Solvent strength, measurement

In this work correlations between mobile phase solvent strength and chromatographic retention of a number of different solute families will be presented. The first solvent strength measurements on ternary mobile phases will also be presented. Finally, a retention mechanism for packed column SFC is proposed. 

Figure 1. Solvent strength measured with two different solvatochromic dyes. On both scales solvent strength is a non-linear function of composition. Note that E q is more sensitive to hydrogen bonding than Ej. ...

Figure 3. Plot of the log of the retention time vs. solvent strength measured with Nile Red for 5 phenols at 50° and 138 bar (outlet). The percent methanol in carbon dioxide is indicated at top of figure. Column 4.6 x 200 mm, 5 /im Lichrosorb Diol. Flow 2.5 ml/min.

Figure 8. Solvent strength measurements as in Figure 7, except methylene chloride and acetonitrile replace methanol. With these lower polarity modifiers, the additive appears to be more polar.

The conceptual basis for understanding the connection between isocratic and gradient elution is well established and is called "linear solvent strength theory".22 27 Linear solvent strength theory proposes that, for a given solute, mobile phase, and column, if one measures the retention time of an analyte at two organic component concentrations, it will be possible to predict the retention time with any other mobile phase composition. The k value that would be observed in pure water, kw, is related to the actual k by the relationship... 

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

Solvent strength is a measure of the affinity of a solvent for the adsorbent. Complete lists are known as eluotropic series and will include mixtures of solvents as well as the pure solvents above. 

Solvent strength is n measure of the molecular polarity of a liquid. 

The regression coefficients of descriptors denote the system (combination of mobile and stationary phases) response to these interactions. These coefficients can be measured, however the procedure is time consuming and inappropriate for practical purposes. According to the linear solvent strength theory (LSST) the retention of the analyte depends on the volume fraction (cp) of the organic modifier in binary mobile phase systems ... 

Kauri-butanol number A measurement of solvent strength for hydrocarbon solvents the higher the Kauri-butanol (KB) value, the stronger the solvency the test method (ASTM D1133) is based on the principle that Kauri resin is readily soluble in butyl alcohol but not in hydrocarbon solvents and that the resin solution will tolerate only a certain amount of dilution and is reflected as cloudiness when the resin starts to come out of solution solvents such as toluene can be added in a greater amount (and thus have a higher KB value) than weaker solvents such as hexane. 

When an ion-pair recombines, it may form an excited state which can luminesce. The intensity of luminescence is a direct monitor of the competing ratio of recombination and luminescence. With steady-state conditions, the luminescence intensity is proportional to the rate of recombination. For instance, Morrow et al. [380] have radio lysed solutions of pyrene in cyclohexane. Solvated electrons and pyrene cations are produced. On recombination, an excited singlet state is produced which can fluoresce. If two pyrene molecules are in (or near) contact when one or other molecule is in the excited singlet state, then excimer fluorescence may be observed. The intensity of fluorescence can be decreased by application of an electric field, since fewer ion-pairs recombine to form the excited state. Jarnagin [381] and Holroyd and Russell [382] have photoionised iVjA iV. iV -tetramethyl-p-phenylenedia-mine (TMPD) with light (of photon energy 5.5—6 eV) in hydrocarbon solvents and measured the photocurrent at various electric field strengths. 

The Hildebrand solubility parameter 8, (total solubility parameter) is a rough measure of solvent strength ... 

Steady-State Solvatochromism. The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21-28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28). To quantify solvent strength, researchers (21-28) adopted the Kamlet-Taft x solvent polarity scale (50-55). This scale best correlates solvatochromic effects on a- x and x- x electronic absorption transitions. 

The bulk solvent strength of binary and ternary supercritical mixtures, measured with the solvatochromic dye Nile Red (E ),... 

Nile Red was recently introduced as a solvatochromic dye for studying supercritical fluids (10). Although not ideal, Nile Red does dissolve in both nonpolar and polar fluids and does not lose its color in the presence of acids, like some previously used dyes. Major criticisms of Nile Red include the fact that it measures several different aspects of "polarity" simultaneously (polarizability and acidity (15)) yet it is insensitive to bases (10). However, in chromatography other single dimension polarity scales, like P, are routinely used. Measurements with Nile Red and other dyes indicate that the solvent strength of binary supercritical fluids is often a non-linear function of composition (10-14). For example, small... 

Tertiary systems. With methanol/carbon dioxide mixtures the addition of even the most polar additives has only a small impact on the mobile phase solvent strength as measured with Nile Red. With TFA concentrations below 1 to 2 % in methanol, ternary mixtures of TFA/methanol/carbon dioxide produce the same apparent solvent strength as binary methanol/carbon dioxide mixtures. As much as 5 or 10 % TFA in methanol is required to noticeably increase the solvent strength of TFA/methanol/carbon dioxide mixtures above those for binary methanol/carbon dioxide mixtures, as shown in Figure 4. 

Measurements with Nile Red were ineffective in measuring the solvent strength of even pure bases (10). Measurements with tertiary systems containing less than 1 % t-butylammonium hydroxide in methanol showed no difference from methanol/carbon dioxide mixtures. 

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