The Separation Mechanism

Further insight into the separation is given by the separation of lanthanides with 4 mM HIBA at pH 4.3 as the complexing agent (Fig. 10.12). Using published formation constants, the fraction of rare earths present in various chemical forms was calculated by a well-known method under the same conditions of pH and HIBA concentration used for the CE separation in Fig. 10.12. The calculated distribution of chemical species for each rare earth is shown in Table 10.4. 

The predominating species are the free metal ion the l l complex (probably ML2 ), the 2 1 complex (probably ML ), and the 3 1 complex (probably ML3). A small fraction of the higher rare earths is also present as the 4 1 complex. Another striking feature is that the average number of ligands associated with a rare earth (n) increases rapidly with increasing atomic number. This occurs in a linear manner as demonstrated by a plot of n against atomic number. 

The proposed mechanism necessitates a very fast rate of equilibrium between the free metal ions and the various complexed species. This condition is fulfilled with lactate and HIBA for the metal ions studied. However, metal ions that have slow com-plexation kinetics cannot be determined by a partial complexation CE system. For example, aluminum(IIl) gave no peak in a lactate system. 

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Despite the fact that solutions of acetyl nitrate prepared from purified nitric acid contained no detectable nitrous acid, the sensitivity of the rates of nitration of very reactive compounds to nitrous acid demonstrated in this work is so great that concentrations of nitrous acid below the detectable level could produce considerable catalytic effects. However, because the concentration of nitrous acid in these solutions is unknown the possibility cannot absolutely be excluded that the special mechanism is nitration by a relatively unreactive electrophile. Whatever the nature of the supervenient reaction, it is clear that there is at least a dichotomy in the mechanism of nitration for very reactive compounds, and that, unless the contributions of the separate mechanisms can be distinguished, quantitative comparisons of reactivity are meaningless. 

Table 2 provides a comparison of membrane structures. Between these two tables, you should get an idea of the operating conditions viz., membrane structural types, the driving forces involved in separation, and the separation mechanisms. 

All SEC columns have to be designed and synthesized by the polymer chemist to meet the specific requirements of the separation mechanism (3). With regard to efficient SEC separations, there are a number of important aspects to consider ... 

In thinking about performing multidimensional separations within the framework of unified chromatography, we must think about using all available tuning opportunities to maximize the differences in the separation mechanisms in the successive parts of the process. The following is just one example. 

Unlike gas chromatography, in which the mobile phase, i.e. the carrier gas, plays no part in the separation mechanism, in HPLC it is the relative interaction of an analyte with both the mobile and stationary phases that determines its retention characteristics. Hence, it is the varying degrees of interaction of different analytes with the mobile and stationary phases which determines whether or not they will be separated by a particular HPLC system. 

Guttman, A, On the Separation Mechanism of Capillary Sodium Dodecyl Sulfate-Gel Electrophoresis of Proteins, Electrophoresis 16, 611, 1995. 

Multidimensional methods thus involve a combination of single mechanisms and systems. In any multidimensional (usually 2D) approach, it is desirable that each dimension be as pure as possible in terms of selectivity of the separation mechanism. In comprehensive 2D separations, the precision (or chromatographic resolution) becomes a limiting factor and is ultimately determined by the quality of the separation in both dimensions. 

The use of LC as a sample pretreatment of GC has received considerable attention. The separation mechanisms in LC and GC are complementary, providing a powerful combined tool for the separation of complex... 

For future studies on MOF-based slurry systems, there is basic selection of criteria that needs to be satisfied by both MOF and the liquid solution. The selection of the MOF possessing the appropriate pore size for the preparation of the slurry system is very important to guarantee that the size of the liquid is large enough and does not occupy the pores which leaves no space for C02 to adsorb. Moreover, the structural stability of the MOF in the aqueous solution is essential so that it does not lose its porous framework nor its surface area. The selection of the liquid candidate is crucial, as it should not provide any extra mass transfer resistance for C02 molecules. Further, experimental and computational investigations are still required to understand the separation mechanism in slurry mixtures and to have insight into the different types of interactions between the gas, liquid, and solid materials. 

The challenge in effectively utilizing the multidimensional peak capacity is to find different types of columns that can uniformly spread the component peaks across the separation space. This challenge means that the separation mechanism of the two columns should be as dissimilar as possible or uncorrelated. A number of experimental studies have been undertaken to examine this effect (Liu et al., 1995 Slonecker et al., 1996 Gray et al., 2002). Chapter 3 examines the effect of correlation on peak capacity in detail using simulation techniques. 

This chapter examines another measure of the space used by 2D separations subject to correlation. Some researchers use the words, peak capacity, to express the maximum number of zones separable under specific experimental conditions, regardless of what fraction of the space is used. By definition, however, the peak capacity is the maximum number of separable zones in the entire space. No substantive reason exists to change this definition. The ability to use the space, however, depends on correlation. In deference to previous researchers (Liu et al., 1995 Gilar et al., 2005b), the author adopts the term, practical peak capacity, to describe the used space. The practical peak capacity is the peak capacity, when the separation mechanisms are orthogonal, but is less than the peak capacity when they are not. The subsequent discussion is based on practical peak capacity. 

To illustrate the separation mechanism in high/low pH RP-RP 2DLC (Fig. 12.2c), the peptides are divided into the three groups according to their p/. The p/ values were calculated using the method of Shimura et al. (2002). As expected, the acidic peptides... 

Figure 1. Schematic of an FFF channel with the separation mechanism for normal FFF shown in detail. Reprinted from [7] Beckett, R. and Hart, B. T. Use of field flow fractionation techniques to characterize aquatic particles, colloids and macromolecules . In Environmental Particles. Vol. 2, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems. Series eds. Buffle, J. and van Leeuwen, H. P., pp. 165-205. Copyright 1993 IUPAC. Reproduced with permission...

The extracting solvent in this scenario is the chromatographic mobile phase, while the sample solvent is the stationary phase. Liquid-liquid partition chromatography is based on this idea. The mobile phase is a liquid that moves through a liquid stationary phase as the mixture components partition or distribute themselves between the two phases and become separated. The separation mechanism is thus one of the dissolving of the mixture components to different degrees in the two phases according to their individual solubilities in each. 

The stationary phase consists of porous polymer resin particles. The components to be separated can enter the pores of these particles and be slowed from progressing through this stationary phase as a result. Thus, the separation depends on the sizes of the pores relative to the sizes of the molecules to be separated. Small particles are slowed to a greater extent than larger particles, some of which may not enter the pores at all, and thus the separation occurs. The mobile phase for this type can also only be a liquid, and it too is discussed further in Chapter 13. The separation mechanism is depicted in Figure 11.11. 

Name the four types of chromatography described in this chapter and give the details of the separation mechanism of each. 

The overall rate of decrease in concentration of particles of any size is given by Eqs. (7.6) and (7.10) by assuming additivity of the separate mechanisms... 

It is, however, also to be noted that deviations from linearity of the aforementioned plots may be readily observed, which may indicate smooth changes in the separation mechanism. If the k vs. percentage of modifier dependency is investigated over the entire or at least a wider range, [/-shaped curves may be obtained, in particular with acetonitrile as modifier. While the drop of retention factors with increase of modifier percentage at low modifier contents may follow the described RP-behavior, the trend... 

The separation mechanism is based on stereoselective ion-pair formation of oppositely charged cationic selector and anionic solutes, which leads to a difference of net migration velocities of the both enantiomers in the electric field. Thus, the basic cinchona alkaloid derivative is added as chiral counterion to the BGE. Under the chosen acidic conditions of the BGE, the positively charged counterion associates with the acidic chiral analytes usually with 1 1 stoichiometry to form electrically neutral ion-pairs, which do not show self-electrophoretic mobility but... 

It appears that the use of HILIC for the separation of basic compounds is increasing and it can provide a useful alternative selectivity to RP, with polar compounds being retained more than nonpolar compounds. The compatibility of HILIC eluents with MS detection seems to be a particular advantageous feature of the technique. Improved understanding of the separation mechanism may lead to its increased use. 

The mobile phase controls HPLC separation While the HPLC stationary phase provides retention and influences the separation mechanism, it is the mobile phase which controls the overall separation. HPLC method development efforts focus on finding a set of mobile phase conditions that provide adequate separation of the analyte peak(s) from other components in the sample. 

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