Stagnant mobile phase mass transfer

While PLB were introduced first (14,15) more recently small PB 5-15 M diameter) have become of major interest. This is a result of the higher separation speeds found with such particles. Not only is the "stagnant" mobile phase mass transfer problem reduced, as in PLB, but solute mixing in the flowing stream is enhanced as a result of the smaller distance between the particles. The performances achieved with the small particle columns are equivalent to those obtained with capillary columns in gas chromatography (13), Examples illustrating the separation speed of such columns will be presented in the applications section of this paper. 

Figure 15.2D shows the contribution of stagnant mobile-phase mass transfer to molecular spreading. With porous column-packing particles, the mobile phase contained within the pores of the particle is stagnant, i.e., it does not move (in Fig. 15.2D one such pore is shown for particle 6). Sample molecules move into and out of these pores by diffusion. Those molecules that happen to diffuse a short distance into the pore and then diffuse out, return to the mobile phase quickly, and move a certain distance down the column. Molecules that diffuse further into the pore, spend more time in the pore and less time in the external mobile phase. As a result, these molecules move to a shorter distance down the column. Again there is an increase in the molecular spreading. 

D. Stagnant Mobile Phase Stagnant mobile-phase mass transfer has been identified as one of the major contributors to peak dispersion in liquid chromatography28 37,38 and is also represented by the C term of the van Deemter equation. As shown in Figure 1.16, the presence of immobile... 

Despite many advantages, CEC columns packed with microparticulate sorbents do have some limitations such as the relatively large void volume between the packed particles and the slow diffusional mass transfer of solutes into the stagnant mobile phase present in the pores of the separation medium [83,84]. Alternative approaches to alleviate the problem of mass transfer and intraparticular void volume are the concepts of monolithic chromatographic beds and open-tubular columns. In mono-... 

FIGURE 16 Schematic representation of the origins of zone-broadening behavior and mass transfer effects of a polypeptide or protein due to Brownian motion, eddy diffusion, mobile phase mass transfer, stagnant fluid mass transfer, and stationary-phase interaction transfer as the polypeptide or protein migrated through a column packed with porous particles of an interactive HPLC sorbent. 

In addition to elucidation of molecular structures, NMR can also extract valuable information about physicochemical parameters. Because of the omnipresence of protonated solvents in CE/CEC, mobile-phase events can be monitored with NMR. Early studies using E-NMR involved the calculation of diffusion coefficients, electrophoretic mobilities, and viscosity [27]. Stagnant mobile-phase mass transfer kinetics and diffusion effects [60] and fluid mass transfer resistance in porous media-related chromatographic stationary phases [61] have been studied with NMR spectroscopy. NMR imaging of the chromatographic process [62] and NMR microscopy of chromatographic columns [63] have also been reported. Several applications of NMR to on-line studies of CE/ and CEC/ NMR are highlighted. 

The third term, C, is a measure of the resistance to mass transfer between the stationary and the mobile phase. It includes ihe contributions by both the stationary phase and the stagnant mobile phase in the pores of the particles in the column bed. This term is complex, but, to a first approximation, it is inversely proportional to the diffusion coefficient, D, and directly proportional to the second power of the distance a solute molecule should travel to get from the mobile phase to the interaction site in the particle. For a totally porous particle, this distance is proportional to the mean particle diameter. 

Figure 4.1 Band-broadening processes in porous irregular microparticles, (a) eddy diffusion analyte molecules take different routes to circumnavigate the particles. They also move more quickly through wide channels than through narrow channels, (b) diffusion in the mobile phase. The short bracket indicates initial band width, the long bracket indicates final band width, (c) mass transfer. On the left is shown mass transfer in stagnant mobile phase in pores, and that due to the adsorption/desorption process. The narrow band represents initial band width, the broad band final band width. On the right is shown mobile phase mass transfer caused by laminar flow.

One approach to minimize mass-transfer resistance in a stagnant mobile phase employs specially designed particles with a bimodal network of pores. The larger pores (>1000 A) facilitate convective transport of the mobile phase inside the particles, whereas the small pores (<500 A) are explored by the sample components by diffusion only and provide the necessary surface area for adequate sorption capacity. 

Fourth Cause Mass Transfer between Mobile, Stagnant Mobile , and Stationary Phases... 

The migration of molecules between the stationary phase and the mobile phase is driven by random movement or diffusion, a factor which is deleterious to high resolution in all forms of chromatography. Since resolution in SEC is solely dependent on diffusion, unlike the other forms of chromatography, optimisation of stationary phase particles is important to improve mass transfer. Factors deleterious to mass transfer can be divided into three separate types those attributable to stagnant mobile phase in the pores of the particles, those caused by differential penetration of the solute molecules into the stationary phase and, finally, longitudinal diffusion between the particles (Snyder and Kirkland, 1979). 

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