Chromatographic selectivity

Polyamide is available commercially as polyamide-6 (based on e-aminocapro-lactam, Macherey-Nagel) and as polyamide-11 (based on 11-aminoundecanoic acid, Merck). The lipophilic properties of these are different, thus altering their chromatographic selectivity. 

Chromatographic selectivity The degree to which compounds are separated on a particular chromatographic system. 

The most common method for varying the chromatographic selectivity for neutral molecules in RPC is to change the type of organic modifier in the mobile phase. In numerous studies using binary mobile phases, equation (4.15) has been shown to describe reasonably well the variation of solute retention with the volume fraction of organic solvent in an aqueous-organic mobile phase... 

Aqueous micellar solutions, i.e. solutions containing a surfactant at a concentration above its critical micelle concentration, have been studied extensively during the last decade, in part from curiosity, and because of the possibility of providing unique chromatographic selectivity compared to conventional RPC [345-349]. Above the critical micelle concentration individual surfactant molecules self-aggregate to form structures known as micelles which are microscopically... 

The final step of method development is validation of the HPLC method. Optimisation of chromatographic selectivity [110], performance verification testing of HPLC equipment [591], validation of computerised LC systems [592] and validation of analysis results using HPLC-PDA [34] were reported. The feasibility of automated validation of HPLC methods has been demonstrated [593]. Interlaboratory transfer of HPLC methods has been described [594]. 

PJ. Schoenmakers, Optimization of Chromatographic Selectivity A Guide to Method Development, Elsevier Science Publishers, Amsterdam (1986). 

In both cases, either conventional FTIR transmission or diffuse reflection detection may be used. Because TLC and the postspectroscopic evaluation are not linked directly, few compromises have to be made with regard to the choice of the solvent system employed for separation. Chromatographic selectivity and efficiency are not influenced by the needs of the detector. The TLC plate allows the separation to be made in a different site from the laboratory where the separated analytes are evaluated. The fact that the sample is static on the plate, rather than moving with the flow of a mobile phase, also puts less demand on the spectrometer. The popularity of TLC-IR derives in part from its low cost. 

Reversed-phase chromatography employs a nonpolar stationary phase and a polar aqueous-organic mobile phase. The stationary phase may be a nonpolar ligand, such as an alkyl hydrocarbon, bonded to a support matrix such as microparticulate silica, or it may be a microparticulate polymeric resin such as cross-linked polystyrene-divinylbenzene. The mobile phase is typically a binary mixture of a weak solvent, such as water or an aqueous buffer, and a strong solvent such as acetonitrile or a short-chain alcohol. Retention is modulated by changing the relative proportion of the weak and strong solvents. Additives may be incorporated into the mobile phase to modulate chromatographic selectivity, to suppress undesirable interactions of the analyte with the matrix, or to promote analyte solubility or stability. 

Yan, C. and Martire, D.E., Molecular theory of chromatographic selectivity enhancement for blocklike solutes in anisotropic stationary phases and its application. Anal Chem., 64, 1246, 1992. 

Specificity may be achieved through sample preparation, chromatographic selectivity, the selectivity of the detection method or combinations of these. It is often tempting to utilise the most selective detection method available (such as MS or MS/MS), since this can reduce the effort required in optimising the sample preparation and chromatography. However, whilst this is often the most expedient approach in early development, it may not always be suitable to transfer expensive, highly technical methods and instrumentation into a manufacturing environment if this is required. 

Uf course, the enhancement of chromatographic selectivity by secondary chemical equilibria is neither new nor confined to reversed-phase systems. Most widespread probably has been the exploitation of protonic equilibria by appropriately ati usting the pH of the eluent so that the degree of ionization of the eluite is altered. Generally the ionized and neutral forms of an eluite are retained differently (2( 7. 208). Formation of metal complexes of certain eluites has also been utilized for modulating retention behavior for higher selectivity. 

PJ. Schoeiunakers, Optimisation of Chromatographic Selectivity. Amsterdam, the Netherlands Elsevier, 1986. 

The equilibrium in Equation 15.40 leads to the following equation for the chromatographic selectivity coefficient [11,12] ... 

Achievement of Other Types of Chromatographic Selectivity of Peptide Separation and Purification... 

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