Separation selectivity variations

Variation of solute retention and separation selectivity as a function of the eluent temperature at constant organic modifier concentration, pH and ionic strength. Solutes tryptophane enantiomers column alpha-... 

The development of HPLC methods where the mobile-phase pH is close to the analyte Ka is not recommended because of potential distorted peaks (which can be amended by the increase of the salt or buffer concentration) these methods may not be rugged and will not be easily transferable to other laboratories (manufacturing facility). Any minor variations in the mobile-phase pH in this case can lead to the significant variations in the analyte retention and separation selectivity. 

Analysis of the statistical data (presented as dots in Figure 6.8) shows that the separate selections by each section have some discrepancy. This discrepancy of calibration points is explained by the presence of the so-called error of the measurement variations, which is more often known as the laboratory error. 

Figure 3 Variation in neutral salt additives may have an impact on separation selectivity by altering the cation from sodium (A) to ammonimn (B). Peaks (1) ribonuclease (2) myoglobin (3) 6-lactoglobubn (4) trypsin inhibitor, (5) B-lactoglobulin B - reference [22],...

ILs are especially attractive for this type of separation because they possess high thermal stability, a broad liquid temperature range, and extremely low vapor pressure, thereby reducing possible column bleeding. By variation of the cation and the anion, the interaction between the IL film and the vaporized compounds can be adjusted in such a way that both polar and nonpolar compounds interact with the IL film, thereby leading to high separation selectivity that otherwise could not be achieved using non-lL stationary phases [10-24]. 

This technique can achieve efficient separations of polar and nonpolar analytes through the variation of both the stationary and mobile phases allowing variation of separation selectivity. LC has been applied to the determination of several phenolic... 

As well as varying the ionic strength of the eluent, the separation selectivity Km in CIC can be changed by variation of column temperature. Sorption heat relates the retention factor k with the column temperature by the van t Hoff equation ... 

The stability constants of and Ba " complexes of (55)-(57) were determined by pH-metric titrations and are shown in Table 2.4. The ionic radii of (1.33 A) and Ba (1.35 A) are very similar and hence cavity/ionic radius size effects are minimized. Table 2.4 reveals that (50) [2.2.2] cryptand has a large preference for Ba whereas (56) [2.2b.2b] forms complexes of identical stability and (57) [2.2.Cg] has a marked selectivity for K. This observed Ba /K selectivity variation can be explained in the following way. The presence of the two benzene substituents in (56) as compared to (50) has the effect of increasing the thickness of the organic layer separating the complexed guest cation from the solvent, hence reducing favourable interactions of the bound cation with the solvent. The net result is a destabilization of the complexes with and Ba , but this effect is more pronounced for the divalent cation than for the monovalent one (12). 

Computer simulation refers to the use of a computer for predictions of separation as a function of experimental conditions. In most cases, computer simulation requires two or more experimental runs with a given sample, in order to calibrate the computer prior to making predictions. Computer simulation can be used in H PLC method development to select optimum final conditions for the separation of a given sample, so that fewer actual experiments are required and better HPLC methods result Ideally, computer simulation should (a) allow for the simultaneous variation of as many as two experimental conditions that affect separation selectivity, (b) be applicable for both isocratic and gradient elution, and (c) be able to predict the further effect on separation of changes in flow rate, column dimensions and particle size. 

Surface-selective flow membranes made of nanoporous carbon, which is a variation of molecular sieving membranes, were developed by Rao et al. (1992) and Rao and Sircar (1993). The membrane can be produced by coating poly(vinylidene chloride) on the inside of a macroporous alumina tube followed by carbonization to form a thin membrane layer. The mechanism of separation is by adsorption-surface-diffusion-desorption. Certain gas components in the feed are selectively adsorbed, permeated through the membrane by surface diffusion, and desorbed at the low-pressure side of the membrane. This type of membrane was used to separate H2 from a mixture of H2 and CO2 (Sircar and Rao, 2000), and their main advantage is that the product hydrogen is at the high-pressure side eliminating the need for recompression. The membrane, however, is not industrially viable because of its low overall separation selectivity. In addition, since the separation mechanism involves physical adsorption, operation at low temperatures is required. 

To accomplish any separation of two cations (or two anions) of the same net charge, the stationary phase must show a preference for one more than the other. No variation in the eluant concentration will improve the separation. However, if the exchange involves ions of different net charges, the separation factor does depend on the eluant concentration. The more dilute the counterion concentration in the eluant, the more selective the exchange becomes for polyvalent ions. 

The more a deviates from unity for a given pair of ions, the easier it will be to separate them. If the selectivity coefficient is unfavorable for the separation of two ions of the same charge, no variation in the concentration of H+ (the eluant) will improve the separation. 

The task of process synthesis is to evaluate these schemes and select the best. The creative possibilities of design do not stop here. For example, these separations can be carried out by heat integration, by multiple-draw columns, or by use of processes other than distillation many other variations are possible. 

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