Secondary adsorption effects

In this equation, we find the different source and the selectivity between the two solutes, which can differ by j the difference in energy of adsorption (5i 55), molecular g-size (As2—Asi), and secondary adsorption effect (Aeas)i (Aeas)2- These considerations are valuable for a "S large number of compounds, but in the case of some g isomers, the second terms of Eq. 13 can be considered practically equal to zero and only difference sources for selectivity remain in the difference in energy of adsorption ... 

In PVA-coated capillaries it was possible to separate at pH 2.5 the standards of poly-2-vinylpyridinium hydrochloride (p(2-VPy)) in the molecular mass range between 1500 and 1,730,000 g mol-1 with dextran T70 as sieving matrix [20]. An example is shown in Fig. 4, where a 5% solution of dextran T70 has been used. The efficiency of the monomolecular basic marker 4-aminopyridine is excellent, demonstrating the exclusion of secondary adsorptive effects at the capillary surface. Hence, the broad peaks of the polymeric standards are due to their polydispersity. As in CE the width of the peaks depends on their migration velocity through the detection window, no direct comparison of broadness of the individual peaks and analyte polydispersity is possible. However, for each individual peak the methods applied in SEC for calculation of the different molecular mass averages can be applied. 

A change in separation temperature is potentially useful as still another means of changing separation selectivity or relative values. As discussed in Section 6-2A [Eq. (6-6a)], however, a change in adsorbent activity (which is equivalent to a change in separation temperature) normally has only a minor effect on separation selectivity. If two sample components have identical A values at one temperature, Eq. (12-3) predicts that their A values will be the same at all temperatures. Small differences in A should be amplified by separation at lower temperatures, however. Equation (12-3) predicts that separation order will be independent of temperature, just as Eq. (6-6) predicts that separation order is independent of adsorbent activity. These are only approximate relationships, however, and secondary adsorption effects [i.e., values in Eq. (6-10)] can result in pronounced changes in relative separation order with changing temperature. 

X have been measured for the adsorbent in question, and when the adsorbent is not too active (a < 0.8), relatively accurate, predictions can be expected. It is assumed, of course, that secondary adsorption effects involving large values of Ag s or are either absent or incorporated into the experimental value of 5 . When one or more of the above conditions is not met, however, the accuracy of calculated /f values ranges from fair to semiquantitative ( 0.2-0.5 log units). A few examples will illustrate the calculation of X values from data provided in this book. 

Electrostatic and adsorption effects conspire to make aqueous GPC more likely to be nonideal than organic solvent GPC. Thus, universal calibration is often not obeyed in aqueous systems. Elence, it is much more critical that the standard chosen for calibration share with the polymer being analyzed chemical characteristics that affect these interactions. Because standards that meet this criterion are often not available, it is prudent to include in each analysis set a sample of a secondary standard of the same composition and molecular weight as the sample. Thus, changes in the chromatography of the analyte relative to the standards will be detected. 

Weak adsorption is understood to be chemisorption simply determined by charge transfer, in Mulliken s sense (137) for instance, adsorption of xenon to transition metals (138) as compared to physical adsorption determined by dispersion forces. In terms of this secondary ensemble effect the influence of alloying is smaller for strong adsorbates than for weak adsorbates, if adsorption on the same sites is considered. 

In this paper we have introduced the secondary ensemble effect, which ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect will in general lead to a decrease in heat of adsorption upon alloying. 

Retention on these supports is adaquetely described by the adsorption displacement model. Nevertheless, the adsorption sites are delocalized due to the flexible moiety of the ligand, and secondary solvent effects play a significant role. The cyano phase behaves much like a deactivated silica toward nonpolar and moderately polar solutes and solvents. Cyano propyl columns appear to have basic tendencies in chloroform and acidic tendencies in methyl tertiobutyl ether (MTBE)... 

Mass Spectral Techniques. Samples for isotope ratio analysis are typically converted to sulfates or sulfides, then to S02(g) for analysis on a mass spectrometer (MS). The precision of the S02 measurement is commonly reported as 0.1 to 0.2 0/00 (16.241. yet systematic errors of 1 0/00 or larger may result from 1) memory effects due to adsorption of S02 on the walls of the MS, and 2) secondary isotope effects due to the existence of two stable isotopes of oxygen, 160 and lsO (251. Both of these errors can be eliminated by using SF6 rather than S02 as the analyte in the MS (25.261. However, existing sulfur fluorination procedures are relatively dangerous and tedious, making the SF6 method less desirable as a routine environmental technique (261. 

Most chromatographic systems with absence of the secondary equilibria effects (such as analyte ionization, specific interactions with active adsorption sites, etc.) show linear dependencies of the logarithm of the retention factors on the inverse temperature, as shown in Figure 2-11. 

The decreased heat of dissociative adsorption of O2 to Pt by alloying with Au has this as an explanation. Also the weaker interaction energy of C atoms to such alloys compared to non-alloyed Pt is a consequence of this ensemble type effect. It is called the secondary ensemble effect. 

The above picture of water/oxide interface does not obviously show the simultaneous, primary and secondary adsorption on non-dissociated water molecules. In their review, Etzler and Drost-Hausen wrote [89] Furthermore, as mentioned elsewhere in this paper (and other papers by the present author and associates), it is obvious that vicinal water is essentially unaffected by electrical double layers . Several properties of the vicinal water appear to be similar for various solid surfaces characterized by various point of zero charge (PZC) values (the paradoxical effect ). It is therefore to be expected that the contribution to the changes of the heat of immersion with changing pH, produced by the secondarily adsorbed vicinal water, is negligible. 

Xanthone, flavone and similar compounds. A difference in chloroform specificity from 2-methoxyethanol is again demonstrated In Figure 11. Xanthydrol is eluted before xanthone and flavone with chloroform as the modifier. Perhaps this shows the useful coupling of a proton donor solvent and proton acceptor solute (a large secondary solvent effect in an adsorption system (43)), the Interaction that was hoped for with the selection of chloroform as one of the selectivity triangle modifiers and apparent here because of a less strong adsorption of the xanthydrol on the fully active silica than some of the other basic solutes used in the preliminary studies. 

The chemical description of this interaction is still to be determined. It appears that there exists some threshold solvent power (defined either by the pure carbon dioxide density or the modifier identity and concentration in a modifier/carbon dioxide mixture) at which the solvent can begin to compete successfully with a particular stationary phase for a particular solute. Whether this involves a deactivation of active sites amenable to specific solute adsorption on the silica surface or a secondary solvent effect (43) where the mobile phase interacts with the solute as well as with the adsorption surface is unknown. 

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