Column moving phase

In column chromatography the mobile (moving) phase is a liquid that carries your material through an adsorbant. I called this phase the eluent, remember Here a gas is used to push, or carry, your vaporized sample, and it s called the mobile phase. 

The term liquid chromatography (LC) is applied to any chromatographic procedure in which the moving phase is liquid, as opposed to gas chromatography (GC) where a gas is utilized as a mobile phase (see discussion in Chapter 14). Classical column chromatography (see Section 15.1), paper chromatography—a forerunner of thin-layer chromatography (see Chapter 13), and HPLC are all examples of LC. This should clarify why it is inappropriate to further abbreviate HPLC to LC unfortunately, it is still commonly done. 

The difference in movement rates of various compounds through a column is attributed to differential migration in HPLC. This can be related to the equilibrium distribution of different compounds such as X, Y, and Z between the stationary phase and the flowing solvent(s), or mobile phase. The speed with which each compound moves through the column (ux) is determined by the number of molecules of that compound in the moving phase, at any moment, since sample molecules do not move through the column while they are in the stationary phase. The molecules of the solvent or mobile phase move at the fastest possible rate except in size exclusion chromatography, where molecular... 

When the fraction of molecules X in the moving phase is zero (M — 0), no migration occurs and ux is zero. If the fraction of molecules X in the moving phase is 1 (i.e., all molecules of X are in the mobile phase, M — 1), then molecules X move through the column at the same rate as solvent molecules and ux = u. As a result, M is also the relative migration rate of compound X. 

The internal volume of a column is occupied by three substances, the mobile phase, the stationary phase and the support. The term mobile phase is a misnomer as it impfies that all the mobile phase is moving, which is not so. The mobile phase within the pores is also stationary and thus constitutes part of the stationary phase. Nevertheless, it is such a well established term, it will still be used to denote the total mobile phase in the column, moving and sialic. The term moving phase will be used for liial fraction of the mobile phase that actually moves, whereas the term static phase will be used for that fraction of the mobile phase that is trapped in the pores, or in the interstices of the support particles, and does not move. 

Carrier gas. Synonymous with mobile or moving phase. The phase that transports the sample through column. 

Components of the sample are retained in the column for different lengths of time due to adsorption-desorption, solution-dissolution, chemical affinity, size exclusion, and other mechanisms of varying nature. Various components are continually washed from one part of the stationary phase and recaptured by another by the moving phase. Different components elute in groups from the column with respect to time from injection. Dispersion in the system causes the bands of components to emerge with... 

Fig. 4. Minimum requirements for a gas chromatographic system include (I) a column which contains the substrate or stationary phase. 12) a supply nt inert carrier gas (moving phase) which is continually passed through the columns. (3) a means lor maintaining pressure and flow constant. (4) a means of admitting or injecting the sample into die carrier gas stream. (5) a detector which senses the sample components as they elute, and >) a display (recorder). The carrier gas may be any gas that does not react with the sample nr adversely artect the detector. Helium, hydrogen, nitrogen, and argon are often used...

The injection of a sample in a chromatographic column may result in more peaks than there are components in the mixture if the mobile phase contains one or several additives. These additional peaks result from the perturbation of the additive equilibrium between the two phases caused by the injection of a sample. It may be assumed that there is a competitive equilibrium of the sample and the modifier. Solutes enter the column, moving with the velocity of the mobile phase and not with the equilibrium velocities dictated by the equilibrium between mobile and stationary phases. System peaks are visualized with an appropriate detector, particularly a refractive index detector. This may cause trouble for the analyst, since the system peak may exhibit k values more than 1 (37). 

The results are presented in Fig. 36. The dependence of h on V + W in columns with continuous motion of ion exchanger is similar to the dependence shown in Fig. 35. The dependence of h upon the linear rate of solution motion for both types of column with phases moving alternately and for columns with a fixed bed are virtually the same when performing the operations of ion-exchanger bed transfer under expansionless... 

HPLC is performed on analytical columns packed with a commercially available solid phase containing long hydrocarbon chains (e.g. Cg, Cig) chemically bound onto silica. Chemicals injected onto such a column move along at different rates because of the different degrees of partitioning between the mobile aqueous phase and the stationary hydrocarbon phase. The HPLC method is not applicable to strong acids and bases, metals complexes, surface-active materials, or substances that react with the eluent. The HPLC method is applicable when the log Kow value falls within the range 0 to 6 (OECD 117, 1989). The HPLC method is less sensitive to the presence of impurities in the test compound compared to the shake-flask method. 

The moving phase in liquid chromatography is generally referred to as the mobile phase. In liquid chromatography, the mobile phase flows through or across the stationary phase as a result of a pressure difference (P) across the column. 

This is essentially the same as the distribution coefficient and the chromatographic process consists essentially of thousands of dynamic equilibria. However, there is not sufficient time for the sample to reach fully its equilibrium distribution between the two phases. The sample remaining in the mobile phase is carried down to a fresh portion of the column where it moves onto the stationary phase from the mobile phase. As concentration of the solute in the mobile phase from this first portion decreases, solute in this region of the column moves back from stationary phase to mobile phase, in keeping with the distribution coefficient, and is carried down to the second portion of the column where once again equilibrium is (nearly) achieved. This does not take place as a series of discrete steps but as a continuous dynamic process (Fig. 6). 

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