Desorbent solution

FigureBl.7.2. Schematic representations of alternative ionization methods to El and PI (a) fast-atom bombardment in which a beam of keV atoms desorbs solute from a matrix (b) matrix-assisted laser desorption ionization and (c) electrospray ionization.

Increasing the value of N (reciprocal power term) reduces the effect of the desorbed solute and agrees ultimately with the case of zero solute desorption. 

A typical sequence followed in this test series consists in injecting (1) a micellar slug of one pore volume of aqueous solution of 4% of the preceding pseudobinary system (2% sulfonate/2% Genapol) (2) a slug of desorbent solution corresponding to a fixed amount of additive (e.g. equal to 1 PV at a concentration of 0.5 %) (3) at least 1.5 PV of brine with no additive. 

Figure 7 shows that for F = 0.5, a sorbing (or desorbing) solute will have reached 95% of the ultimate sorption capacity at (Tq,)1/2 = 0. 38. If we assume R 1, then Equation 38 can be rearranged to give... 

Spectrometer was used. Normal operating conditions were used normal slit, IX expansion, 12-minute scan speed (4000-200 wave numbers), and normal gain, in accordance with Perkin-Elmer setup instructions. A Beckman 2 cm path length, near infrared silica cell (holds 8 ml sample) was used to hold the desorbing solution in the sample compartment (Figure 1). 

Analysis of Samples on Charcoal. Each charcoal sampling tube was scored and broken open. The front and backup sections were placed in individual 2-mL serum vials. One milliliter of solvent, carbon disulfide or acetonitrile, was added and the vials were immediately crimped shut. After 30 minutes with occasional agitation, 5 yL of the desorbed solution was injected into the gas chromatograph. The quantity of vinyl acetate in the sample was read from a standard curve. 

Examples of K+ and NH4 adsorption-desorption plots for the two-cation system (K-Ca NH4-Ca) and the three-cation system (K-NH4-Ca) are shown in Figure 4.42. These data show that there is a significant hysteresis or relaxation effect in the 24-hr desorption process. This hysteresis-relaxation effect in the case of K-Ca exchange is more pronounced without added NH4 than with added NH4 in the desorbing solution. 

The rate of adsorption r, is proportional to the concentration in solution, [C], (at equilibrium in this case) and the amount of adsorption sites left vacant by the desorbing solutes. Now, let us determine these vacant adsorption sites. On a given trial of the experiment, the number of adsorption sites filled by the solute may be quantified by the ratio XIM, as mentioned previously. The greater the concentration of the solute in solution, the greater this ratio will be. For a given type of solute and type of carbon adsorbent, there will be a characteristic one maximum value for this ratio. Call this (Z/M) ,(. Now, we have two ratios XJM, which is the ratio at any time and (Z/M) ,p which is the greatest possible ratio. The difference of these two ratios is proportional to the number of adsorption sites left vacant consequently, the rate of adsorption r, is therefore equal to ks[C][(XIM) i, - (XIM)], where is a proportionality constant. 

Linear change or step change in mobile phase composition may produce differential migration of concentrated solutes. The average velocity of each desorbed solute is proportional to its fractional equilibrium mobile phase concentration. Therefore gradient elution has been normally used to remove adsorbed components from membranes. Gradient elution is convenient because it is difficult to determine, a priori, the modifier concentration required to selectively elute just the desired species. 

Desorption of strongly-bound ions often takes longer to attain equilibrium than adsorption. Several factors may contribute to adsorption/desorption hysteresis including differences in the aqueous composition of the adsorbing and desorbing solution, diffusion, time, and the nature of the adsorption bond. 

The hysteresis often observed in the adsorption and desorption of As has important implications for remediation strategies designed to decrease As concentrations to levels that meet drinking water supply criteria. The initial efficiency may be high as the readily desorbable fraction is removed from solution however, efficiencies may decrease significantly if As adsorption is not readily reversible. Changing the chemistry of the desorbing solution can facilitate or inhibit As desorption. 

This technique is known as the stationary port technique since the feed solution and the desorbent solution are always added at the same port and the product streams and the recycle stream are always removed from another port. Technichem and Finn Sugar manufacture chromatography systems which utilize the stationary port technique. 

Such conditions correspond to saturation, hence no further molar exchange occurs. When this happens to the whole bed, the bed must be regenerated, for example by passing a hot, inert fluid through the bed, thereby desorbing solute. 

Desorption of samples. Prior to analysis, 1.0 mL of desorbing solution is pipetted into each sample container. The desorbing solution consists of 0.05 iL internal standard per mL of carbon disulfide. The sample vials are capped as soon as the solvent is added. Desorption should be done for 30 minutes with occasional shaking. 

Desorbing solution with internal standard. This solution was prepared by adding 20 xL of dimethylformamide to 100 mL of toluene. 

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