Chromatographic separation principles

The retention in SEC, in which the solute partition takes place between the common eluent phases in two different environments, interstitial and pore space, is expressed is follows  

In the SEC separation process, the separation condition is usually chosen to minimize the enthalpic interaction of polymer solutes with the packing materials. Therefore in the ideal SEC condition (ATf=0) KgpQ is a function of the conformational entropy loss (AS 0) of polymer chains when transferred into the restricted pore space  

If this condition is fulfilled, SEC separates the polymers exclusively in terms of the size of polymer chain in the eluent relative to the size of the pores. This condition constitutes the background of the Cassasa theory of Kgpc [ 14] and the universal calibration method [15]. 

On the other hand, the retention in IC is generally expressed as follows. 

Enthalpy and entropy compensation phenomena have been found not only in the chromatographic analysis of polymers but also in many kinds of kinetic and equilibrium processes for which a linear relationship exists between AH and AS [52, 53]. In the LCCC analysis of synthetic polymers, it is generally assumed that the retention of polymers is balanced by unfavorable entropic effect due to the size exclusion mechanism (AS 0) and the favorable enthalpic effect due to the solutestationary phase interaction (AH 0) [54—56]. Consequently, it is expected that a polymer species at the LCCC condition elutes independent of its molecular weight 

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The first applications of CDs as chiral selectors in CE were reported in capillary isotachophoresis (CITP) [2] and capillary gel electrophoresis (CGE) [3]. Soon thereafter, Fanali described the application of CDs as chiral selectors in free-solution CE [4] and Terabe used the charged CD derivative for enantioseparations in the capillary electrokinetic chromatography (CEKC) mode [5]. It seems important to note that although the experiment in the CITP, CGE, CE, and CEKC is different, the enantiomers in all of these techniques are resolved based on the same (chromatographic) principle, which is a stereoselective distribution of enantiomers between two (pseudo) phases with different mobilities. Thus, enantioseparations in CE are commonly based on an electrophoretic migration principle and on a chromatographic separation principle [6]. 

HPLC remains the dominant technique for chiral separation in industry. CE has become well accepted in academic laboratories. Current GC, HPLC, and CE instruments are automated. Chiral separation in CE relies on a chromatographic separation principle. Nevertheless, there are significant differences, as shown in Table 3, between these techniques. The property of... 

Another capillary technique, also with significant potential but again just used for analytical enantioseparations, is CEC [173, 174]. This hybrid technique relies on electrophoretic migration and chromatographic separation principles. The enantioseparation of several chiral compounds in non-aqueous CEC using polymethacrylate-type (Chiralpak OP) packing material is shown in Fig. 10 [174]. 

Explain how micellar electrokinetic chromatography combines electrophoretic and chromatographic separation principles. Why is the MEKC micelle called a pseudostationary phase ... 

While the migration principle, i.e., the driving forces moving the analytes through the separation capillary, is based on electrophoretic mechanisms the chiral separation is based on enantioselective interactions between the analyte enantiomers and a chiral selector and is, therefore, a chromatographic separation principle. The fact that the selector is in the same phase as the analytes in CE and not part of a stationary phase that is immiscible with the mobile phase as found in chromatography does not represent a conceptional difference between both techniques. The chiral selector in CE is also called pseudophase as it is not a physically different phase and may also possess an electrophoretic mobility. Enantioseparations in CE have also been termed capillary electrokinetic chromatography . 

As mentioned above, enantioseparations in EKC rely on a chromatographic separation principle. Despite this fact, there are significant differences between these techniques. Responsible for all differences between chromatographic and electrophoretic enantioseparations is the property of the electrophoretic mobility to be selective for the analytes residing in the same physical phase [2]. Another important point is that in chromatographic techniques, except in the case of a chiral mobile phase additive (CMPA), the analyte is virtually immobile when associated with a chiral selector. In EKC the analyte selector complex is commonly mobile. 

Manual transfer of the chromatographically separated substance to the detector . These include, for example, the detection of antibiotically active substances, plant and animal hormones, mycotoxins, insecticides, spice and bitter principles and alkaloids. The frequency distribution of their employment is shown in Figure 54 [295]. 

Another case to which we can apply the principle of microscopic reversibility is the isomerization of cis- and rrans-(H20)4CrClJ and the loss of Cl- from each. This system has been studied both by careful chromatographic separation of the components20 in quenched solutions and by simultaneous multiwavelength spec-trophotometric determinations.21 The scheme is as follows, where the subscript c indicates cis, t trans, and m the monochloro complex ... 

Since reproducibility of the flow system is critical to obtaining reproducibility, one approach has been to substitute lower-performance columns (50-to 100-p packings) operated at higher temperatures.1 Often, improvements in detection and data reduction can substitute for resolution. Chemometric principles are a way to sacrifice chromatographic efficiency but still obtain the desired chemical information. An example of how meaningful information can be derived indirectly from chromatographic separation is the use of system or vacancy peaks to monitor chemical reactions such as the titration of aniline and the hydrolysis of aspirin to salicylic acid.18... 

Principles and Characteristics In electrophoresis the separation of electrically charged particles or molecules in a conductive liquid medium, usually aqueous, is achieved under the influence of a high electric field. This differs from chromatographic separations... 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Principles and Characteristics Traditional analytical approaches include off-line characterisation of isolated components, and the use of several chromatographic separations, each optimised for a specific spectroscopic detector. Neither LC-NMR nor LC-MS alone can always provide complete structure determinations. For example, MS may fail in assigning an unequivocal structure for positional isomers of substituents on an aromatic ring, whereas NMR is silent for structural moieties lacking NMR resonances. Often both techniques are needed. 

Principles and Characteristics The use of a liquid chromatographic separation as a means of preparing samples for subsequent analysis by another chromatographic separation is well established. The goal of such cleanup separations is to reduce the complexity of the... 

The LC/MS/MS method utilizes the principle of three-dimensional separation to achieve excellent selectivity based on chromatographic separation (reversed-phase, size-exclusive, ionic, etc.), the unique mass-to-charge ratio of the analyte s parent ion, and the fragment ion. A sample clean up... 

Chromatographic separation and mass spectrometry are necessary to identify and measure quantitatively individual organic halogen components, but these are not practical for routine assay, and more general measurements are usually used. The first is the measurement of total organohalogen (TOX), and the second is of adsorbable organohalogen (AOX). These methods are based upon the principle... 

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