This technical article discusses what is meant when a column is referred to as end-capped, and what the function of the end-capping is. To understand end-capping, we need to step back and look at the bonded phase on the HPLC column. Reversed-phase HPLC columns usually comprise a silica particle with a stationary phase, such as a C18 hydrocarbon bonded onto the surface.
Silica is an amorphous polymer of silicon and oxygen. This polymer terminates at the surface of the particle as –Si-OH groups, commonly called “silanols.” These silanol groups serve as attachment sites for the bonded phase. A silane reagent, such as Cl(CH3)2SiC18H37 is reacted with the silanol to form a silyl ether (–Si-O-Si-). The bulk of the C18 group prevents bonding to all of the exposed silanols. This results in a surface that looks much like that on the left side of Figure 1, where there is a fairly high population of unbounded silanols, often termed “residual silanols.” The residual silanols are somewhat acidic and can be overly reactive with sample components, especially basic analytes, so it is preferable to reduce the population of residual silanols.
By reading the full article, you will discover why most reversed-phase HPLC columns are end-capped these days and what other purposes end-capping serves.
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