Nonspecific sorption

Some authors have suggested the use of fluorene polymers for this kind of chromatography. Fluorinated polymers have attracted attention due to their unique adsorption properties. Polytetrafluoroethylene (PTFE) is antiadhesive, thus adsorption of hydrophobic as well as hydrophilic molecules is low. Such adsorbents possess extremely low adsorption activity and nonspecific sorption towards many compounds [109 111]. Fluorene polymers as sorbents were first suggested by Hjerten [112] in 1978 and were tested by desalting and concentration of tRN A [113]. Recently Williams et al. [114] presented a new fluorocarbon sorbent (Poly F Column, Du Pont, USA) for reversed-phase HPLC of peptides and proteins. The sorbent has 20 pm in diameter particles (pore size 30 nm, specific surface area 5 m2/g) and withstands pressure of eluent up to 135 bar. There is no limitation of pH range, however, low specific area and capacity (1.1 mg tRNA/g) and relatively low limits of working pressure do not allow the use of this sorbent for preparative chromatography. 

Vapor permeation of membrane-supported films of IZn has also been studied [31]. Examined as permeants were benzene, toluene, 4-picoUne, 2-picoline, cyclohexane, and methylcyclohexane. In pair-wise comparisons modest permeation selectivities were observed (i.e., factors of 1.1 to 9, depending on the permeant pairs compared). The competitive transport measurements were made at equal vapor pressures for the component pairs. The selectivities decrease if comparisons are made at equal reduced vapor pressures, P/Po, where Po is the vapor pressure at saturation. Nonspecific sorption of volatile compoimds at a given partial pressure generally inversely correlates with values for the saturated vapor pressure. 

Nonspecific hydration, or hydration of the lattice without first-order phase transitions, also must be considered. Cox et al. [40] reported the moisture uptake profile of cromolyn sodium, and the related effects on the physical properties of this substance. Although up to nine molecules of water per molecule of cromolyn sodium are sorbed into the crystalline lattice at 90% relative humidity, the sorption profile does not show any sharp plateaus corresponding to fixed hydrates. Rather, the uptake profile exhibits a gradual increase in moisture content as relative humidity increases, which results in... 

The match between the spectrum of the paste and one of the solid compounds may verify, e.g. surface precipitation of sparingly soluble fhydrjoxide as the mechanism of sorption. On the other hand, the match between the spectrum of the paste and the model solution spectrum (at a high percentage of uptake) suggests nonspecific (electrostatic) adsorption. 

Thus, current versions of the model allow sorption of inner-sphere complexes directly to surface hydroxyls whereas outer-sphere complexes are located at the [3-plane. Recent modifications have included an extension to allow parameter estimation of surface site densities, surface acidity constants and site densities using Bom solvation and crystal chemical theory (Sverjensky, 1993, 1994 Sverjensky Sahai, 1996 Sahai Sverjensky, 1997a,b) and to treat electrolyte ions as nonspecific adsorbing species that screen charge in the p-plane (Robertson Leckie, 1997). 

Significantly less data is found in the literature on the sorption of nitramines. Unlike those of TNT, sorption isotherms of RDX and HMX are usually well described by a linear equation (Equation 2.4), thus indicating either a nonspecific or a specific but nonsaturated sorption process [142,147,152,154,169] ... 

Fig. 4.7.3. Schematic drawing of effects of concentration of immobilized affinity ligands and of uneven support surface on nonspecific and specific sorption.

TABLE 2,5, Division of Initial Heats of Sorption into Specific (Dipole + Quadrupole) and Nonspecific (Dispersion f Repulsion + Polarization) Contributions... 

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