To establish an innovative analytical method based on high resolution sampling two-dimensional chromatography (HiRes 2D-LC) for determination of the content of vitamin D₃ in vitamin D drops.

Two-dimensional liquid chromatography was used. Thermo HYPERSIL Gold Silica (100 mm×2.1 mm, 1.9 μm) column was used in the first dimension with n-hexane-n-amyl alcohol (996∶4) as mobile phase. The flow rate was 0.2 mL·min^-1. The samples were injected and tested at the wavelength of 265 nm. The column temperature was 40 ℃. In the second dimension liquid chromatography, ShimPack Velox Hilic (50 mm×2.1 mm, 2.7 μm) was used as the column with n-hexane-n-pentanol-isopropanol (98∶1∶1) as the mobile phase. The flow rate was 0.5 mL·min^-1. The samples were injected and tested at the wavelength of 265 nm. The column temperature was 40 ℃. A six-position 14-way valve and was equipped with 2 multi-center cutting valves was equipped to make multiple consecutive cuts of the pre-vitamin D₃ peak and vitamin D₃ peak.

The calibration curves showed a good linearity at the range of 1.018 4-5.092 mg·mL^-1 (r≥0.999 8). The precision test showed the RSDs of the peak area of pre-vitamin D₃ and vitamin D₃ were 0.95% and 0.40%, respectively. The repeatability test showed the RSD of vitamin D₃ content was 0.41%. The average recovery rate (n=9) was 101.4%. The test solution was stable at 4 ℃ and 10 ℃ for 12 h, and the RSDs were 0.58% and 0.66%, respectively. The contents of vitamin D₃ in the samples of vitamin D drops measured by this method were 100.4%, 101.6%, 100.9%, 101.6%, 102.7% and 101.6%, which was basically consistent with the results measured by the fourth method in General Chapter 0722 of ChP 2020 Vol Ⅳ.

This method has a good specificity and high sensitivity to accurately determine the content of vitamin D₃ in vitamin D drops.