Causes of peak broadening

The shape of the chromatographic peak in gas-liquid chromatography is generally the result of random, diffusion-type processes which occur in the column. 

If a diffusible, originally tightly limited system, e.g. a volume of gas, 

In the separation column there are several simultaneously occurring but by their nature different processes which have the same effect as diffusion. 

The phenomenon is called dispersion, the spreading of an originally compact substance peak. In a model described by Giddings (random value model) the various processes can be treated uniformly for calculation and finally combined in an effective diffusion coefficient which describes the overall peak broadening in the column. 

As long as sample molecules remain in the mobile phase, i.e. the carrier gas, they will continue to diffuse. Since the concentration of the peak in the direction of the column, at its exit we are interested only in the proportion of diffusion in the direction of the column. In the event of free diffusion the variance produced would be  

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At present this may only be modelled approximately, as discussed in Chapter 4, rather than fully incorporated into the dynamical theory. This is an important cause of peak broadening in diffictrlf materials such as strained layers. 

Gaussian functions are mathematically tractable, and in fact real chromatographic peaks can be close to Gaussian. However, the Plate Theory does not address the question of the causes of peak broadening and thus limitations on... 

Multipath effect in chromatography, the cause of peak broadening through diffusion... 

CONDUCTING POLYMERS Fundamentals and Applications 3. Identify all possible causes of peak broadening in a CV of a CP. 

Introduction of a suppression device between the column and the detector can be expected to cause some degree of peak broadening due to diffusional effects. The shape of the analyte band will also be influenced by hydrophobic adsorption effects, especially when the adsorption and desorption processes are slow. Examination of peak shapes and analyte losses can therefore provide important insight into the use of suppressors with organic analytes which are weakly acidic or weakly basic. It can be expected that peak area recovery rates after suppression are governed by a combination of hydrophobic interactions with the suppressor and permeation through the membranes with the balance between these mechanisms being determined by eluent composition, suppression conditions and analyte properties. 

Detector configuration arrangement of multiple detectors in series or in parallel can cause additional peak broadening, flow rate variations, back pressure variations,... 

Table 7.3 shows that there are fewer stages of fructose in the new column ( FG-4 ) than in the old column ( FG-2 ). This causes a peak broadening of fructose, thus worsening the separation of glucose and fructose (Fig. 7.3). 

Extra-column volumes should be kept as low as possible because they affect the separation. They are one cause of band broadening and of tailing. Their contribution to band broadening is additive. If each part adds 5% to peak width the decrease in separation performance is not negligible at all. Whereas it can be difficult to modify the instrument with regard to injector and detector, it is possible to use adequate tubing to connect the parts. Adequate means capillaries with an inner diameter of 0.17 or... 

It is obvious that a standard 8 pi detector cell causes a peak broadening of more than 1 % for the early eluted peaks on a 25 cm x 4.6 mm column. 

Figure 10.10. An example of peak broadening caused by instrument dispersion (extra-column band-broadening), which affects the early eluting peaks more severely. Figure courtesy of Academy Savant.

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