Pore flow in CEC

Effects of Pore Flow in CEC with Porous Particles... 

Relevant for the discussion of the effects of pore flow in CEC is the total or average EOF through narrow channels. The effect of electrical double-layer overlap on EOF is usually expressed in an electroosmotic flow screening factor P, which is defined as the ratio of the EOF velocity to that obtained without double-layer overlap, as can be found from the Smoluchowski equation ... 

A number of papers have appeared in which the importance of pore flow in CEC is discussed [13-22], Often, these discussions are supported by rather indirect experimental evidence for the existence and the magnitude of the pore flow, such as by its assumed effect on the separation efficiency. 

Since electroosmotic flow can exist in both the interparticle and intraparticle spaces, numerous studies have focused on the existence of intraparticle flow in CEC. Several groups have investigated the existence of electroosmotic flow in wide-pore materials [41-44], A model was developed to estimate the extent of perfusive flow in CEC packed with macroporous particles [41] by employing the Rice and Whitehead relationship. Results showed the presence of intraparticle EOF in large-pore packings (> 1000 A) at buffer concentrations as low as 1.0 mM. Additional parameters had been investigated [43,44] to control intraparticle flow by the application of pressure to electro-driven flow. Enhancement in mass transfer processes was obtained at low pore flow velocities under the application of pressure. The authors pointed out that macroporous particles could be used as an alternative to very small particles, as smaller particles were difficult to pack uniformly into capillary columns. 

The pores within the HPLC particles used in CEC have a distribution in the range of 8-10 nm. This will not in general support EOF, owing to the doublelayer overlap that occurs in these narrow pores, and in CEC using these particles flow is assumed to occur in the interstitial region of the space between particles. However, if the packing material contains fine particles of the order of <2 pm, then these may pack into the interstitial space and so in turn cause double-layer overlap and so prevent EOF. 

The effects of pore flow in size-exclusion electrochromatography (SEEC) are even more apparent than in reversed-phase CEC. The solutes typically separated in SEEC are slowly diffusing macromolecules such as synthetic polymers. For these solutes the enhanced diffusion effect becomes relevant even at low pore flow velocities and at low pore-to-interstitial flow ratios. 

The effect of pore size on CEC separation was also studied in detail [70-75]. Figure 9 shows the van Deemter plots for a series of 7-pm ODS particles with pore size ranging from 10 to 400 nm. The best efficiency achieved with the large pore packing led to a conclusion that intraparticle flow contributes to the mass transfer in a way similar to that of perfusion chromatography and considerably improves column efficiency. The effect of pore size is also involved in the CEC separations of synthetic polymers in size-exclusion mode [76]. 

Further, when electric field is applied to a capillary that is closed at one end, EOF as shown in Fig. 1.15 is set up in the tube. This situation is very similar to that of a dead end pore in a packing particle in CEC, where flow is generated at the pore wall and causes the electrolyte to accumulate at the dead end, followed by pressure driven flow at the center in the reverse direction. Unlike in pressure driven flow where mass transfer through such a pore would depend entirely on the diffusive flux, in CEC this flow would greatly enhance the mass transfer. 

The reasons for formation of bubbles in packed column CEC has not been explained satisfactorily. Bubbles may be formed due to local differences in EOF velocity (e.g. between unpacked and packed sections of the capillary [6,7]), by local differences in field strength (leading to "hot spots"), by release of gas trapped in the pores or electrochemically formed [8]. Whichever mechanism applies, it was suggested by early workers in CEC, to pressurize the inlet and outlet vials, in order to keep the gas dissolved [3,9,10], Once the bubbles form, the detector base line becomes very noisy and the current unstable. This may lead to break down of the current and the flow stops. Robson et al. illustrated that using pressurization of the solvent vials CEC can be carried out routinely at high fields with high speed and high efficiency [11],... 

EFFECTS OF PORE FLOW ON PEAK BROADENING IN CEC 3.1. The Enhanced Diffusion Effect... 

A high electroosmotic flow through the stationary-phase particles may be created when the appropriate conditions are provided. This pore flow has important consequences for the chromatographic efficiency that may be obtained in CEC. From plate height theories on (pressure-driven) techniques such as perfusion and membrane chromatography, it is known that perfusive transport may strongly enhance the stationary-phase mass transfer kinetics [30-34], It is emphasised... 

This is the result of both a lower effect on the diffusivity and a lower Cs contribution to the total plate height. Moreover, for retained solutes (k > 0), the gain in mass transfer is less pronounced [18]. Nevertheless, the enhanced diffusivity effect of pore flow is highly interesting since it indicates that CEC may have a high potential for solutes with a low diffusion coefficient. 

For CEC in the absence of pore flow, an A term equal to 1 dp has been proposed, while at fully perfusive conditions values of 0.2dp have been reported [18,23], In the limit of fully perfusive conditions (oo= 1), the disturbances of... 

Later work on macroporous particles in CEC indicated a much stronger effect of pore flow on the separation efficiency [16], In this study it was shown that at moderate ionic strengths in the range of 0.1-10 mM, fully perfusive behavior could be created with particles having pore sizes of 50-400 nm. Plate... 

It is clear that a significantly pore flow can be generated in electrochromatography when porous particles are employed as the stationary phase. This pore flow may strongly enhance the separation efficiency that may be obtained in CEC,... 

When the pore size, ionic strength, and electrical field strength are optimized for a high pore flow velocity and a high pore-to-interstitial flow ratio, reduced plate heights well below unity can be achieved in CEC of low-molecular-mass compounds. When such conditions can be created in combination with the use of small particles (dp < 1 pm), plate heights below 1.0 pm will be possible. 

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