Types of liquid chromatography

The present chapter is largely concerned with HPLC, together with a summary of developments in quantitative thin-layer chromatography, but a brief account of the various types of liquid chromatography is given first together with a guide to the choice of appropriate separation mode. 

There are four main types of liquid chromatography which require discussion. 

Liquid-solid chromatography (LSC). This process, often termed adsorption chromatography, is based on interactions between the solute and fixed active sites on a finely divided solid adsorbent used as the stationary phase. The adsorbent, which may be packed in a column or spread on a plate, is generally a high surface area, active solid such as alumina, charcoal or silica gel, the last 

In general, the compounds best separated by LSC are those which are soluble in organic solvents and are non-ionic. Water soluble non-ionic compounds are better separated using either reverse-phase or bonded-phase chromatography. 

Normal /, /i -Oxydipropionitrile Carbowax (400, 600, 750, etc.) Glycols (ethylene, diethylene) Cyanoethylsilicone Saturated hydrocarbons, e.g. hexane, heptane aromatic solvents, e.g. benzene, xylene saturated hydrocarbons mixed with up to 10 per cent dioxan, methanol, ethanol, chloroform, methylene chloride (dichloromethane) 

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In contrast to vapour phase chromatography, the mobile phase in liquid chromatography is a liquid. In general, there are four main types of liquid chromatography adsorption, partition, ion-chromatography, and gel filtration. 

Thin-layer chromatography (TLC) is a type of liquid chromatography in which the stationary pease is in the form of a thin layer on a flat surface rather than packed into a tube (column). It is a member of a family cf techniques that include some types of electrophoresis and paper chromatography, more generally referred to as planar chromatccraphy. Since we will not discuss electrophoresis in this section, and since TLC has virtually superseded paper chromategr pby in most analytical... 

Due to the fact that different end groups can be formed during the polycondensation, the reaction products may exhibit a functionality-type distribution in addition to the molar mass distribution. Although SEC is suitable to analyze the molar mass distribution, it does not yield information on different end groups. For the determination of the functionality-type distribution, other types of liquid chromatography must be used. 

Answer the following with normal phase or reverse phase. For which type of liquid chromatography is a C18 column used Which is similar to adsorption chromatography in terms of the polarity of the stationary phase ... 

One column can be used for different types of liquid chromatography by changing the eluent components. As an example, a column packed with octadecyl-bonded silica gel has been used for size-exclusion liquid chromatography with tetrahydrofuran (THF), normal-phase liquid chromatography with n-hexane, and reversed-phase liquid chromatography with aqueous acetonitrile. Examples of the chromatograms are shown in Figure 1.4. 

Equation (1.27), which has been expanded to different types of liquid chromatography (Knox equation), shows that there is an optimum flow rate for each separation and that this does indeed correspond to the minimum on the curve represented by equation (1.27). The loss in efficiency that occurs when the velocity is increased represents what occurs when trying to rush the chromatographic separation by increasing the flow rate of the mobile phase. However, intuition can hardly predict the loss in efficiency that occurs when the flow is too slow. To explain this phenomenon, the origins of the terms A, B and C have to be considered. Each of these parameters has a domain of influence that can be seen in Fig. 1.9. Essentially, this curve does not depend on the nature of the solute. 

Using Figure 25-14, suggest which type of liquid chromatography you could use to separate compounds in each of the following categories ... 

Pure silica is used as a stationary phase in various types of liquid chromatography.28 In size exclusion chromatography29 the separation of polymer compounds is effectuated on basis of diffusion rate of variably-sized molecules through a microporous silica packing with uniform pore size. 

Reverse-phase liquid chromatography. Describes the type of liquid chromatography that uses a nonpolar stationary phase and a polar mobile phase. 

Chromatography is a separation process in which the sample mixture is distributed between two phases in the chromatographic bed (column or plane). One phase is stationary whilst the other passes through the chromatographic bed. The stationary phase is either a solid, porous, surface-active material in small-particle form or a thin film of liquid coated on a solid support or column wall. The mobile phase is a gas or liquid. If a gas is used, the process is known as gas chromatography the mobile phase is always liquid in all types of liquid chromatography, including the thin-layer variety. 

Another, more recent equation, the Knox equation, is applicable to various types of liquid chromatography and includes the adjusted height h ... 

There are many ways to classify the types of liquid chromatography. One of these is discussed below. Four types of high pressure liquid chromatography to be discussed here are liquid-solid, bonded reversed phase, ion-exchange, and paired-ion. These are all based on the differences in chemical properties of the materials to be separated. 

Size exclusion effects can influence the measurement of chromatographic parameters in all types of liquid chromatography. Size exclusion effects occur because of the existence of pores in the stationary phase which are filled with mobile phase and are too small for large sample molecules to enter. The efficiency of the column is clearly influenced by these effects as the rate of equilibration between the mobile and stationary phases depends on the rate of diffusion of the sample molecules into the pores. As the length of the alkyl chain attached to the particles constituting the stationary phase is increased, the rate of diffusivity into the pores is decreased due to restricted access. It is thus clear that an optimum situation must be achieved whereby the access of the sample molecules to the stationary phase is... 

The third type of liquid chromatography, partition chromatography, is performed by placing the sample on a column of solid support that has been impregnated with a liquid. The sample is then flowed down the column with a second liquid as the mobile phase that is immiscible in the liquid used to moisten the column. Therefore, as the sample flows down the column, the various components are partitioned between the solid and liquid phases, based upon their solubility in the two solvents, and thus elute at different times. 

Methods of Separation and Purification of Polysaccharides There are two main types of liquid chromatography techniques that are employed for the purification of polysaccharides, namely, ion-exchange chromatography and size-exclusion chromatography (SEC) [99]. Ion-exchange chromatography is widely used for the fractionation of polar and nonpolar polysaccharides. SEC is used for the purification of polysaccharides on the basis of molecular weights. 

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