Retention processes

Thermo-diffusion calculations analyze the migration of hazardous material from compartment to compartment to release in containment. These calculations use physico-chemical parameters to predict the retention of hazardous materials by filtration, deposition on cold surfaces and other retention processes in the operation. Containment event trees aid in determining the amount, duration and types of hazardous material that leaves the containment. 

The co plAxlty of the retention proceas In RPC haa encouraged activity in non-chroaatographlc techniques to evaluate parameters appropriate for predicting retention as a function of mobile phase composition. Preliminary studies have indicated that solvatoChroaic methods can provide some useful insight into the retention process. The scale of solvent strength, based on... 

So far the models proposed to explain retention in RPC have largely remained the province of the physical chemist. The mathematical difficulty of using these models and their lack of a simple conceptual picture of the retention process in familiar chromatographic terms has diminished Interest in their use compared to simple empirical rules for trial and error optimization of separations. 

Jere et al. also reported on the stereoretentive C-H bond activation in aqueous phase catalytic hydrogenation of alanine (19). They demonstrated that hydrogenation of the carboxylic functionality is a stereo retentive process. Racemization occurs through a distinct process. 

The adsorption/desorption kinetics of Zn on soils with the removal of organic matter and Fe oxides were studied by Kingery et al. (1999) using traditional batch experiments on the same soil used by Hinz and Selim (1999). Adsorption of Zn on the three soils was kinetically characterized by three kinetic rates a very fast initial retention process within 30 min, an intermediate retention process from 30 min to 8 hours, followed by the very slow process (Fig. 5.5). About 50% of Zn was adsorbed on the three soils within the initial first 30 min. Initially, Zn desorption increased with time during the first 20 minutes from both the untreated soils and the soils without organic matter and Fe oxides. However, after 20 minutes of adsorption, net Zn adsorption from the whole soil and the soil with the... 

Variations in retention and selectivity have been studied in cyano, phenyl, and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline, and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile, or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns were correlated with polar group selectivities of mobile-phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers [84],... 

Peptides larger than 10 to 20 residues adopt conformations in solution through the interplay of hydrogen bonding, electrostatic and hydrophobic interactions, positioning of polar residues on the solvated surface of the polypeptide, and sequestering of hydrophobic residues in the nonpolar interior. Protein shape is dynamic, changing continuously in response to the solvent environment. The retention process in RPLC is initiated as the protein approaches the stationary-phase surface. Structured water associated at the phase surface and adjacent to hydrophobic contact surfaces on the polypeptide is released into the bulk mobile... 

FIGURE 1.3 Enantiomer separation of the chiral acid iV-acetyl-a-allyl-glycine on CHIR-ALPAK QN-AX (a) and CHIRALPAK QD-AX (b) by an enantioselective anion-exchange retention process. Chromatographic conditions Column dimension, 150 x 4 mm ID eluent, 1 % (v/v) glacial acetic acid in methanol flow rate, 1 mLmin temperature, 25°C detection, UV 230 nm. (Reproduced from M. Lammerhofer, et ah, Nachrichten aus der Chemie, 50 1037 (2002). With permission.)... 

A stoichiometric model can conveniently be invoked to explain the ion-exchange retention process [43 6]. As discussed in detail in these cited papers on ion-exchange theory, useful information about the involved ion-exchange process can be deduced from plots of log k vs. the log of the counterion concentration [X], which commonly show linear dependencies according to the stoichiometric displacement model (Equation 1.1)... 

The exact mechanism controlling shape-selective retentive processes is not fully understood, although it is clear that the pure partitioning and adsorption models cannot account for differences in retention for isomer separations or the range of selectivityobserved for columns of various surface coverages and alkyl chain lengths. 

Sadek, P.C., Carr, P.W., Doherty, R.M., Kamlet, M.J., Taft, R.W., and Abraham, M.H. Study of retention processes inreversed-phase high-performance liquid chromatography by the use of the solvatochromic comparison method. Anal. Chem., 57(14) 2971-2978, 1985. 

In conclusion, the different shapes of isotherms describing equilibrium distributions of a contaminant, between geosorbents and aqueous or gaseous phases, depend on the sorption mechanism involved and the associated sorption energy. At low contaminant concentration, all models reduce to essentially linear correlation. At higher contaminant concentration, when sorption isotherms deviate from linearity, an appropriate isotherm model should be used to describe the retention process. 

Alternative mechanistic schemes can also be devised to account for nonlinear van t Hoff plots. A particularly sinq>le case occurs when more than one binding site on the stationary phase surfoce is involved in the retention of eluite. The retention process can formally be modeled for the... 

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