Chromatographic properties selectivity

The understanding of retention and selectivity behaviour in reversed-phase HPLC in order to control and predict chromatographic properties ai e interesting for both academic scientists and manufacturers. A number of retention and selectivity models are the subject of ongoing debate. The theoretical understanding of retention and selectivity, however, still lags behind the practical application of RP HPLC. In fact, many users of RP HPLC techniques very often select stationary phases and other experimental conditions by experience and intuition rather than by objective criteria. 

The reaction conditions can be selected so as to be able to separate substances with the same or similar chromatographic properties (critical substance pairs) by exploiting their differing chemical behavior, thus, making it easier to identify them. Specific chemical derivatization allows, for example, the esterification of... 

The remainder of this section deals primarily with selectivity optimization in isocratic liquid chromatography and with gradient elution Before entering these subjects proper, however, a discussion of the relevant chromatographic properties of solvents is in order as a framework for the intuitive selection of the preferred solvent or solvent mixtures for selectivity optimization. 

A second motivation to include CE methods is the excellent performance of chiral CE, which is often the first choice technique to separate stereoisomers. Such method can be used complementarily to avoid potential co-elution of isomers or related products, e.g., degradation products, with similar chromatographic properties. Practically, one can fractionally collect the peak volume, lyophilize it, and dissolve the resulting mass in an appropriate solvent. The pre-concentrated sample can then easily be analyzed with a selective and efficient CE method. Another option is to develop an on-line coupling between HPEC and CE to facilitate the analysis. " ... 

Gas chromatographic methods have been successfully used for the determination of penicillin molecules bearing neutral side-chains in milk and tissues (95, 97), but cannot be used for amphoteric -lactams. Gas chromatography of penicillin residues is further complicated by the necessity for derivatization with diazomethane. This derivatization step is particularly important because it not only leads to formation of the volatile penicillin methyl esters but also improves their chromatographic properties (thermal stability and decreased polarity). Using a fused-silica capillary column in connection with a thermionic nitrogen-selective detector, excellent separation and sensitivity figures were obtained. 

In normal-phase or adsorption chromatography, the chromatographic properties are functions of the specific area. Retention factors increase with the specific surface area. The parameter of the specific surface area of the packing could be of great importance when selectivity and efficiency have to be improved. One very short column packed with a silica of high specific surface area will yield the same results as a long column packed with a silica of low specific surface area (18). 

Other properties which have contributed to the attractiveness and versatility of the sol-gel doping approach are the chemical, photochemical and electrochemical inertness as well as the thermal stability of the matrix the ability to induce electrical conductivity16 the richness of ways to modify chemically the matrix and its surface as well as the above-mentioned controllability of matrix structural properties the enhanced stability of the entrapped molecule1,17 the ability of employing the chromatographic properties of the matrix for enhanced selectivity and sensitivity of reactions with the dopant4 the simplicity of the entrapment procedure the ability to obtain the doped sol-gel material in any desired shape (powders, monoliths, films, fibers) and the ability to miniaturize it18,19. 

The final stage of the residue analysis procedures involves the chromatographic separation and instrumental determination. Where chromatographic properties of some food residues are affected by sample matrix, calibration solutions should be prepared in sample matrix. The choice of instrument depends on the physicochemical properties of the analyte(s) and the sensitivity required. As the majority of residues are relatively volatile, GC has proved to be an excellent technique for pesticides and drug residues determination and is by far the most widely used. Thermal conductivity, flame ionization, and, in certain applications, electron capture and nitrogen phosphorus detectors (NPD) were popular in GC analysis. In current residue GC methods, the universality, selectivity, and specificity of the mass spectrometer (MS) in combination with electron-impact ionization (El) is by far preferred. 

In general, standard methods applicable to a vast majority of compounds of interest to ensure throughput capabilities are critical for LC/MS screens. Although not optimized for specificity, standard conditions provide a systemic measure of control. This control results in data that has high quality, reliability, and comparability. With a strategic selection of compounds that have similar molecular weights, structural features, and chromatographic properties, the detection selectivity and precision are satisfactory for this particular type of analysis. 

Structural Isomers. Chromatograms illustrating the separation of ortho, meta and para isomers of cresol (22) and and xylene ( O)on RP columns are shown in Figures 4 and 5. They enable a comparison of the chromatographic properties and selectivities due to <. - and -CD complexation between positional isomers of the above compounds.Similar behaviour was observed for ortho,meta and para isomers of fluoronitrobenzene, chloronitrobenzene, iodoni-trobenzene, nitrophenol, nitroaniline, dinitrobenzene (22), nitrocinnamic acid (22) some mandelic acid derivatives (19,21,34) and ethyltoluene (28). Both [Pg.225]

Human serum albumin and bovine serum albumin are closely related proteins and, consequently, the chromatographic properties of the CSPs based on these proteins are similar. The only difference between the two phases appears to be due to inherent differences in stereoselectivity between HSA and BSA. For example, on the HSA-CSP (S)-warfarin elutes before (K)-warfarin, whereas on the BSA CSP the opposite elution order is observed (85). This is consistent with the enantioselectivities of the native proteins (106). However, even though there are differences between the CSPs, the selectivity, mobile-phase effects, and chromatographic properties of the HSA CSP and BSA CSP are so similar that the two phases will be discussed together. 

For the removal of EPS from final solutions of bioproducts, selective adsorption has proven to be the most effective technique. Therefore considerable effort is being put into the development of adsorbents capable of retaining high EPS selectivity under physiological conditions (ionic strength of fi=Q.Q5-Q.2, neutral pH). Recently, numerous cationic polymer adsorbents have been developed for removing EPS from protein solutions. This article will elucidate the chromatographic properties... 

T. Toyo oka, Use of derivatization to improve the chromatographic properties and detection selectivity of physiologically important carboxylic adds, J. Chro-matogr. B 671 91-112 (1995). 

The mobile phase must obviously be chosen for its chromatographic properties it must interact with a suitable stationary phase to separate a mixture as fast and as efficiently as possible. As a general rule, a range of solvents is potentially able to solve any particular problem, so selection must be based on different criteria ... 

The reaction conditions can be selected so as to be able to separate substances —with the same or sinailar chromatographic properties (critical substance pairs)... 

Consistent chromatographic behavior with respect to chromatographic parameters (retardation properties, selectivity, separation efficiency). 

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