Adsorption exchange step

Research has shown that ascorbic acid can be produced from hulls of immature walnuts by extracting the hull with 0.2% sulfur dioxide solutions, and purifyiag the extract by adsorption on and elution from anion-exchange resias (see Ion exchange). Eluates from the anion-exchange step are concentrated, purified by organic solvent fractionations, decolorized, and crystallized (35). 

The reaction is carried out in close-loop reactor connected to a mass spectrometer for 1S02, 180160 and 1602 analyses as a function of time [38], The gases should be in equilibrium with the metallic surface (fast adsorption/desorption steps 1 and f ) If the bulk diffusion is slow (step 6) and the direct exchange (step 5) does occur at a negligible rate, coefficients of surface diffusion Ds can be calculated from the simple relationship between the number of exchanged atoms Ne and given by the model of circular sources developed by Kramer and Andre [41] ... 

Enzyme Purification. The purification of the xylanolytic enzymes began with adsorption on a cation exchanger (CM-Sepharose FF) at pH 4.0. The final purification was accomplished by another ion exchange step as described previously for xylanase (24), / -xylosidase (25), a-arabinosidase (26) and acetyl esterase (27). 

The results for Pt(5 3 3) (Figs. 11 and 26) are consistent with the increased sticking probability observed for a Maxwellian source of H2 on Pt(9 9 7) over Pt(l 1 1) [81]. It also provides direct evidence for an additional channel to dissociative adsorption through step sites which was invoked to explain the enhanced rate of H2 + D2 exchange reaction at Pt(3 3 2) over that observed on Pt(l 1 1) surfaces investigated using a Maxwellian beam source [82]. 

This concept represents a new theory for the action of process corrosion inhibitors. In contrast to the old filming amine theory the new one is based on a specific adsorption mechanism involving an ion exchange step. It can correctly predict the decrease of protection efficiency with increasing pH. It also explains structural differences between inhibitors such as increased efficiency of propylenediamine derivatives over primary amines and decreased efficiency with decreasing molecular weight of the amine group compound. 

The overall reaction can be seen as analogous to an Eley-Rideal modification of the LHHW model that involves a reaction between an adsorbed reactant with an unadsorbed reactant from the bulk phase (see Chapter 5). We treat the ion-exchange step as the adsorption of the first reactant on the inactive catalyst sites to form active sites, and the organic phase reaction step as the reaction of the second reactant with the adsorbed reactant at the catalyst sites. 

A protein is bound to the affinity adsorbent under conditions sufficiently mild to bind as few contaminating proteins as possible. The sample is usually in equilibration buffer and may be either a crude cell extract, culture supernatant fluid or obtained after a precipitation or ion-exchange step. Adsorption is better from more concentrated samples and higher flow rates can be applied, but the concentration is limited by the viscosity of the sample. It is acceptable to apply dilute samples as these are concentrated on the adsorbent. Overall, the volume of sample is not crucial provided that it is not so large that time becomes a limiting factor, unless the protein is weakly... 

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

Another example of vims clearance is for IgM human antibodies derived from human B lymphocyte cell lines where the steps are precipitation, size exclusion using nucleases, and anion-exchange chromatography (24). A second sequence consists of cation-exchange, hydroxylapatite, and immunoaffinity chromatographies. Each three-step sequence utilizes steps based on different properties. The first sequence employs solubiUty, size, and anion selectivity the second sequence is based on cation selectivity, adsorption, and selective recognition based on an anti-u chain IgG (24). 

Ion-exchange reactions are reversible. A regeneration procedure restores the resin to the ionic form it was in prior to the adsorption step. 

After the second extraction/stripping cycle, the plutonium is concentrated by evaporation or by preferential adsorption (qv) on ion-exchange resins. As in the case for uranium, the newer faciHties, such as THORP, use only a single purification step. 

Isolation. Isolation procedures rely primarily on solubiHty, adsorption, and ionic characteristics of the P-lactam antibiotic to separate it from the large number of other components present in the fermentation mixture. The penicillins ate monobasic catboxyHc acids which lend themselves to solvent extraction techniques (154). Pencillin V, because of its improved acid stabiHty over other penicillins, can be precipitated dkecdy from broth filtrates by addition of dilute sulfuric acid (154,156). The separation process for cephalosporin C is more complex because the amphoteric nature of cephalosporin C precludes dkect extraction into organic solvents. This antibiotic is isolated through the use of a combination of ion-exchange and precipitation procedures (157). The use of neutral, macroporous resins such as XAD-2 or XAD-4, allows for a more rapid elimination of impurities in the initial steps of the isolation (158). The isolation procedure for cephamycin C also involves a series of ion exchange treatments (103). 

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