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

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 (C ₈) 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°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-C ₈ 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-C ₈ phase were investigated to demonstrate the chromatographic utility of this ultra-stable phase. Inverse size exclusion chromatography and flow studies of the HC-C ₈ and the sterically protected C ₁₈ 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-C ₈ stationary phase behaves as a typical reversed phase material. The HC-C ₈ stationary phase offers unique chromatographic selectivity for certain classes of analytes compared to both alkyl and phenyl bonded phases