Single-crystalline 3D Covalent Organic Framework For Gas Chromatography

Food flavor compounds,such as those found in coffee and wine,are known for their complexity and diversity.Gas chromatography is the main method for their identification and analysis,and the separation in commercial gas chromatography columns mainly relies on van der Waals forces and polarity.However,for some structurally similar substances,complete separation is difficult to achieve,making it difficult to further comprehensively analyze their flavor characteristics.Therefore,the preparation of high-performance gas chromatography columns for the separation and analysis of flavor substances in food is crucial.Covalent organic frameworks（COFs）are a new class of porous crystalline organic polymers formed by covalent bonding.COFs possess several advantages,such as good thermal stability,large surface area,tunable pore size,and ease of post-modification,making them ideal candidates as gas chromatography stationary phases for gas phase separation.However,current COFs used as gas chromatography stationary phases are polycrystalline and have incomplete structures and irregular morphologies.In contrast,single-crystalline COFs have completed and ordered crystal structures,as well as regular morphologies,offering the potential to study the impact of COF stationary phase particle size on chromatographic separation.Unfortunately,relevant research in this area has not been reported to date.To address this issue,we designed and prepared single-crystalline three-dimensional COF-303 capillary gas chromatography columns with varying particle sizes,and evaluated their separation performance and potential applications in food flavor compound analysis.The main research findings are as follows:1.Single-crystalline COF-303 capillary columns（10 m×0.53 mm）with particle sizes ranging from 0.4μm to 1.6μm were successfully synthesized,and the effect of particle size on the gas phase separation performance of single-crystalline COF-303-capillary was investigated.The four types of capillary columns produced were all non-polar columns,and as the particle size increased,the separation efficiency and column efficiency for separating xylene,dichlorobenzene,and pinene isomers decreased.Thermodynamic studies showed that van der Waals forces and hydrogen bonding played a major role in the separation process.In addition,the flexible structure of COF-303 led to a decrease in isomer separation with increasing particle size.Kinetic studies showed that the eddy diffusion and longitudinal diffusion coefficients of the single-crystalline COF-303 capillary columns were relatively stable,but the mass transfer resistance increased with particle size,indicating that the increase in mass transfer resistance（0.30 ms～0.62 ms）was the main reason for the decrease in column efficiency of the large particle COF-303 gas chromatography column.In relative retention measurements of isomers within different capillary columns in run-to-run（n=8）and day-to-day（n=8）retention time,the highest relative deviations were 0.06%and 0.39%,respectively.After five temperature programming cycles,the maximum reduction in column efficiency and separation factor was 2.0%and 1.4%,respectively,demonstrating that the COF-303-capillary had good repeatability and stability.2.The results of the previous chapter showed that the COF-303-capillary with a particle size of 0.4μm had the best separation performance.Therefore,further exploration was conducted on the separation performance of the 0.4μm COF-303-capillary and its potential applications in the separation of food flavor substances.The single-crystalline COF-303-capillary（0.4μm）produced not only achieved efficient separation of n-alkanes and n-alcohols,but also showed good separation performance with Grob reagent and an alkaline property.Moreover,it achieved baseline separation（resolution of 1.68～11.75）of xylene,dichlorobenzene,trimethylbenzene,pinene,nitrotoluene,and chloroaniline isomers,with a maximum column efficiency of 7898 plates·m^-1（for o-xylene）.Compared with the polycrystalline COF-303,commercial capillary chromatography columns DB-5 and HP-FFAP columns,the single-crystalline COF-303-capillary（0.4μm）produced showed superior separation performance.It achieved baseline separation of some alcohols,pyrazines,and ester isomers that are difficult to separate in coffee and wine flavor substances within 5 minutes,and the separation of pyrazine substances（resolution of 2.04～11.06）was better than that of the commercial DB-5 column（resolution of 0.88～3.36）.