Difference between the normal and end capped columns

[reddyashwin]

Dear all,

what the difference between normal reversed phase and endcapped columns

regrads,
ashwin

[Andy Alpert]

The most common reversed-phase columns are manufactured by reacting silica with a silane containing an alkane sidechain. If the alkane is a C-18 group, then it's so large that steric hindrance between adjacent C-18 groups prevents the reaction with some of the silanol groups that lie between them on the surface of the silica. Such materials are termed nonendcapped. Many of the unreacted silanols can be capped by a second reaction with a silane containing a small alkyl group; usually, C-1 (i.e., methyl). These products are called endcapped. Endcapped materials have a higher carbon load on the surface and avoid most of the reactions with silanols that can lead to skewed peaks. Some legacy applications were developed with nonendcapped materials, and so those must be used to reproduce those results. Also, while endcapped materials are better for most applications, there are a few in which the nonendcapped materials are better. One example is the separation of tartaric from quinic acid in fruit juice. Those can be separated to baseline isocratically with a nonendcapped C-18 column, but few if any endcapped columns can do so.

Andy Alpert
PolyLC Inc.

[Gerhard Kratz]

In addition to Andy's comment: even with a very small end-capping group statistically 50% of the rest silanol groups will stay non endcapped. What ever marketing people try to suggest with key words like total endcapping, doubble endcapping or what so ever, there are still SiOH groups on the surface. And for some compounds these free SiOH groups are necessary to get a baseline separation.