Preparation Of Metal-organic Framework Polymer Monolithic Columns For On-line Enrichment And Purification Of Ursolic Acid

Ursolic acid(UA)is a pentacyclic triterpene belonging to the C30 isoprenoid compound which is widely found in herbs,leaves,flowers and fruits.In recent years,due to their various activities and low toxicity,it has become a research hotspot.Studies confirmed that UA had a variety of pharmacological effects,such as stabilization,anti-inflammatory,anti-cancer,anti-ulcer,anti-HIV,cardioprotection,neuroprotection and immune enhancement.Due to the complex matrix of plant samples and the presence of substances with similar structures and polarities,the extraction and purification of UA is difficult.Therefore,sample pretreatment is a key part of the UA analysis process.On-line solid phase extraction(SPE)is the most commonly used sample pretreatment technology,which can reduce the consumption of organic solvents,shorten the sample pretreatment time,simplify the operation process,realize the automatic separation,enrichment,purification,analysis and detection of samples,and improve sensitivity and reproducibility.Metal-organic frameworks(MOFs)are formed by the coordination of metal cations(or cationic clusters)with multifunctional organic ligands,which are one of the most promising advanced porous materials with large specific surface area,high porosity,uniform pore size and structural diversity.By incorporating MOF into the polymer monolith,the specific surface area and adsorption properties of the polymer monolith are significantly improved.In this paper,two kinds of MOF-polymer monolithic columns were prepared by redox method and successfully applied to the on-line purification,enrichment and analysis of UA in Chinese herbal medicine samples.In the first experiment,a MOF-polymer monolithic column was prepared by redox initiation using modified MOF-199 and N-methylolacrylamide(NMA)as co-monomers.The obtained monolithic column was characterized by scanning electron microscopy(SEM)and nitrogen adsorption-desorption isotherm measurement.It was used as SPE absorbent for the on-line enrichment of UA by high performance liquid chromatography.The MOF-polymer monolithic column showed high adsorption capacity and good permeability.Under the optimum conditions for extraction and determination,the calibration equation was y=79.854x+0.1939;the linear range was 0.001–0.9 mg/mL;the linear regression coefficient was 0.9993;the limit of detection(LOD)and the limit of quantification(LOQ)were 0.17?g/mL and 0.57?g/mL,respectively;the inter-day and intra-day accuracies were<6.44%;the recoveries were in the range of 86.52–105.26%.In the second experiment,a MOF-polymer monolithic column was prepared by in-situ radical polymerization with derivatized UiO66-2COOH and N-isopropylacrylamide(NIPAAm)as co-monomers.The obtained monolith possessed characteristics of relatively uniform porous structure and a high surface area(84.16 m~2/g).The monolith was used as SPE absorbent for the on-line purification and enrichment of UA in Chinese herbal medicine samples by HPLC.The relative standard deviation(RSD)of intra-day and inter-day assays for five plant samples were in the range of 1.00–3.67%and 2.43–5.76%,respectively;the recoveries were in the range of 92.2–103.28%.The results indicated that the on-line SPE-HPLC method based on two kinds of MOF-polymer monolithic columns had certain ability of enrichment and purification of UA in various Chinese herbal medicine samples such as Prunella vulgaris,Plantago,and Hedyotis diffusa.This SPE-HPLC method also had the advantages of good repeatability,high recoveries and low detection limits.