Adsorbent linear capacity

With increasing solute amount values of the height equivalent to a theoretical plate increase and k values decrease if other parameters are fixed. Estimate of the maximum sample amount that can be charged to a column so as to avoid loss of column performance occurring due to overloading, can be made from the value of the adsorbent linear capacity, 0°. On this basis, it is found that the maximum sample size for porous silica is about 2 x 10-4 g of sample per gram of silica. 

Fig. 4-3. Illustration of the effect of adsorbent heterogeneity on adsorbent linear capacity for a hypothetical two-site case.

Changes in separation temperature or solvent ate lather limited as means for increasing adsorbent linear capacity. These-Variables can affect linear capacity only by reducing sample /f valuesi but separation eventually becomes poor if sample adsorption is reduced sufficiently (small K effect). In some adsorption systems the achievement of adequate linear capacities by means of temperature or solvent variation would occur only for sample KP values so small as to seriously compromise separation. 

Fig. 4-4. Adsorbent linear capacity as a function of absorbent type and relative deactivation by water (19,19d). (a) Narrow pore silica (Davison Code 12, 190 preactivation) (b) medium pore silica (Davison grade MS, 190 preactivation) (c) wide pore silica (Davison code 62, 190 preactivation) (d) medium pore alumina (Alcoa F-20,...

To assure linear isotherm separation, sample size must be kept below some maximum value which we will call the linear capacity of the adsorbent bed. Linear capacity can be defined as the sample size 6(, i (grams of san le per grams of adsorbent) which is just sufficient to cause a 10% change in R (or K), relative to the linear isotherm value IR (or /f"). This is illustrated in Fig. 4-2. A similar definition of linear capacity in terms of... 

Equation (4-4a) predicts that heterogeneity will decrease with increasing separation temperature, and linear capacity should therefore increase with increases in temperature. Barrer and Rees (7) have provided a detailed discussion of the dependence of isotherm linearity on temperature in gas-solid systems, and it is now generally accepted that isotherm linearity normally improves with increasing temperature [e.g.. Ref. (/i)]. Numer- ous examples of increased isotherm linearity with increase in temperature have been reported for liquid-solid systems as well [e.g., Ref. (14)]. It must be kept in mind, however, that polar adsorbents generally lose water... 

Polar adsorbents such as alumina, silica, and other metal oxides are normally deactivated by water to increase linear capacity. Maximum linear capacity usually occurs for 30 to 60% surface coverage, which corresponds to 1-2% water per lOOm /g of adsorbent surface. The linear capacity of water-deactivated adsorbents is typically 5 to 100 times greater than for the original undeactivated adsorbent, assuming the starting adsorbent was dried to remove all physically adsorbed water (see Section... 

We have so far assumed a single sample compound in discussing the linear capacity of a chromatographic system. As the number of sample components increases, and samples become more complex, a greater fraction of the adsorbent bed will be in contact with sample at a given time... 

Deactivation of adsorbent, see Adsorbent, HETP, Linear capacity Delocalized adsorption, 270 Demixing, see Solvent Detection of sample bands, 349-350 Development, see Bed development Diatomaceous earth, 172 Diazaaromatics, see Pyridine derivatives Dielectric constant, see Solvent strength Diffusion coefficient, calculation of, 102 Diffusion of sample, contribution to HETP, 102... 

Langmuir isotherm, 55-58 linear capacity of, 80 Linear capacity Od.i, 78-79 vs. adsorbent deactivation, 87-89 vs. adsorbent type, 89-90 vs, band width, 90 vs. K 83-87 vs. sample structure, 86 vs. sample type, 90-91 vs. solvent, 87 vs. temperature, 86-87 Linear Elution Adsorption Chromatography (LEAC), 353-354 Linear isotherm, 77-79 See also Linear capacity advantages of, 77... 

In Section 4-1, it was shown that isotherm linearity is determined by 6, the fractional coverage of the surface by sample. The maximum value of 0 for isotherm linearity (the linear capacity 0o.i) in turn depends upon adsorbent heterogeneity specifically, the types of adsorbent sites (as defined by their Ki values) and their relative concentrations (iV,). Nowhere have we considered the variation of linear capacity with the extent of sample adsorption, i.e., the dependence of 60.1 on A . For a Langmuir isotherm Eq. (4-1) shows that linear capacity corresponds to 10% surface coverage, regardless of the value of X . Similarly, for a surface which contains several different site types (differing in values), it may easily be shown that if the Kf values of all sites are increased by a constant factor Q, K° is... 

A technique akin to adsorbent deactivation is thb modification of the total surface by covering the adsorbent with, sev al. monolayers of some compound or by reacting all the surface sites. In eitfier case a new surface results, of generally higher linear capacity and totally different adsorption characteristics. This general technique has so far beeit jeserved for adsorbents used in gas-solid chromatography (see Section 9-2B). 

Different adsorbents show wide variation in surface area and surface heterogeneity, with resulting differences in linear t paCity. The linear capacity of an activated silica will normally exceed that of an activated alumina by 100-fold. Optimally deactivated silicas have 5-15 times the capacity of an optimally deactivated alumina (see Fig 4-4). Figure 4-4 shows that the linear capacities of different silicas also vary. Differences... 

In selecting the right adsorbent for a given separation we must take into account the adsorbent s effect on (1) linear capacity, (2) column efficiency or HETP values, (3) sample recovery as related to chemisorption or adsorbent-catalyzed sample reactions, and (4) selectivity (i.e., the dependence of sample K° values on the adsorbent). The role of the adsorbent in determining linear capacity, column efficiency, chemisorption, and sample reaction is discussed in Chapters 4, 5, and 13. The present chapter describes the general effect of adsorbent type and activityf on sample values (linear isotherm distribution coefficients). Chapter 7 will provide details on the selectivity and related properties of individual adsorbent types. 

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