To classify liquid chromatography, there are several ways. If this classification relies upon nature of the separation process and the stationary phase, then it can be adsorption chromatography, ion-exchange chromatography, and size exclusion chromatography. In relation to the first type, two modes are defined relies on polarity to two phases, i.e. normal phase and reverse-phase chromatography, as mentioned types about 90% cover of all chromatography applications and for the separation of components the column are used.
Columns are usually made of SS, around 50 and 300mm in length and within 2 and 5 mm, they are typically packed with a stationary phase of 3 to 10 µm size. Columns of the internal distance of less than 2 mm are acceptable regularly in the form of microbore HPLC columns.
Types of packed gels:
Silica gel is the most popular packing material used. Silica gels are available in two types it is a spherical and irregular shape, Spherically shaped gels are most commonly used. There is a pore on the surface of the silica gel used in liquid chromatography. By contains the pores, it gives a bigger surface area than without holes. The most silica columns nowadays have spherical types and small size particles have been developed. The 5μm is a most commonly used size, but small size 1.5 to 3μm gel is also used in the HPLC column. Small gels are packed in small column housing and therefore retention time is reduced.
Silica gels were always used in the earlier stage of HPLC development. But, the polymer-based column is getting well-liked. Polyethylene and polypropylene are generally known polymers. Like silica gel, polymer gel columns are available in very small particles. Polystylene, Polymethacrylate, polyhydroxymethacrylate, Polyvinyl alcohol are types of polymers.
In addition to silica and polymer gels, the gels used are natural substances like Cellulose, chitosan agroceae, and dextrin and ceramics for example zirconia and hydroxyptite are used in liquid chromatography for the separation of analytes but is have very limited use.
There are several types of columns used in the separation of components with the help of the mobile phase, which is based on the separation mode used in liquid chromatography. Here some types of separation modes are mentioned.
Reversed-phase chromatography (RPC), Normal-phase chromatography (NPC), Hydrophilic interaction chromatography (HILIC), ion exchange chromatography (IEC), ligand exchange chromatography (LEC), ion exclusion chromatography (IEC), Gel permeation chromatography (GPC), Gel Filtration Chromatography (GFC), Affinity chromatography, Chiral chromatography.