Adsorption problems

Some GPC analysts use totally excluded, rather than totally permeated, flow markers to make flow rate corrections. Most of the previously mentioned requirements for totally permeated flow marker selection still are requirements for a totally excluded flow marker. Coelution effects can often be avoided in this approach. It must be pointed out that species eluting at the excluded volume of a column set are not immune to adsorption problems and may even have variability issues arising from viscosity effects of these necessarily higher molecular weight species from the column. 

Variations of resistance with frequency can also be caused by electrode polarization. A conductance cell can be represented in a simplified way as resistance and capacitance in series, the latter being the double layer capacitance at the electrodes. Only if this capacitance is sufficiently large will the measured resistance be independent of frequency. To accomplish this, electrodes are often covered with platinum black 2>. This is generally unsuitable in nonaqueous solvent studies because of possible catalysis of chemical reactions and because of adsorption problems encountered with dilute solutions required for useful data. The equivalent circuit for a conductance cell is also complicated by impedances due to faradaic processes and the geometric capacity of the cell 2>3( . 

It exhibits a single oxidation process (Ea = + 0.59 V, vs. SCE) affected by some adsorption problems. These adsorption phenomena, which typically affect the electrochemical response of these derivatives, sometimes make it difficult to ascertain by controlled potential coulometry the effective number of electrons involved in the oxidation step. In this case, the (approximate) number of electrons involved per molecule of dendrimer, nd, can be roughly calculated by comparing the cyclic voltammetric responses of the dendrimer with that of the ferrocene monomer using the following empirical equation.27,40... 

Capillary wall Generally, the most straightforward approach is to use an uncoated fused silica capillary. But sometimes this is not possible because of adsorption problems to the capillary wall, or other wall properties are needed to control the electroosmotic flow. In literature, there are multiple examples. Besides permanently coated capillaries, there are several descriptions of dynamic coatings available, e.g., triethanolamine, Triton X-100, Polybrene, and quaternary ammonium salts. The advantage of these dynamic coatings is that the coating can be renewed between injections, which could improve repeatability and reproducibility of the separation. 

Before an in-depth discussion of mass transfer models and coefficients we need to be explicitly clear that all mass transfer models are approximations that allow us to solve the partial differential equations (pde) describing an adsorption problem. There are a great many sources that derive and present the partial differential equations that describe adsorption of gases appropriate for column separations. The Design Manual For Octane Improvement, Book I [7] was among the earlier works to show them. The forms as presented by Ruthven [2] are shown here owing to the consistent and compact nomenclature that he has employed. There are a wider array of forms to choose from in the literature including [6, 7]... 

As a point of reference there are a great many adsorption problems where the full form (Eq. 9.9) may not be necessary. For small concentration changes across a mass transfer front and for sufficiently high velocity that axial dispersion is not of serious consequence we can write a much simpler first order pde as shown below ... 

The process sketched out in this section can be used to create an adsorber design. There are often times when one wants to examine the performance of an existing unit. When performance analysis is needed there are several alternatives. Depending on the specifics of the problem there may be an analytical solution for the adsorption problem and that may enable the creation of a satisfactory descrip-hon of the process to use in understanding phenomena that are observed in operation. 

Linearity of response versus absolute amount injected must be confirmed for each different sample type and for each different set of chromatographic operating conditions. This linearity cannot be assumed. Nonlinearity may result from column overload, detector overload, or adsorption problems. 

Adsorption problems are generally indicated by the calibration curve not psssing through the origin, and in some cases by nonlinearity of the curve. A change of the column may be the answer. Perhaps increased temperature will reduce the problem to a workable level. Even though it is not desirable, some adsorption can be tolerated and still give quantitative results but frequent recalibration is critical. 

Barrer, R. M. and Grove, D. M. Trans. Faraday Soc. 47 (1951) 826, 837. Flow of gases and vapours in a porous medium and its bearing on adsorption problems I. Steady state of flow, II. Transient flow. 

The solution is assumed ideal. It is incompressible all lattice sites are filled with some species of molecule. All species of molecules at the lattice sites are of equal (or nearly equal) size. For physical reasons, only one molecule can occupy each lattice site. Since there is a distribution of adsorptive energies within the zeolite, corresponding to the locally varying electrostatic field, the adsorption problem is approached from the standpoint of a superposition of several solutions - all the sites in each being identical. The number of solutions that must be considered equals the number of different adsorptive energy sites that are found within the zeolite. 

Precautions should also be taken with the aqueous solvents used to prepare standard working solutions in order to avoid adsorption problems, especially at low concentrations. To minimize adsorption during standard curve and validation sample (VS)/QC preparations, aliquots of the high-concentration stock solutions should be spiked promptly into the control blank plasma to prepare the standards and VS/QC. Once the compounds are in an environment of protein solutions, the adsorption problem is negligible. If there is a problem, however, adding or pre-rinsing with a solution of protein or a chaotropic agent (e. g., Tween, Triton X-100 or CHAPS) may help to alleviate the problem. 

Note Copper columns often cause adsorption problems incompatible with amines, anilines, acetylenes, terpenes, steroids, and strong bases. 

Polymers, advanced materials Micelles, colloids, vesicles Adsorption problems Surfactants... 

It is widely accepted that localized adsorption is a result of large variation in potential energy from point to point on the adsorbent surface, and that the lateral interaction in such systems must be affected to some extent by the existence of these variations (3). This paper consists in a more quantitative formulation of these concepts than has been available previously. Furthermore, it is shown that, for adsorption on crystallographically perfect surfaces, a considerable amount of information about the nature of the potential field at the surface of the solid can be obtained from a proper treatment of the experimental data. This approach to the adsorption problem can also be extended to give the formal equations applicable to adsorption on a heterogeneous surface however, it seems unlikely that much practical use could be made of these equations because of the very large number of unknown parameters which appear. 

The major application of coatings are in CZE and isoelectric focusing (see Chapter 5). All coatings discussed above have been used in CZE, with only a few exceptions. These coatings help solve the protein adsorption problem, with the EOF approaching zero. A large database is available to further assist the readers with their coating problems.13... 

In retrospect, the early adsorption models identified most of the significant aspects of the adsorption problem. The relationship between adsorption energy and average chain configuration was explored extensively. The assumptions necessary for the calculations, however, limited the utility of these models in predicting the behavior of real systems and, hence, their credibility. Improved analyses of excluded volume and equilibrium required a more comprehensive mathematical framework. 

The carrier liquid can also influence the FFF results as it can alter solute-solute or solute-wall interactions as well as the extension of a polymer in solution [266]. This in turn influences the diffusion and retention of the sample. Limited efforts have been made to describe these phenomena so that their influence still cannot be quantitatively treated. The practical importance of solvent effects becomes clear for the example of proteins which can be switched from positive to negative polyelectrolytes by pH variation. In any case, the pH should be chosen outside the range of the isoelectric point to avoid adsorption problems. 

Aryl-ether-based dendrons were used as stoppers in a series of rotaxanes containing one to three bipyridinium units surrounded by polyether macrocycles (compounds 74-76) [150], Electrochemical investigation in acetonitrile shows that a first reversible cathodic wave is due to the bipyridinium units, which are all reduced simultaneously and independently at the same potential. Electrode adsorption problems prevented accurate measurements for a second reduction process, also due to the bipyridinium units. On oxidation, two not fully reversible processes are observed, involving both the polyether macrocycle surrounding the bipyridinium units and the dioxybenzene units of the dendritic stoppers. 

Another adsorption problem that has been extensively studied using MD and Monte Carlo simulations is that of the wetting of a weakly adsorbing surface by a rare gas such as argon [29]. Qualitatively, wetting behavior is determined by the... 

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