol in complex matrices on a large scale and promotes natural-based materials in pharmaceutical analysis field.