Hi) What sorbents are used in HPLC? What requirements should they meet?
As sorbents are usually used:
silica gel, aluminum oxide, are used in normal-phase chromatography. The retention mechanism in this case is usually adsorption;
silica gel, resins or polymers with grafted acidic or basic groups. Scope of application – ion exchange and ion chromatography;
silica gel or polymers with a given pore size distribution (exclusive chromatography);
chemically modified sorbents (sorbents with grafted phases), prepared most often on the basis of silica gel. The retention mechanism is adsorption or distribution between the mobile and stationary phases. The scope of application depends on the type of grafted functional groups. Some types of sorbents can be used in both reverse and normal phase chromatography;
chemically modified chiral sorbents, for example, cellulose and amylose derivatives, proteins and peptides, cyclodextrins, chitosans used for the separation of enantiomers (chiral chromatography).
Sorbents with grafted phases can have different degrees of chemical modification. As grafted phases are most often used:
- octadecyl groups [Si-(CH2)17-CH3] (sorbent octadecylsilane (ODS) or C18);
- octile groups [Si-(CH2)7-CH3] (octilsilane or C8 sorbent);
- phenyl groups [Si-(CH2)n-(C6H5)] (sorbent phenylsilane);
- cyanopropyl groups [Si-(CH2)3-CN] (CN sorbent);
- aminopropyl groups [Si-(CH2)3-NH2] (NH2 sorbent);
- diol groups [Si-(CH2)3-OCH(OH)-CH2-OH] (sorbent diol).
The analysis is most often performed on nonpolar grafted phases in reverse-phase mode using sorbent C18.
In liquid chromatography, a number of rather strict requirements are imposed on the sorbent material.
Firstly, the column material must have sufficient strength and rigidity, which does not depend much on the presence and composition of the eluent in the column.
Secondly, the sorbent must have a sufficiently developed homogeneous surface and a narrow fractional composition of particles.
Thirdly, the sorbent should not enter into irreversible chemical interactions with both the components of the eluent and the shared sample.