Selectivity hydrophobic

It was pointed ouF that while ionomer-modified electrodes have greater selectivity than naked electrodes, ionomers provide only a general form of selectivity (charge type and mass or hydrophobicity selectivity), and special methods were suggested to improve specific selectivity. For example, it was thought that applying an additional cellulose acetate layer over the Nafion film would make the electrode selective with respect to dopamine specifically (in contrast to other neurotransmitters). To make the electrode system specifically selective to o-nitrophenol (in contrast to /(-isomer), a complexation with cyclodextrin was performed, which increased the selectivity ratio one order of magnitude with respect to the o-isomer. ... 

Where nucleic acids are concerned, the enhanced hydrophobicity of abiotic polyfluorinated aromatic bases (e.g., tetrafluorobenzene or tetrafluoroindole deoxyribose derivatives) was exploited as an alternative to natural hydrogen bonding to achieve selective and stable nucleic acid base pairing in duplex DNA [85], The DNA replication was examined using polyfluorinated-nucleotide analogs as substrates. A DNA polymerase active site was able to process the polyfluorinated base pairs more effectively than the analogous hydrocarbon pairs, demonstrating hydrophobic selectivity of polyfluorinated bases for other polyfluorinated bases [86]. 

The separation factor relies on the design of the active sites on the resins and the solution chemistry. Mechanisms contributing to chemical selectivity reportedly include the hard-soft acid-base principle (HSAB), hydrophilicity/hydrophobicity selectivity, and pore size recognition. These mechanisms have common characteristics and their effects are thus not easy to distinguish. 

Hydrophobicity Selection of medium for hydrophobic interaction chromatography... 

Figure 5. A schematic representation of the hydrophobic selection operated by lipid vesicles towards a library of peptides. The hydrophobic peptide(s) is selected out (bound) and in the presence of a membrane-bound condensing agent, can be polymerized on the membrane surface.

Ion-pair HPLC mode is a superposition of two competitive processes ion-exchange and reversed-phase. Component retention is strongly dependent on the type of ionpairing agent, its concentration, and most of all, on the history of the used column. The virgin reversed-phase (RP) column does show the hydrophobic selectivity in the ion-pair mode. However, with time, the adsorbent surface can become covered with a dense layer of adsorbed surfactant. This may irreversibly transform the RP column into an ion-exchange one. 

M.G. Khaledi, Hydrophobic Selectivity in Micellar and Hydro-Organic RPLC, Anal. Chem., 60 876 (1988). 

The properties that essentially influence the selectivity of the reversed phase are hydrophobicity, hydrophobic selectivity, silanophihc activity, shape selectivity, polar selectivity, and metal content. These properties are different for each different support and, therefore, essentially determine the separation. 

The hydrophobic selectivity is the methylene group selectivity of the stationary phase, that is, the selectivity of two compounds differing by a methylene group, which are retained predominantly due to their hydrophobicity. This selectivity is to be used, for example, in the case of neutral, hydrophobic analytes for the comparison of columns and assessment of their similarity, and has a much higher significance than the hydrophobicity treated in Section 4.4.1, and later sections. 

Analyte pairs that reveal the similarity of the phases with respect to selectivity for the current issues should be chosen. Assume that in one case the separation of both simple neutral molecules as well as aromatics is relevant see Figure 4.10. Ethylbenzene/fluorenone is a measure for the hydrophobic selectivity and chrysene/perylene for the aromatic selectivity. XTerra MS and AQUA as quite hydrophobic phases show good hydrophobic selectivity. 

When our analytes and our analytical goal are known, then we can select the column directed toward this aim. If, for example, we want to separate organic acids differing primarily in the chain length of the aliphatic rest, that is, in then-hydrophobic selectivity, then it is best to try a classical endcapped, hydrophobic reversed phase, because this provides the methylene selectivity necessary for this problem. 

Samples of boiled rice, com starch, and tapioca starch were analyzed in the search for a folate-free food matrix. These samples were treated with a-amylase in a water bath at 75°C for 1 h and then extracted with phosphate buffer (pH 6.1). Folates were then selectively concentrated using SPE on styrene divinylbenzene cartridges, which have a broad hydrophobic selectivity for both polar and apolar organics but exhibit enhanced interaction with aromatic compounds when compaied with traditional Cu reversed-phase sorbents. In addition, the assay has been used to measure the two folate levels in commercially produced bread samples from differait brands in Australia [101]. 

Of HPLC methods, RP-HPLC has been used most to analyze plant proteins. Its resolution equals or exceeds that of most methods it is also fast, reproducible, sensitive, quantifiable, and gives good recovery. Most important, however, it complements other methods as it fractionates proteins based on difleient surface hydrophobicities. Selection of optimal RP-HPLC columns requires consideration of many factors, including support type, hydrophobic ligand, pore size, particle size, column dimensions, and silanization [46,48]. 

Hydrophobic selectivity The ability of a phase to separate two apolar components. 

Undoubtedly, apolar is an flexible term, especially if two analytes are involved, which is true in the case of separation factors. We use in our work a total of six quite different pairs of analytes for the investigation of the hydrophobic selectivity of phases. Without going into too much detail, after a lot of measurements and chemometric analysis, it was established that the representation in Figs. 15, 2b and 16 represents the selectivity of phases quite well. Here, some commercially available phases are listed according to decreasing hydrophobic selectivity, or polar selectivity, respectively (Fig. 16). 

The hydrophobicity/hydrophobic selectivity also needs to be considered from another point of view. By means of the values in Table 3, the influence of the coverage on the separation behavior of different phases is shown, which are prepared using the same silica gel matrix. In Table 4, the retention and separation factors of several analytes are listed. In all cases, hydrophobic and polar versions of the phases are used. 

Information from Test 1 The graphical presentation of the ethylbenzene/ fluorenone separation factors shown in Figs. 2a, 2b and 15 highlights the hydrophobic characteristics of the phases. The phases in the left half of each figure are hydrophobic in character (more precisely they have good hydrophobic selectivity), whereas the phases in the right half could be described as polar. 

Fig. 19b. Selectivity chart 2 ( hydrophobic selectivity ) for comments, see text.

Hydrophobicity or hydrophobic selectivity 0-CH2 Retention factor between pentylbenzene and butylbenzene, = kp /k. This is a measure of the surface coverage of the phase as the selectivity between alkylbenzenes differentiated by one methylene group is dependent on the ligand density. 

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