Plate height zone broadening

Band broadening arises from three principal mechanisms, one of which depends on the mean velocity of the carrier and two of which are independent of . The latter two represent nonidealities in instrument and sample. The total variance of an eluting peak is the sum of variances contributed by each bandbroadening mechanism. Expressed as plate height H, which is the total variance divided by column length L (12), the zone broadening is described by... 

The broadening of zones by molecules caught in different flow lines is the most fundamental mechanism of band dispersion in F(+) methods. In some cases ordinary diffusion up and down the separation path also contributes measurably to zone broadening. In the simplest case ordinary diffusion contributes a variance of a2 = 2Dt and thus a plate height, a2/L, of... 

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. 

The kinetic contributions to zone broadening are evaluated by fitting data for the column plate height, as a function of the mobile-phase velocity, to a mathematical model describing the relationship between the two parameters. Several models have been used in the above experiment, but those by de Ligny and Remijnsee and Knox and Pryde, and developed by Guiochon and Siouffi are most widely used and, at least for a first approximation, allow for comparison and determination of the differences between TLC and column chromatography... 

All separation processes are inherently accompanied by zone broadening, which is due to the dynamic spreading processes dispersing the concentration distribution achieved by the separation [1]. As long as the relative contributions of these dispersive processes decrease, the efficiency of the separation increases. A conventional empirical parameter describing, quantitatively, the efficiency of any separation system is the number of theoretical plates per separation unit, A, or the height equivalent to a theoretical plate (the theoretical plate height) H defined by... 

Fig.l Zone broadening and its correction, (a) Schematic demonstration of the evaluation of the efficiency of the separation system of FFF (b) dependence of the theoretical plate height on the linear flow velocity (c) schematic demonstration of the application of the correction for the zone broadening on a model fractogram. 

Several dispersive processes contribute to zone broadening longitudinal diffusion, nonequilibrium and relaxation processes, spreading due to the external parts of the whole separation system, such as the injector, detector, connecting capillaries, and so forth. It has theoretically been found [2] that the resulting efficiency of the FFF, characterized by the height equivalent to a theoretical plate, can very accurately be described by... 

The van Deemter rate theory identified three major factors that cause band or zone broadening during the chromatographic process the eddy diffusion or the multi-path effect (A-term), longitudinal diffusion or molecular diffusion of the analyte molecules (B-term), and resistance to mass transfer in the stationary phase (C-term). The broadening of a zone was expressed in terms of the plate height, H, and was described as a function of the average linear velocity of the mobile phase, u. 

In open tubular colvunns, because the thin hquid film is deposited directly on the wall of the column rather than the sohd supports, the A term is zero, therefore ehminating one of the major contributor to zone broadening. Comparing to packed columns, the resistance to mass transfer is also reduced in both the hquid phase due to the apphcation of very thin film of the stationary phase, and in the mobile phase due to the apphcation of very narrow internal diameter columns. The typical open tubular colmnns have an internal diameter of 0.25 mm and a film thickness of 0.25 pm. A combination of ah these factors makes for the fact that capillary GC columns have much lower plate height value and substantiaUy more theoretical plates. The effect of carrier gas and hnear velocity on capihary column efficiency is illustrated in Figure 4, which shows a family of van Deemter plots for common carrier gases. 

When zone broadening such as that shown in Figure 2.6 is present, plate theory is not adequate to describe it. In zone broadening, the plate height can be expressed in centimeters by taking the square of the variance per unit length of the column so that... 

Skoog and West describe three causes of zone broadening eddy diffusion, longitudinal diffusion, and nonequilibrium mass transfer [1], The Van Deemter equation was developed to relate the flow rate and plate height ... 

The extent of a solute zone broadening determines the chromatographic eflB-ciency of the separation, which is expressed in terms of the number of theoretical plates n), or the height equivalent to a theoretical plate (//) given by the ratio of the column length (L) to the number of theoretical plates H=L/ri). The column efficiency can be easily calculated from the profile of a chromatographic peak as ... 

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