Mixed mode mechanisms

For example, the ratio of the n-octanol/watcr distribution coefficient of the nondissociated species to that of the ionic species is nearly 10,000 for 3-methyl-2-nitrophenol, but only about 1000 for pentachlorophenol because of the greater significance of the hydrophobicity of the ionized form of pentachlorophenol. The logarithm of the -octanol/water distribution coefficient of pentachlorophenol as the phenolate is about 2 (determined at pH 12, and 0.1 M KC1), which indicates significant distribution of the ionized form into the n-octanol phase [8,37], Extraction of such highly hydrophobic ionogenic organic compounds can result from mixed-mode mechanisms that incorporate both the hydrophobic and ionic character of the compound. 

Zakaria and Brown (Zl) have found that, whereas nucleoside and base retention mechanisms can be adequately explained in terms of solvophobic considerations, nucleotide retention behavior can best be explained in terms of a mixed-mode mechanism. In an acidic mobile phase, it has been observed that ribonucleotides elute in order of increasing negative charge. This elution pattern is atypical for the reversed-phase... 

Occasionally, one inadvertently may use a mixed-mode mechanism in an SPE method. A good example is the sorption of triazine herbicides onto C-18 bonded phases that are monofunctional and are not endcapped. In this case, the basic compound has the potential for hydrogen bonding to the silanol sites of the silica gel, as well as reversed-phase sorption into the C-18 bonded phase. 

Uptake of water and polar solvents by the column From mixed mobile phases, polar solvent(s) are preferentially adsorbed on the surface of polar adsorbents, sometimes giving rise to multilayer solvent adsorption on the adsorbent support. In such a case, the retention is contributed to by a liquid-liquid partition mechanism between the adsorbed liquid layer and the bulk mobile phase, in addition to the adsorption. Such a mixed-mode mechanism can be intentionally utilized for separation of strongly polar or even ionic compounds. 

Antibodies, especially monoclonal antibodies, play an important role in many research, diagnostic and therapeutic applications. HPLC has proved to be a powerful technique for the analysis, separation, and purification of antibodies. A variety of chromatographic methods based on physicochemical separation mechanisms, biological affinity, and mixed-mode mechanisms are available to achieve successful antibody purification. Each method and each combination of methods has its particular strengths and weaknesses. 

Antibiotics have a wide diversity of chemical stmctures and range ia molecular weight from neat 100 to over 13,000. Most of the antibiotics fall iato broad stmcture families. Because of the wide diversity and complexity of chemical stmctures, a chemical classification scheme for all antibiotics has been difficult. The most comprehensive scheme may be found ia reference 12. Another method of classifyiag antibiotics is by mechanism of action (5). However, the modes of action of many antibiotics are stiU unknown and some have mixed modes of action. Usually within a stmcture family, the general mechanism of action is the same. For example, of the 3-lactams having antibacterial activity, all appear to inhibit bacterial cell wall biosynthesis. 

Hybrid (mixed mode) ventilation A system that makes use of a mix of natural and mechanical ventilation. May be further subdivided into seasonal hybrid, e.g., natural ventilation in summer and mechanical ventilation in winter or spatial hybrid, e.g., mechanical ventilation in core areas and natural ventilation at the perimeter. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

Ion exchange resins based on poly(styrene-divinylbenzene) backbones display mixed mode retention mechanisms. The ion exchange functionality (sulfonic acid or carboxylic acid for cation exchangers and quartemary or primary, secondary, or tertiary amines for anion exchangers) contributes to the ionic mechanism and the backbone polymer to hydrophobic retention. This is exemplified... 

Method development in SPE begins with an analysis of the sample matrix as well as the sample analytes. One of the most efficient mechanisms of SPE is to use mixed-mode ion exchange sorbents. If the analytes are bases, cation exchange is the most selective SPE mode. If the analytes are acids, anion exhange is the most efficient. However, it is sometimes easier to capture the matrix components and allow the analytes to pass through the sorbent unretained. 

Ferguson et al. [18] reported on the application of a mixed-mode HPLC separation, coupled with ESI-MS for the comprehensive analysis of NPEOs and nonylphenol (NP) concentrations and distributions in sediment and sewage samples. The mixed-mode separation, which operates with both size-exclusion and reversed-phase mechanisms, allows the resolution of NPEO ethoxymers prior to introduction to... 

Lerch, R.N., E.M. Thurman, and E.L. Kruger (1997). Mixed-mode sorption of hydroxylated atrazine degradation products to soil A mechanism for bound residue. Environ. Sci. Technol., 31 1539-1546. 

The TDSCF approximation is a good starting point for a mixed quantum mechanical/classical treatment. Let us assume that R is the classical and r the quantum mechanical mode. Then, the wavefunction r(r t) describing the vibration of the fragment molecule is a solution of the time-dependent Schrodinger equation... 

Note While the principal separation mechanism is ion exchange, the organic moieties on many of these phases can interact through nonpolar interactions as well. Thus, many phases are mixed mode. 

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