Chiral Chromatography Resolution Can Be Enhanced by Using Microfluidic Precision Manufactured Macroporous Silica Microspheres

Among others, polysaccharide-functionalized macroporous silica microspheres are predominantly employed as chiral stationary phases (CSPs) in HPLC. Conventionally, such macroporous phases are prepared through pore-widening post-treatments of polydispersed mesoporous silica microspheres and typically exhibit a broad distribution of pore size and multimodal pore structures. This post-treatment manufacturing strategy and the resultant enlarged pores have limited the chiral separation performance of current CSPs. Herein, a droplet microfluidic synthesis platform was utilized to integrate sol-gel chemistry with a phase-separation process, in order to realize precision manufacture of monodisperse macroporous silica with monomodal pore structure and narrow pore size distribution, in a single shot. Following microfluidic synthesis, a monodisperse chiral stationary phase (Mono-CSP) functionalized with cellulose tris(3,5-dimethylphenylcarbamate) was prepared and compared with the classical Chiralcel OD-H column with the same polysaccharide functional group but based on conventional macroporous silica. Van Deemter curves demonstrated pronouncedly improved peak efficiencies, as a result of the reduced eddy diffusion on Mono-CSP. Systematic comparison with the Chiralcel OD-H phase indicates that such monodispersed CSP realized significantly improved resolution in enantioseparation. In particular, the Mono-CSP presented universally enhanced retention toward racemates of diverse properties, including acidic, neutral, and basic compounds. Of all the 42 enantiomers investigated, spanning alcohols, ketones, aldehydes, amines, esters, ethers, and organic acids, 90.5% showed higher retention factors (<i>k</i>₁), and 93% realized enhanced resolution on the Mono-CSP than on the polydispersed Chiralcel with the same chiral stationary phase chemistry. The current microfluidic precision manufacturing methodology holds promise for extension to the preparation of multiple types of high-performance chiral chromatographic stationary phases.