Synthesis and characterization of hypercrosslinked, surface-confined, ultra-stable silica-based stationary phases

Abstract

The synthesis and chromatographic characterization of a highly crosslinked self-assembled monolayer (SAM) stationary phase whose acid and thermal stability were significantly improved relative to a sterically protected octadecylsilane (ODS) stationary phase were recently described [B.C. Trammell, L. Ma, H. Luo, D. Jin, M.A. Hillmyer, P.W. Carr, Anal. Chem. 74 (2002) 4634]. Unfortunately, this highly crosslinked SAM phase is much more silanophilic than a conventional sterically protected octadecyl silane phase. 29Si CP-MAS NMR analysis shows that the high concentration of silanol groups in the self-assembled monolayer causes the increased retention and poor peak shape of basic solutes. In this work dimethyl-chloromethyl-phenylethylchlorosilane (DM-CMPES), a silane with only a single reactive silyl chloride group was tested as an alternative to chloromethyl-phenyethyltrichlorosilane (CMPES) as the basis for forming the starting phase. Most importantly this "conventional" silanization step (i.e., a non-SAM silanization) was followed by a Friedel-Crafts reaction using aluminum chloride as the catalyst and styrene heptamer as the multi-valent crosslinker to form the surface DM-CMPES groups into a network polymer which is fully confined and attached to the surface. An octyl (C8) derivative of the hypercrosslinked (HC) dimethyl-chloromethyl-phenylethyl (DM-CMPES) surface-confined stationary phase was synthesized to demonstrate the potential of a Friedel-Crafts based approach to making high efficiency, acid and thermally stable polymerized phases on silica with selectivity closer to conventional aliphatic phases. The stability of the retention factors of these phases under very aggressive conditions (5%, (v/v) trifluoroacetic acid and 150 degrees C) are compared to that of a sterically protected octadecylsilane (ODS) phase. The comparisons show that the long term stability of highly crosslinked DM-CMPES phases in acid is superior to the conventional phase. The HC-C8 phase is even more stable in acid than the HC-styrene heptamer DM-CMPES phase on which it is based. Additionally, the efficiency and peak shape of several prototypical bases under acidic (0.1% TFA, pH 2.0) elution conditions are discussed. The column dynamics and thermodynamic characteristics of the HC-C8 phase were investigated to demonstrate the chromatographic utility of this ultra-stable phase. Inverse size exclusion chromatography and flow studies of the HC-C8 and the sterically protected C18 stationary phases indicate the absence of pore plugging and quite good (nearly 100,000 plates/m) chromatographic efficiency. Further chromatographic investigations show that the HC-C8 stationary phase behaves as a typical reversed phase material. The HC-C8 stationary phase offers unique chromatographic selectivity for certain classes of analytes compared to both alkyl and phenyl bonded phases.

Application of silica-based hyper-crosslinked sulfonate-modified reversed stationary phases for separating highly hydrophilic basic compounds

“…Because our − SO 3 -HC-C 8 -L phase is synthesized by sulfonating a highly acid stable HC-C 8 [26][27][28][29][30], it has very good acid stability even at high temperatures. This greatly increased acid stability has allowed us to explore temperature effects on this novel phase.…”

“…The new sulfonated reversedphase was prepared by introducing an exceedingly small number of sulfonyl groups (approximately 0.11μmoles/m 2 ) to a hyper-crosslinked reversed-phase [26][27][28][29][30]. After this minor degree of sulfonation, the hydrophobicity of the new phase was still quite comparable to that of ordinary C 8 phases.…”

Synthesis and characterization of silica-based hyper-crosslinked sulfonate-modified reversed stationary phases

“…A novel type of acid stable silica-based reversed phase was recently developed in our laboratory [40][41][42][43]. Orthogonal polymer forming reactions are used to prepare a very thin layer of hyper-crosslinked (HC) aromatic network that is entirely confined to the silica surface.…”

Optimization of the synthesis of a hyper‐crosslinked stationary phases: A new generation of highly efficient, acid‐stable hyper‐crosslinked materials for HPLC

“…Although the chemistry involved in making the crosslinked functionalized aromatic layer is similar to that employed by Davankov [46][47][48], our study is fundamentally different in that Davankov's materials are totally organic polymers, whereas our materials are based on silica modified with a surface confined layer of a highly crosslinked aromatic polymer. The fully connected polymer network prevents the loss of bonded phase, which leads to superior hydrolytic stability in acid when these new phases are compared with conventional silica-based phases [44,45]. At the same time, the excellent mass transfer properties of monomeric-bonded phases were preserved and none of the adverse kinetic effects [42][43][44][45] of depositing a polymer has been observed.…”

“…In previous articles [42][43][44][45], we developed a novel type of acid-stable, silica-based platform. Orthogonal polymerization reactions were used to prepare a monolayer of hypercrosslinked (HC) aromatic network that is entirely confined to the surface.…”