Resolution with solvent peaks

Typical normal-phase operations involved combinations of alcohols and hexane or heptane. In many cases, the addition of small amounts (< 0.1 %) of acid and/or base is necessary to improve peak efficiency and selectivity. Usually, the concentration of polar solvents such as alcohol determines the retention and selectivity (Fig. 2-18). Since flow rate has no impact on selectivity (see Fig. 2-11), the most productive flow rate was determined to be 2 mL miiT. Ethanol normally gives the best efficiency and resolution with reasonable back-pressures. It has been reported that halogenated solvents have also been used successfully on these stationary phases as well as acetonitrile, dioxane and methyl tert-butyl ether, or combinations of the these. The optimization parameters under three different mobile phase modes on glycopeptide CSPs are summarized in Table 2-7. 

The effect of altering the acetonitrile concentration was observed as a reduction in retention times as the solvent strength increased, peak height increased correspondingly. The resolution between the peaks was slightly reduced with increased solvent strength but this was insufficient to cause peak overlap and hence deterioration of quantitative results. 

To determine the effect of mobile-phase composition on the sorption behavior of TGs on reverse-phase columns, two mixtures were employed acetonitrile/ethanol (80 20) and aceto-nitrile/methanol (80 20). A very rapid analysis resulted, with excellent peak shape and adequate resolution, when ethanol was used as the secondary solvent. Substituting an equal amount of methanol for ethanol resulted in increased solute retention, poor detector response, and asymmetrical peaks. Methanol forms a monomolecular layer on octadecyl-derived silica, which may explain the increase in solute retention caused by methanol. Also, the use of methanol would... 

The existence of clathrate-like water structure adjacent to the hydrophobic surfaces of macromolecules is an attractive hypothesis. Models have been proposed which have received some support from thermodynamical arguments [808]. However, this concept has proved ineffective as a basis for the interpretation of the structure associated with the many electron density solvent peaks, separated by 2.8 A to 3.0 A, which are frequently observed on the Fourier X-ray maps close to the surface of a protein [809, 810], Recently, however, some local clathrate-like water has been observed in special cases in the high-resolution studies of the small plant protein, crambin [811], in a hydrated deoxydinucleoside-phosphate drug complex [812], in (Phe4Val6) antamanide hydrate [8131 and in an oligodeoxy-nucleotide duplex [814],... 

UPLC (uses smaller columns with smaller particle size and higher resolution and solvent pressure), which has recently been introduced, allows for better resolution of chromatographic peaks, shorter analysis time, and less solvent consumption. 

Figure 6-42 (a) Water suppression with excitation sculpting on 2-mM sucrose in 9 1 H2O/D2O. (b) The residual solvent peak has been eliminated by further processing. (Reproduced from T. D. W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, Pergamon Press, Amsterdam, 1999, p. 365.)... 

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