According to the separation mechanism, liquid chromatography can be divided into four categories

According to its separation mechanism, liquid chromatography can be divided into four types Adsorption chromatography the stationary phase of adsorption chromatography is adsorbent The separation process of chromatography is carried out on the surface of adsorbent and does not enter the interior of stationary phase Unlike gas chromatography, mobile phase (i.e solvent) molecules also adsorb on the adsorbent surface On the surface of adsorbent, the sample molecules compete with the mobile phase molecules in adsorption, so the selection of mobile phase has a great influence on the separation effect Generally, gradient elution method can be used to improve the efficiency of chromatographic separation In the analysis of polymers, adsorption chromatography is generally used to separate additives, such as azo dyes, antioxidants, surfactants, etc., and also to analyze the composition of petroleum hydrocarbons The mobile phase and stationary phase of this kind of chromatography are both liquid The sample molecules quickly reach equilibrium distribution between the two liquid phases The separation is based on the difference of partition coefficient of each component in the two phases, which is similar to the extraction process The commonly used fixatives are β, polyethylene glycol (peg400-4000), trimethylene glycol (TMG) and squalane (SQ) Using the same method as gas chromatography (GC), the fixed solution was coated on the porous surface of the carrier, but it was easy to lose in use At present, bonded stationary phase is widely used In this solid phase, the fixative is not coated on the surface of the carrier, but is chemically bonded to some organic group on the surface of the pure silica gel particles For example, the reaction between chlorooctadecylsilane and hydroxyl (- OH) on the surface of silica gel can form a monoalkylated surface The advantage of this kind of fixed liquid is that it is not easy to be eroded by the mobile phase In the partition chromatography, the mobile phase can be pure solvent or mixed solvent for gradient elution The polarity of mobile phase should be different from that of stationary phase to avoid the solubility between them Generally, it can be divided into positive phase distribution and reverse phase distribution Ion exchange chromatography ion exchange resin is usually used as stationary phase Generally, the sample ions and the fixed phase ions can be exchanged reversibly Due to the different exchange capacity of each component ion, the chromatographic separation can be achieved Ion exchange chromatography is a newly developed modern analytical technology, which has been widely used in the analysis of amino acids and proteins It is also suitable for the separation and analysis of some inorganic ions (such as NO3 -, SO42 -, Cl -, and inorganic cations such as Na +, Ca2 +, Mg2 +, K +, etc.) Gel chromatography is currently applied to both aqueous solutions and organic solvents When the eluent is used as an aqueous solution, it is called gel filtration chromatography It has many applications in the biological field When organic solvent is used as eluent, it is called gel permeation chromatography, and it is applied more widely in polymer field The above four different types of liquid chromatography can choose different methods according to different sample properties.