Kevin D. Wyndham

Porous Hybrid Organic‐Inorganic Particles in Reversed‐Phase Liquid Chromatography

Abstract Reversed‐phase chromatographic media have recently become available that are based on porous hybrid organic‐inorganic particles. The present paper reviews hybrid particles that are made from organosilanes (organic moiety) and tetraalkoxysilanes (inorganic moiety). The hybrid particles are defined and classified within the context of a broader definition of hybrid materials. First syntheses and chromatographic evaluations are discussed for this class of hybrid packing materials. Publications are then described, which characterize two distinguishing chemical properties of hybrid particles vs. silica gel: 1) less acidic silanols, and 2) markedly longer lifetimes in alkaline mobile phases. These properties are achieved without sacrificing mechanical strength, as is found for fully organic particles, i.e., polymers, with the same chemical features. Literature reports are then reviewed that employ hybrid based reversed‐phase column packings for HPLC. Topics covered include fundamental retention mechanism studies, methods development studies, and applications made possible with the hybrid based products. Further review is presented on the use of theses hybrid particles for UPLC. The hybrid particles afford good mechanical strength without sacrificing retention and loading capacity, as is found for non‐porous particles. Applications employing hybrid based particles in the UPLC mode are then reported.

Chromatographic evidence of silyl ether formation (SEF) in supercritical fluid chromatography.

In this article, we propose that silyl ether formation (SEF) is a major contribution to retention and selectivity variation over time for supercritical fluid chromatography (SFC). In the past, the variations were attributed to instrumentation, but high performance SFC systems have shed new light on the source of variation. As silyl ethers form on the particle surface, the hydrophilicity is decreased, significantly altering the retention and selectivity observed. SEF is expected to occur with any chromatographic particle containing silanols but is slowed on hybrid inorganic/organic particles. The SEF reaction is between alcohols on the particle surface and in the mobile phase solvent. We have found that storage conditions of a column are paramount, which can either prevent or accelerate the process. Because SEF exists as an equilibrium between the liquid phase and the particle surface, the process is also reversible. The silanols can be hydroxylated (regenerated) to their original state upon exposure to water. The next generation of stationary phases will either advantageously utilize SEF or effectively mitigate its effects. Mitigation of SEF would be a significant improvement in SFC that has the potential to vault their performance to levels of similar reproducibility and reliability observed for high performance liquid chromatography (HPLC). Further research in SEF may lead to a better understanding of the mechanism of interaction between the solutes and chromatographic surface.

Highly efficient capillary columns packed with superficially porous particles via sequential column packing.

Highly efficient capillary columns packed with superficially porous particles were created for use in ultrahigh pressure liquid chromatography. Superficially porous particles around 1.5μm in diameter were packed into fused silica capillary columns with 30, 50, and 75μm internal diameters. To create the columns, several capillary columns were serially packed from the same slurry, with packing progress plots being generated to follow the packing of each column. Characterization of these columns using hydroquinone yielded calculated minimum reduced plate heights as low as 1.24 for the most efficient 30μm internal diameter column, corresponding to over 500,000plates/m. At least one highly efficient column (minimum reduced plate height less than 2) was created for all three of the investigated column inner diameters, with the smallest diameter columns having the highest efficiency. This study proves that highly efficient capillary columns can be created using superficially porous particles and shows the efficiency potential of these particles.