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Although the number of new types of stationary phases increases annually (ion stationary phase, chiral chromatographic phase, etc.). Reversed-phase high-performance liquid chromatography (RP-HPLC) are still most widely used in all chromatographic phases, especially C18 and C8. The choice of employing C18 and C8 columns for analysis is mostly based on their well documented and wide ranging applicability as well as their reliability concerning high selectivity, reproducibility, and stability (ruggedness). However, the separation of highly polar analytes remain problematic due to the poor retention of those analytes on conventional reversed phase. To better isolate polar compounds such as alkalis, an alternative technique to RPLC was proposed by Alpert in 1990 and is called the hydrophilic interaction chromatography (HILIC). However, HILIC is not very suitable for separating nonpolar compounds. Therefore, the mixed mode RPLC/HILIC stationary phase has been emerged as an alternative. And it becomes more and more popular in medicine and biomedical applications due to its unique selectivity. Cyano functionalized silica based materials have been used as HPLC stationary phases in separation science for long time. The unique ligand structure with polar nitrile group attached to the terminal of an alkyl chain bonded on silica allows the cyano phase to be used either in normal phase or reversed-phase mode. Although the cyano-bonded phase has been commercialized, it is still not widely used due to its poor stability.
Traditional methods for the preparation of cyano bonded silica involve silylation reaction of silanol groups on the silica support with mono-, di-, or tri-functionalized silane in an organic solvent. Because of the consumption of large volume of organic solvent, this process is not considered to be environmental benign and economically efficient. Several solvent-free, vapor phase deposition methods are available for synthesis of chemically bonded silica phases. Wikström and coworkers developed gas phase silylation method for preparing chromatographic packings. Aminopropyl- or epoxysilane were bonded to porous silica support by heating the support together with gaseous silanes and gaseous triethylamine at reduced pressure. Acylamide stationary phases were synthesized by a successive gas-phase modification of silica gel with γ-aminopropyltriethoxysilane and benzoyl chloride, benzoic or stearic acid. Derivatization of amino phases with carboxylic acids at temperatures above 150°C requires no activation of carboxyl groups or the use of condensing reagents. Ek and coworkers investigated effects of various reaction conditions on the gas-phase deposition of aminopropyl alkoxysilanes on porous silica. Recently, Lazghab and coworkersreported functionalization of porous silica powder in a fluidised-bed reactor with n-octadecyltrichlorosilane, glycidoxypropyl trimethoxysilane, and aminopropyl triethoxysilane. While gas phase silylation is shown to be a viable alternative to traditional solvent based methods, surprisingly, there is no report on its application to preparation of cyano bonded silica phases.
Click chemistry has emerged as a popular synthetic route to multifunctional chromatographic stationary phases. Among the many click reactions, thiol-ene reaction is particularly attractive as it allows the formation of a highly stable thioether between a thiol and an alkene under mild conditions with high yield of conversion. By bonding a vinyl terminated functional group to mercaptopropyl bonded silica, Lindner and coworkers synthesized a wide variety of thioether tethered stationary phases with functional groups ranging from alkyl chains of various length, naphthalene imides, quinuclidines for reversed-phase and mixed mode chromatography. Liang and coworkers bonded cysteine and glutathione to vinyl modified silica to prepare zwitterionic stationary phases for hydrophilic interaction chromatography. This approach has also been used to prepare mixed-mode monolithic capillary columns. |