Chromatographic Evidence of Silyl Ether Formation (SEF) in Supercritical Fluid Chromatography

Abstract

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.

Supercritical fluid chromatography in pharmaceutical analysis

“…It has been proven that a thin layer of co-solvent exists on the surface of the stationary phase [39,49] leading to the contact with free silanols. This proximity can result in silyl ether formation (SEF) coming from the slow condensation between alcohols used as cosolvents and residual silanols [50]. The increasing scarcity of the silanols can lead to loss of retention over time, significantly affecting reproducibility of SFC separations.…”

Development and application of sub‐2‐μm particle CO2‐based chromatography coupled to mass spectrometry for comprehensive analysis of lipids in cottonseed extracts

“…The observation of the silyl ether formation (SEF) poses a problem in regards to column stability, as well as method repeatability and robustness. Although there seems to be an approach published elsewhere to regenerate a deteriorated column due to SEF equilibrium reactions, the results presented herein suggested using a “buffered‐like” modifier composition to prevent retention time shifts and decreases in sensitivity was a better alternative.…”

“…Based on these findings, communications were relayed back to the Consumables Group organization in Waters Corporation. They further researched the phenomenon and found evidence of silyl ether formation (SEF) equilibrium reactions between the alcohols on the particle surface and the mobile phase solvent . Fairchild et al .…”

Maximizing performance in supercritical fluid chromatography using low-density mobile phases

“…However, the advantages of using low-density SFC mobile phases are multiple : 1) the retention increase of semi-volatile compounds allows their separation, 2) faster analyses than vHPLC or micro-HPLC analyses can be achieved 23,24 , 3) the co-elution of these analytes with most sample solvents can be avoided when using flame ionization detection 24 , and 4) no headspace GC analysis is needed since they can can be separated in the presence of heavier compounds by applying a gradient of either organic modifier, flow rate or column back pressure 24 . The only drawback could be the potential band stretching due to the overlap between the sample band and that of the strong sample solvent [25][26][27] and to the above-mentioned thermal effects.…”