Mobile phase diffusivity

As compared to HPLC, cSFC shows higher efficiency, universal and selective detection, minimal derivatisation for separation and the ability to separate thermally labile organic compounds. Often, cSFC analyses are also considerably faster. This arises because higher mobile phase diffusion coefficients translate directly into higher optimum velocities. However, sensitivity, detection dynamic range and sample capacity... 

Diffusion rates in liquids (LC) are typically three to four orders of magnitude less than in gases (GC). The lower mobile-phase diffusivity Dm affects two of the plate-height terms in liquid chromatography given in Table 19.1. First, the B/u term is small. Secondly, the Cmu term is large. The Csu term is small in many LC applications where the stationary phase is only a monolayer of liquid bonded to the surface of a solid... 

Although HETP is a useful concept, it is an empirical factor. Since plate theory does not explain the mechanism that determines these factors, we must use a more sophisticated approach, the rate theory, to explain chromatographic behavior. Rate theory is based on such parameters as rate of mass transfer between stationary and mobile phases, diffusion rate of solute along the column, carrier gas flowrate, and the hydrodynamics of the mobile phase. 

The use of elevated temperature in lEC reduces the mobile-phase diffusion coefficient and concomitantly decreases band spreading. Most mobile phases in lEC are composed of water with salts and thus produce efficiencies which are less than those obtained in modes using organic solvents. Because increased temperatures decrease retention, they may permit the use of lower salt concentrations. Elevated temperatures have been especially effective in amino... 

The use of elevated temperature in lEC reduces the mobile-phase diffusion coefficient and concomitantly... 

Diffusion coefficient in gas phase Diffusion coefficient in liquid stationary phase Diffusion coefficient in mobile phase Diffusion coefficient in stationary phase Activation energy... 

Here, the A term is due to eddy dispersion and flow contribution to plate height and is independent of ly it is a function of the particle size and the packing efficiency. The next term includes B, which depends on the molecular diffusion coefficient in the longitudinal direction, i.e. the mobile-phase diffusivity. The third term includes C, which results from mass transfer between the mobile and stationary phases and has contributions from (1) diffiision in the film around the particle in the column, (2) diffiision in the Uquid phase that is stagnant in the pores and (3) diffusion in the liquid-phase coating on particles. 

Concentration of solute in mobile phase Cm Diffusion coefficient, liquid film Dt... 

In these expressions, dp is the particle diameter of the stationary phase that constitutes one plate height. D is the diffusion coefficient of the solute in the mobile phase. 

To determine how the height of a theoretical plate can be decreased, it is necessary to understand the experimental factors contributing to the broadening of a solute s chromatographic band. Several theoretical treatments of band broadening have been proposed. We will consider one approach in which the height of a theoretical plate is determined by four contributions multiple paths, longitudinal diffusion, mass transfer in the stationary phase, and mass transfer in the mobile phase. 

To increase the number of theoretical plates without increasing the length of the column, it is necessary to decrease one or more of the terms in equation 12.27 or equation 12.28. The easiest way to accomplish this is by adjusting the velocity of the mobile phase. At a low mobile-phase velocity, column efficiency is limited by longitudinal diffusion, whereas at higher velocities efficiency is limited by the two mass transfer terms. As shown in Figure 12.15 (which is interpreted in terms of equation 12.28), the optimum mobile-phase velocity corresponds to a minimum in a plot of H as a function of u. 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

In reeent years, tire use of elevated temperatures has been reeognised as a potential variable in method development. Witlr inereased temperature, aqueous-organie mobile phases separations ean improve, viseosity deereases and diffusion inereases so baek pressures are redueed. At higher temperatures (usually with superheated water > 100 °C under modest pressures) water alone ean be used as the mobile phase and eair provide unique separation opportunities. The absenee of an organie solvent enables the use in HPLC of alternative deteetors sueh as FID or on-line LC-NMR using deuterium oxide as the eluent. 

The dispersion of a solute band in a packed column was originally treated comprehensively by Van Deemter et al. [4] who postulated that there were four first-order effect, spreading processes that were responsible for peak dispersion. These the authors designated as multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. Van Deemter derived an expression for the variance contribution of each dispersion process to the overall variance per unit length of the column. Consequently, as the individual dispersion processes can be assumed to be random and non-interacting, the total variance per unit length of the column was obtained from a sum of the individual variance contributions. 

Driven by the concentration gradient, solutes naturally diffuse when contained in a fluid. Thus, a discrete solute band will diffuse in a gas or liquid and, because the diffusion process is random in nature, will produce a concentration curve that is Gaussian in form. This diffusion effect occurs in the mobile phase of both packed GC and LC columns. The diffusion process is depicted in Figure 6. 

Equation (11) accurately describes longitudinal diffusion in a capillary column where there is no impediment to the flow from particles of packing. In a packed column, however, the mobile phase swirls around the particles. This tends to increase the effective diffusivity of the solute. Van Deemter introduced a constant (y) to account... 

Theoretically, dispersion can take place by diffusion in the stationary phase but, as will be seen, in practice, is much less in magnitude than that in the mobile phase. The theoretical treatment is similar to that for dispersion in the mobile phase using equation (10). 

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