Chromatography band spreading

The effects of various pore-size distributions, including Gaussian, rectangular distributions, and continuous power-law, coupled with an assumption of cylindrical pores and mass transfer resistance on chromatographic behavior, have been developed by Goto and McCoy [139]. This study utilized the method of moments to determine the effects of the various distributions on mean retention and band spreading in size exclusion chromatography. 

Overall, the most effective factor in Equation 5.20 is the particle size. The smaller the particle size, the higher the column efficiency. Equations 5.13, 5.15, and 5.18 are depicted in Figure 5.6 against flow velocity as A, B, and C, respectively. The band spreading is thus influenced by Equation 5.15 at a low flow rate. The band spreading is influenced by Equations 5.18 and 5.19 at a high flow rate. For gas chromatography curve D is obtained. 

The apparent dispersion coefficient in Equation 10.8 describes the zone spreading observed in linear chromatography. This phenomenon is mainly governed by axial dispersion in the mobile phase and by nonequilibrium effects (i.e., the consequence of a finite rate of mass transfer kinetics). The band spreading observed in preparative chromatography is far more extensive than it is in linear chromatography. It is predominantly caused by the consequences of the nonlinear thermodynamics, i.e., the concentration dependence of the velocity associated to each concentration. When the mass transfer kinetics is fast, the influence of the apparent axial dispersion is small or moderate and results in a mere correction to the band profile predicted by thermodynamics alone. 

Figure 23-19 Band spreading from multiple flow paths. The smaller the stationary phase particles, the less serious this problem is. This process is absent in an open tubular column. [Adapted from H m. McNair and E. J. Bonelli, Basic Gas Chromatography (Palo Alto. CA Varlan Instrument Division. 1968).]...

The volume of a chromatography system outside of the column from the point of injection to the point of detection is called the dead volume, or the extra-column volume. Excessive dead volume allows bands to broaden by diffusion or mixing. Use short, narrow tubing whenever possible, and be sure that connections are made with matched fittings to minimize dead volume and thereby minimize extra-column band spreading. 

One of the main advantages of equilibrium theory is the capability to predict some fundamental phenomena that occur in multi-component chromatography such as the displacement effect and the tag-along effect (Chapter 2.6.2). Another application is the use as short-cut methods for preliminary process design. As no effects causing band spreading are included, it is not possible to predict the system behavior exactly. 

Kauffman, A.D Kissinger, P.T. Exfra-column band spreading concerns in posf-column photolysis reactors for microbore liquid chromatography. Curr. Sep. 1998, 17, 9-16. 

Radial compression uses radial pressure applied to a flexible-wall column to lessen wall effects. The mobile phase has a tendency to flow slightly faster near the wall of the column because of decreased permeability. The solute molecules that happen to be near the wall are carried along faster than the average of the solute band, and, consequently, band spreading results. Preparative scale radial compression chromatography columns have been found to possess efficiencies close to those of analytical-scale columns when an adequate radial compression level is used. Radial compression technology also helps lower the cost by substituting reusable column holders in place of expensive steel columns. 

The time>based contributions to the extracolumn band spreading play a role only for fast chromatography or early-eluting peaks in normal chromatog-... 

Glass columns used for low-pressure chromatography are nearly always equipped with an adjustable inlet plunger that can be moved up and down depending on the hei t of the bed. This way, any void space above the packed bed can be eliminated easily. The same principle can be applied to HPLC columns. When the bed collapses, the void space that is form above the bed contributes significantly to band spreading and peak distortion. However, the bed itself usually has become nonunifotm, and this distorts the peaks as well Thus an adjustable end fitting removes only part of the cause of p distortion and its effectiveness is limited. 

The first factor is the band spreading due to the chromatographic process itself. The same mechanisms are in effect as in linear chromatography. These phenomena have been discussed in Section 2.2. 

Figure 2 Factors affecting band spreading in liquid chromatography.

There is another packed column technique, referred to as porous HDC, or simply liquid exclusion chromatography or EEC, which has also been applied to particle size analysis. In this case the hardware nearly exactly duplicates that used for bulk polymers. Although the elution volume range is enhanced with these systems, the rather excessive band-spreading which results severely limits the resolution and therefore the practical use of the technique. 

Horvath, Cs. Lin, H.J. Band spreading in liquid chromatography General plate height equation and a method for the evaluation of the individual plate height contributions. J. Chromatogr. 1978, 149, 43. 

Because the effect on resolution attributed to band spreading is innately important in chromatography, the loading capacity of a column directly influences the efficiency of separations performed on ion-exchange sorbents. As the concentration of protein injected overloads the column, the plate number decreases due to band broadening, reducing the resolution between separated species. Column efficiency can be increased by using microparticulate supports... 

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