Hashtag: Things you wanna know about endcapping
What is endcapping?
When silica-based stationary phase is bonded with funtional groups it is impossible to reach all silanol groups because of steric hindrance and other factors. The left behind silanol groups are called residual silanol groups and they usually cause peak tailing of basic compounds and need to be silylated. This process of silylation is called endcapping and aimes to reduce silanol activity.
What is double endcapping?
Double endcapping means it is endcapped twice. By applying double (or triple) endcapping silanol interactions can be further minimized and stability at mid pH can be improved. It is difficult to remove every residual silanol group even with endcapping. It is not a 100% process, and there is always a small number of silanols that still remain on the surface. However, they are often non-reactive because of steric hinderance from the endcapping agent and funtional groups. Many manufacturers choose to apply multiple endcapping to minimize silanol activity at their best.
Why do reversed-phase HPLC columns have an endcapping?
Endcapping is most often used with reversed-phase HPLC columns. Endcapping of the HPLC Column minimizes silanol interaction by reducing residual silanol group. It protects the silica support from dissolution, and it also cuts down on unwanted adsorption of basic or ionic compounds. There are different endcapping procedures in modern HPLC column phases. The chart below shows the difference between the traditional stationary phase bonding and TMS endcapping Reaction:
What is the difference between normal reversed phase and endcapped HPLC columns?
When manufacturing reversed-phase columns, it is common to use silica to react with a silane containing an alkane sidechain. If the alkane is too large it may be prevented to react with some of the silanol groups on the surface of the silica. Such materials are called nonendcapped. Those silanol left unreacted can be future reacted through endcapping with a silane containing a small alkyl group. These products are called endcapped.
People have developed some applications with nonendcapped materials, and those must be used to reproduce those results. Fior endcapped materials, marketing people may tell you that there are total endcapped or doubble endcapped, there are still SiOH groups left on the surface. For a baseline saparation these free SiOH groups are necessary for some compounds. Also, though for most applicaitons endcapped materials are better, there are cases that the nonendcapped materials serves best. For example, when you separate tartaric from quinic acid in fruit juice. It can be separated to baseline isocratically uising a nonendcapped C-18 column, but few endcapped columns are up for it.
What are the benefits of Endcapping?
• Undesired interaction of polar or charged analytes (acids, bases ect.) is reduced because the amount of available silanol groups is minimized.
• Endcapped materials have a higher carbon load on the surface.
• Peak tailing can be minimized resulting in better peak shape.
• Minimized silanol interactions on the double endcapped column reduce retention.
• When used in harsh conditions (for example extreme pH), endcapping helps prevent the surface of functionalized silica from being attacked and hydrolized.
When do you choose endcapped or non-endcapped columns?
For the sake of extended column lifetime, make sure the method pH is in pH range recommended by the bonded phase .
In general, endcapped HPLC columns are a good choice for mid-range pH methods. As pH increases, silica dissolution is decreased by endcapping.
Non-endcapped HPLC columns are a good choice for low pH methods.