Phase, mobile stationary

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

From equation 12.1 it is clear that resolution may be improved either by increasing Afr or by decreasing wa or w-q (Figure 12.9). We can increase Afr by enhancing the interaction of the solutes with the column or by increasing the column s selectivity for one of the solutes. Peak width is a kinetic effect associated with the solute s movement within and between the mobile phase and stationary phase. The effect is governed by several factors that are collectively called column efficiency. Each of these factors is considered in more detail in the following sections. 

The distribution of a solute, S, between the mobile phase and stationary phase can be represented by an equilibrium reaction... 

The most widely used particulate support is diatomaceous earth, which is composed of the silica skeletons of diatoms. These particles are quite porous, with surface areas of 0.5-7.5 m /g, which provides ample contact between the mobile phase and stationary phase. When hydrolyzed, the surface of a diatomaceous earth contains silanol groups (-SiOH), providing active sites that absorb solute molecules in gas-solid chromatography. 

For most samples liquid-solid chromatography does not offer any special advantages over liquid-liquid chromatography (LLC). One exception is for the analysis of isomers, where LLC excels. Figure 12.32 shows a typical LSC separation of two amphetamines on a silica column using an 80 20 mixture of methylene chloride and methanol containing 1% NH4OH as a mobile phase. Nonpolar stationary phases, such as charcoal-based absorbents, also may be used. 

A separation process that is achieved by the distribution of the substances to be separated between two phases, a stationary phase and a mobile phase. Those solutes, distributed preferentially in the mobile phase, will move more rapidly through the system than those distributed preferentially in the stationary phase. Thus, the solutes will elute in order of their increasing distribution coefficients with respect to the stationary phase."... 

In planar ehromatography, the fraetions are not always transferred to another separation system, but rather a seeondary separation is developed, orthogonally on the same ehromatographie plate. Therefore, for all substanees not eompletely separated it is possible that baseline separation ean be aehieved by means of a seeond separation proeess with an appropriate mobile (stationary) phase. Figure 8.2 shows that in the seeond dimension a theoretieally unlimited number of seeondary eolumns ean be applied. Beeause of this, the terminology two-dimensional PC is not suffieiently... 

On the basis of the principle of grafted TLC, reversed-phase (RP) and normal-phase (NP) stationary phases can also be coupled. The sample to be separated must be applied to the first (2.5 cm X 20 cm) reversed-phase plate (Figure 8.16(a)). After development with the appropriate (5ti 5yi) mobile phase (Figure 8.16(b)), the first plate must be dried. The second (20 cm X 20 cm) (silica gel) plate (Figure 8.16(c)) must be clamped to the first (reversed-phase) plate in such a way that by use of a strong solvent system (Sj/, SyJ the separated compounds can be transferred to the second plate (Figure 8.16(d)). Figure 8.16(e) illustrates the applied, re-concentrated... 

Now assuming that (k ) is a constant (which, for a given solute, will be true in SEC) and a given column is considered, then the particle diameter and the film thickness are also constants. Furthermore, as the mobile phase and stationary phases are the same liquids (the stationary phase consists of the mobile phase held in the pores), then Dm = Ds. 

Chromatography is essentially a method of separation based on two phases, one stationary and one mobile. If the composition of the mobile phase is not changed during the separation, the term isocratic elution is used. For separation of complex mixtures with wide ranges of polarities, the composition of the mobile phase can be changed during separation, a process known as gradient elution. 

HSCCC is attracting attention based on its high separation scale, 100% recovery of sample, and mild operating conditions. It is a chromatographic separation process based on the partition coefficients of different analytes in two immiscible solvent systems (mobile phase and stationary phase) subjected to a centrifugal acceleration field. 

Species Sample stationary Phase Mobile Phase Remark Reference... 

Stationary phase Mobile phase Flow rate... 

The addition of metal ions to the mobile phase frequently yields Improved separations of solutes capable of forming complexes (conversely the addition of ligands to the mobile phase may allow the separation of metal ions based on differences in the distribution constants of the complexes between the mobile phase and, stationary phase) [353-355]. A number of important... 

BHA, BHT, PG, TBHQ and tocopherols) a variety of stationary phases, mobile phases and detectors can be used [711]. Common antibacterials such as carba-dox, thiamphenicol, furazolidone, oxolinic acid, sul-fadimethoxine, sulfaquinoxaline, nalidixic and piromidic acid can be analysed by GE-RPLC-UV (at 254 nm). Collaborative studies have been reported for the HPLC determination of the antimicrobial sodium benzoate in aqueous solutions [712], Plastics devices used for field collection of water samples may contain polymer additives (such as resorcinol monobenzoate, 2,4-dihydroxybenzophenone or bisphenol A) or cyanobac-terial microcystins [713],... 

In gc there is only one phase (the stationary liquid or solid phase) that is available for interaction with the sample molecules. Because the mobile phase is a gas, all sample vapours are soluble in it in all proportions. In hplc both the stationary phase and the mobile phase can interact selectively with the sample. Interactions such as complexation or hydrogen bonding that are absent in the gc mobile phase may occur in the hplc mobile phase. The variety of these selective interactions can also be increased by suitable chemical modification of the silica surface, hence hplc is a more versatile technique than gc, and can often achieve more difficult separations. 

Stationary phase Mobile phase Detection Reference substances Reference... 

Stationary phase, mobile phase, and column length should yield sufficient sample and peak capacity. 

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