Metal-organic framework ZIF-8 nanocrystals as pseudostationary phase for capillary electrokinetic chromatography

The outstanding properties such as large surface area, diverse structure, and accessible tunnels and cages make metal organic frameworks (MOFs) attractive as novel separation media in separation sciences. However, the utilization of MOFs in EKC has not been reported before. Here we show the exploration of zeolitic imidazolate framework-8 (ZIF-8), one of famous MOFs, as the pseudostationary phase (PSP) in EKC. ZIF-8 nanocrystals were used as the PSP through dispersing in the running buffer (20 mM phosphate solution containing a 1% v/v methanol (pH 9.2)) to enhance the separation of the phenolic isomers (p-benzenediol, m-benzenediol, o-benzenediol, m-nitrophenol, p-nitrophenol, and o-nitrophenol). ZIF-8 nanocrystals in the running buffer were negatively charged, and interacted with the phenolic hydroxyl groups of the analytes, and thus greatly improved the separation of the phenolic isomers. Inclusion of 200 mg L-1 ZIF-8 in the running buffer as the background electrolyte gave a baseline separation of the phenolic isomers within 4 min. The relative standard deviations for five replicate separations of the phenolic isomers were 0.21.1% for migration time and 4.59.7% for peak area. The limits of detection varied from 0.44 to 2.0 mg L-1. The results show that nanosized MOFs are promising for application in EKC.