Reversed phase partitioning mechanism

Is the reversed-phase (partition) mechanism possible for atrazine ... 

POLYAMIDE. Separations are based on formation of hydrogen bonds between the functional groups of the sample (phenols, amino acid derivatives, heterocyclic nitrogen compounds, carboxylic and sulfonic acids) and the carbonyl oxygen of the amide group, as well as a general reversed-phase partition mechanism (water is the weakest solvent). Available as two polycondensation products, polyundecanamide (PA 11) and polycaprolactam (PA 11). 

Successful fractionations of a series of mal todex tr i ns (up to DP 23), cyclodextrins and isomaltodexrins have been achieved using RP columns (50,51), of which water was an eluant. For these particular examples, a reverse phase partition mechanism... 

The plates were Diphenyl-RP-F2S4, like before. The retention mechanisms observed were adsorption with lipophilic solvents and reversed-phase partition mechanism with alcohols (see Table 29). 

The structure of silica gel tends to change with time and this creates problems of irreproducibility in the separations. To remedy this situation and reduce the gel s polarity, the reactivity of silanol groups can be used to covalently bind organic molecules. Bonded stationary phases behave like liquids. However, the separation mechanism now depends on the partition coefficient instead of adsorption (Fig. 3.9). Bonded phases, whose polarity can be easily adjusted, constitute the basis of reversed phase partition chromatography, which is used in the majority of analyses by HPLC. 

Unexpected pH dependencies were explained by (a) competition between negative analyte ions and OH ions for interaction with the electrical double layer and (b) a mixed retention mechanism in which reverse-phase partition or interaction with unreacted silanols from the stationary-phase base may play a significant role. 

There are a number of modes or mechanisms into which chromatography is divided. These include adsorption, normal-phase partition, reversed-phase partition, and ion exchange. Often, the term partition is deleted from the discussions of the differences and similarities of these modes. The word partition initially arose when supports had to be coated with a liquid phase (and the mobile phases saturated with them) to accomplish separations with these two modes. Today, bonded-phase versions of these liquid phases are available, making them easier to use with greater reproducibility. Perhaps it has been the use of these bonded supports that have enabled the name of the mode to be simplified. 

The chromatogram in Fig. 2.5 demonstrates that the retention of (3), (5) and (6) is governed by reversed-phase partitioning processes, whereas for (2) and (4), both reverse-phase and cation-exchange mechanism are operating. Again, this method was validated to be precise, accurate, specific and robust. 

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

Two types of system are used for ion-pair liquid chromatography. When polar stationary phase materials, such as silica gel, are used an ion-pair partition mechanism is applied. When non-polar stationary phase materials, such as octadecyl-bonded silica gel and polystyrene gel, are employed a paired-ion adsorption mechanism is involved. The former is called normal-phase ion-pair partition liquid chromatography, and the latter is called reversed-phase ion-pair liquid chromatography. 

CSPs has, overall, a hydrophobic character (very similar to RP phases with C4-C8 ligands) which stems from contributions of the chiral selectors itself and (capped) linker groups (only a portion of the linkers are utilized for selector attachment) which constitutes a kind of hydrophobic basic layer on the support surface. Hence under typical RP-conditions, hydrophobic interactions between lipophilic residues of the solute and hydrophobic patches of the sorbent may be active and thus a reversed-phase like partition mechanism may be superimposed upon the primary ion-exchange process k = A rp -I- A ix). This A Rp-retention contribution may be especially important for eluents with high aqueous content. 

Sentell, K.B. and Dorsey, J.G., Retention mechanisms in reversed-phase liquid chromatography. Stationary-phase honding density and partitioning, Anal. Chem., 61, 930, 1989. 

LC Tan, PW Carr. Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography. II. Partitioning vs. adsorption mechanism in monomeric alkyl bonded phase supports. J Chromat A 775 1-12, 1997. 

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