Wall adsorption

The mechanism of the UDMH dark decay is unknown, but it is presumed to be heterogeneous in nature. It is probably not wall adsorption, since much slower decay rates were observed previously in the absence of O2 ( ) ... 

Gassner, B., Friedl, W., and Kenndler, E., Wall adsorption of small anions in capillary zone electrophoresis induced by cationic trace constituents of the buffer, /. Ckromatogr., 680, 25, 1994. 

EOF when no disturbing processes such as wall adsorption occurs. The practically obtained migration time of the neutral marker (Teof) can be utilized to calculate the velocity of the EOF ... 

The standard deviation of the Gaussian zones expresses the extent of dispersion and corresponds to the width of the peak at 0.607 of the maximum height [24,25]. The total system variance (ofot) is affected by several parameters that lead to dispersion (Eq. 17.22). According to Lauer and McManigill [26] these include injection variance (of), longitudinal (axial) diffusion variance (of), radial thermal (temperature gradient) variance (of,), electroosmotic flow variance (of,), electrical field perturbation (electrodispersion) variance (of) and wall-adsorption variance (of ). Several authors [9,24,27-30] have described and investigated these individual variances further and have even identified additional sources of variance, like detection variance (erf,), and others... 

Wall-adsorption variance. Analyte-wall interactions are possible in fused silica capillaries depending on the characteristics of the wall and the analytes. Usually it concerns an ionic interaction mechanism, but sometimes hydrophobic interactions are also possible. Wall adsorption is frequently observed in the analysis of large molecules [26,33], but is also observed in separations of small ions [34]. The variance due to wall adsorption is given by the following expression [29] ... 

It is the effective mobility that determines selectivity since the effect of the electroosmotic mobility is equal for all the sample constituents. The effective mobility can be worked out from the apparent mobility if the extent of / eof known. Several methods are applied to determine however, the procedure of applying a neutral marker is commonly used. A marker is a compound that migrates only due to the EOF (neutral). The velocity of a neutral compound represents the velocity of the EOF when no other disturbing processes such as wall adsorption occur. Using the practically obtained migration time of the neutral marker (tgoi) the velocity of the EOF can be calculated ... 

Wall adsorption, although significant mainly in metallic volumetric systems, appears not to result in major problems if a suitably careful calibration is performed. Rapid transfer of heat from the sample to the cell wall is recommended [164],... 

Fluorescence Measurement of Pyrene Wall Adsorption and Pyrene Association with Humic Acids 181... 

If pH extremes cannot be utilized, then either a coated capillary or buffer additives can be used. Without either of these, proteins might adsorb to the inner capillary wall. Adsorption will affect the separation and in some circumstances this may be beneficial. Additives that are often added to CE buffer systems include surfactants, zwitterionic salts, ethylene glycol, methyl-cellulose, organic modifiers, and quaternary amines. Capillary coatings include polyacrylamide, polyethyleneglycol, polyvinylpyrrolidone, and methyl-cellulose. Mazzeo and Krull65 discuss capillary coatings and buffer additives further. 

The numerous reasons which can account for various deviations from the ideal FFF retention theory were discussed in the corresponding sections. Here, additional problems are treated which can complicate FFF measurements and significantly distort the results obtained. General requirements for a successful FFF measurement include precise flow control and flow rate precise temperature measurement precise determination of t0 and tr correct relaxation procedure control of sample overloading and integrity and control of mixed normal and steric retention effects as well as wall adsorption control. Some of these complications cannot be avoided so one must correct for these effects, usually in a sem-iempirical and partially very complicated fashion. 

Covalently bonded coatings Affect EOF reduce wall adsorption Stability problems... 

Other effects may also contribute to band broadening causing reduced achievable plate counts. Besides the already-mentioned wall adsorption, temperature effects (Joule heating) may reduce plate numbers. Sample application can have a strong influence on plate count, especially when large volumes and/or high sample concentrations are injected. Mobility differences between buffer constituents and analyte ions lead to asymmetric (triangular) peaks caused by electrodispersion, which is extremely noticeable with smaller molecules. Differ-... 

Polystryrenesulfonates are already fully dissociated even at low pH values, where the EOF in unmodified fused silica capillaries is strongly suppressed. Consequently, it is possible to separate them in a counter electroosmotic way in free solution at pH values around or below 2.5. It is only possible to separate PSSs in free solution up to a molecular mass of 8000. Schure et al. [23] discussed wall adsorption because they could not separate PSSs at a pH value of 5. It seems... 

C. P. Petroff, P. P. Nair, and D. A. Turner, Int. J. Appl. Radiat. Isot., 15 491 (1954). The Use of Siliconized Glass Vials in Preventing Wall Adsorption of Some Inorganic Radioactive Compounds in Liquid Scintillation Counting. 

For gas transport in small pores (say, less than the 10 nm range) the sizes of which are no longer much larger than those of the gas molecules, the contribution of viscous flow can be neglected and other considerations need to be factored in the model. First, the gas molecules are considered to be hard sphere with a finite size and the gas diffusion process is assumed to proceed in the membrane pores by random walk. The membrane pores are assumed to consist of smooth-wall circular capillaries. In addition to gas molecules colliding with the membrane pore walls, adsorption on the pore wall and the associated surface flow or diffusion are considered. Adsorption also effectively reduces the pore size for diffusion. 

Another approach to reducing the EOF as well as wall adsorption is to add a compound to the running buffer that will compete with the solute for the silanol sites on the surface. These materials must have some affinity for the charged or polar sites on the inner wall and so they must, themselves, be hydrophilic or charged. Nonionic surfactants are hydrophilic to prevent solute adsorption on the wall and block the silanols in order to reduce the EOF. The use of cationic polymers in the buffer results in a reversal of EOF to the anodic direc-... 

As PSSs are fully dissociated even at low pH, they can be directly separated in this medium, because the EOF is strongly suppressed at pH values below 2.5. Wall adsorption of PSS has never been noticed [20]. Some authors discussed wall adsorption [21] because they had not been able to detect PSS at intermediate pH values (pH=5). However, with these conditions at pH 5 the migration of the PSS and the EOF may just be identical in opposite direc-... 

Peak area precision is of great importance in the development of a quantitative assay. The reproducibility of the peak area is dependent upon a number of parameters, all of them instrumentally derived. Chemistry has little impact upon the quantitative reproducibility of an assay except where electrolytic changes in buffer pH can affect the absorption spectra of an analyte or where wall adsorption of the analyte can occur [17]. Most commercial instrumentation allows the use of electroki-netic and pressure injection modes for introduction of sample. 

You have probably noticed that the CE mechanism actually does not include a chromatographic distribution mechanism. Consequently, it is as readily applicable to macromolecules as to smaller ones. Hence, it is valuable for the separation of large biomolecules. Chemical modification of the sihca wall or addition of detergents to the background electrolyte is often required to eliminate wall adsorption of proteins. 

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