Solute capacity factor phase additive

The adjusted retention time fj represents the additional time required for the solute to travel the length of the column. It therefore corresponds to the time spent by the solute in the stationary phase. The ratio of the time spent by the solute in the stationary phase to that spent in the mobile phase is called the solute capacity factor k ... 

Experimental Verification. The changes of capacity factor values k with (3-CD concentration in the mobile phase solution are illustrated in Figure 1 as the behavior of methylphenobarbital enantiomers on RP-18 columns (26). The similar influence of CD on k values was observed for all the studied compounds ( disubstituted benzenes, mandelic acid and its derivatives, some aromatic aminoacids, some barbiturates and hydantoins)(17-19,21-26,28). o(- or -CD additions were always followed by a decrease in the apparent capacity factor (k ) values. These results suggest that the adsorption... 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

Low-molecular-weight carboxylic acids are difficult to extract from aqueous solutions using SPME techniques. Formic through butyric acids are miscible in water, and even caprylic acid (Cs) is soluble to the extent of 68 mg/100 g (7). The low capacity factors of carboxylic acids to nonpolar phases used in capillary gc columns lead to severe fronting of acid peaks, which often can be used to identify their presence in mixtures with other flavor compounds. The same phenomenon also has an effect on the absorption of acids by SPME phases. It is possible to enhance their extraction by SPME fibers, however. Figure 3 shows the relative extraction efficiencies for several carboxylic acids, each at a concentration of 10 ppm in water. The results show the effectiveness of headspace extractions using both the 85- a,m polyacrylate and lOO-pm polydimethylsiloxane fibers alone, and after the addition of 25% NaCl to the solutions. With the excep-... 

A comprehensive overview of preconcentration techniques for uranium (VI) and thorium (IV) prior to analysis was published (Prasada Rao et al. 2006). The multitude of off-line techniques that were reviewed includes liquid-liquid extraction, liquid membranes, ion exchange, extraction chromatography, flotation, absorptive electrochemical accumulation, solid-phase extraction (SPE), and ion imprinting polymers. In addition, online preconcentration methods for uranium, thorium, and mixtures of the two are also briefly surveyed. This overview includes over 100 references and is a good source for finding a suitable preconcentration technique with regard to the enrichment factor, retention and sorption capacity, method validation, and types of real samples. The review article focused on samples in which the uranium was already in solution so that digestion procedures for solid samples were not discussed (Prasada Rao et al. 2006). 

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