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.