Band Broadening and Column Efficiency

The efficiency of a column is a number that describes peak broadening as a function of retention, and it is described in terms of the number of theoretical plates, N. Two major theories have been developed to describe column efficiency, both of which are used in modern chromatography. The plate theory, proposed by Martin and Synge,31 provides a simple and convenient way to measure column performance and efficiency, whereas the rate theory developed by van Deemter et al.32 provides a means to measure the contributions to band broadening and thereby optimize the efficiency. 

Efficiency, N, is defined in terms of the retention time (tR) of the solute, measured at the peak apex, and the standard deviation, cr, of the solute population in the peak measured as the peak width  

The efficiency can be varied by changing physical column parameters such as the length, diameter, and construction material of the container of the column. It can also be varied by changing chemical parameters such 

In its most general form, the van Deemter equation may be written 

11) the C terms represents the contributions to zone broadening from resistance to mass transfer in the stationary phase and the mobile phase, respectively. 

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Hydrodynamic (kinetic) aspects of chromatography, band broadening and column efficiency... 

It has been shown that when the intracolumn effect of mass transfer and diffusion is the main factor controlling band broadening, the column efficiency decreases with the increase of the viscosity of the meth-anol/water mixture on the other hand, when the extra-column effect is the main factor, an increase in viscosity of the eluents will help in improving column efficiency. Column efficiency is also related to the properties of the sample [86]. 

Monoliths Low backpressure, suited for conventional HPLC Higher separation efficiency by column coupling Rugged against delay volume and extra-column band broadening fast column re-equilibration Reduced maintenance on pumps and injector seals Reduced need for sample pre-treatment... 

For the best separation results, the efficiency of the system must be optimized in order to minimize band broadening, and the column should have the capacity to retain the analytes and sufficient selectivity to resolve them. 

The efficiency is the measure of the chromatographic band broadening and the number of the theoretical plates (AO in the column and is usually calculated using the following equation ... 

HPLC theory could be subdivided in two distinct aspects kinetic and thermodynamic. Kinetic aspect of chromatographic zone migration is responsible for the band broadening, and the thermodynamic aspect is responsible for the analyte retention in the column. From the analytical point of view, kinetic factors determine the width of chromatographic peak whereas the thermodynamic factors determine peak position on the chromatogram. Both aspects are equally important, and successful separation could be achieved either by optimization of band broadening (efficiency) or by variation of the peak positions on the chromatogram (selectivity). From the practical point of view, separation efficiency in HPLC is more related to instrument optimization, column... 

Relationships between mobile phase velocity and column efficiency, such as the Knox equation (equation (2.32)), are developed by taking into account all the possible contributions to band broadening. Thus the total peak variance (a oiumn) arising from band broadening in a packed column is given by... 

The rate theory of chromatography describes the shapes and breadths of elution bands in quantitative terms based on a random-walk mechanism for the migration of molecules through a column. A detailed discussion of the rate theory is beyond the scope of this text. We can, however, give a qualitative picture of why bands broaden and what variables improve column efficiency. ... 

Some of the variables that affect column efficiency can be controlled to improve separations. The extent of band broadening, and thus colunm efficiency, depends on the amount of contact time between the mobile and stationary phases. Therefore, colunm efficiency depends on the flow velocity of the mobile phase. Plots of plate height vs. velocity are called van Deemter curves. The plots are fit to the van Deemter equation... 

Accurate temperature control is required however, inclusion of heat exchangers increases the dead volume which causes band broadening and limits column efficiency. 

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