Random walk and nonequilibrium theory

B = Longitudinal molecular diffusion in both mobile and stationary phases, and C = Kinetic or mass transfer term originating in the stationary phase. 

In actual practice, there are two basic considerations that prevail upon in gas chromatography, namely  

However, the resolution or extent of separation of any two peaks from a column is solely dependent upon both retention and column efficiency. 

Although separations may be caused by elution, frontal and displacement analyses, yet the elution technique is the most common. This method makes use of a stream of carrier-gas flowing through the column. Precisely, a sample is injected into the carrier-gas as a plug of vapour that is swept into the head of the packed chromatographic column. Separation of components that comprise the sample results from a difference in the multiple forces by which the column materials tend to retain each of the components. 

Upon emerging from the column, the gaseous phase immediately enters a detector attached to the column. Here, the individual components register a series of signals that appear as a succession of peaks above a base line on the chromatogram. 

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We will look at the three variables that may cause zone spreading, that is, ordinary diffusion, eddy diffusion, and local nonequilibrium. Our approach to this discussion will be from the random walk theory, since the progress of solute molecules through a column may be viewed as a random process. 

Clearly, departures from equilibrium—along with the resultant zone spreading—will decrease as means are found to speed up equilibrium between velocity states. One measure of equilibration time is the time defined in Section 9.4 as teq, equivalent to the transfer or exchange time between fast- and slow-velocity states. Time teq must always be minimized this conclusion is seen to follow from either random-walk theory or nonequilibrium theory. These two theories simply represent alternate conceptual approaches to the same band-broadening phenomenon. Thus the plate height from Eqs. 9.12 and 9.17 may be considered to represent simultaneously both nonequilibrium processes and random-walk effects. 

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