High-Density Octadecyl Chemically Bonded Core–Shell Silica Phases for HPLC: Comparison of Microwave-Assisted and Classical Synthetic Routes, Structural Characterization and Chromatographic Evaluation

In this study, different grafting methods were examined to develop high-density chemically modified octadecyl core–shell silica particles for HPLC applications. The influence of the reagents and the experimental conditions were evaluated, either through conventional heating or by microwave irradiation. For both experimental synthetic processes, chemically bonded phases were prepared by grafting the functionalized silane (octadecyldimethylchlorosilane or octadecyldimethyl(dimethylamino)silane) and, on the other hand, using a hydrosilylation procedure. The bonded silica phases were fully characterized by elemental analysis, diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy, 29Si cross-polarization magic angle spinning NMR and 1H magic angle spinning NMR; and the granulometric distribution was evaluated by SEM. The maximum grafting rate (4.4 µmol m−2) was determined by direct silanization with hexamethyldisilazane. The best grafting rate (3.8 µmol m−2) for octadecyl functionalization was obtained using octadecyldimethyl(dimethylamino)silane. The Tanaka test was used to estimate the chromatographic behavior of the overall set of synthesized bonded stationary phases, and their chromatographic performances were compared with similar columns available on the market. In this article, we used microwave irradiation methods to drastically reduce the reaction time for a comparable grafting rate and equivalent chromatographic performances.