Solvent strength lowering

As we continue lowering the pressure, GC is the final limiting case when the mobile phase has zero solvent strength over the entire column length and where temperature is the only effective control parameter. Gas chromatography is shown in Figure 7.3. 

In principle, enhanced sensitivity can be achieved when the column focusing of the analyte is possible. The sample is preferably dissolved in a solvent with lower eluotropic strength compared to that of the mobile phase at the start of a chromatographic run. A solvent with... 

Density is a factor in the solvating power of a supercritical fluid the more dense the fluid, the more powerful its solvent strength. Since changing the temperature and pressure within the supercritical phase changes the density, a supercritical fluid can be made to possess a wide range of solvent power. This property together with its increased diffusion and lower viscosity makes supercritical fluid an attractive extraction medium. 

The influence of ionic strength on yCii is great in solvents of lower permittivities. When we compare ionic activities in different solvents, we have to consider this activity coefficient, in addition to the transfer activity coefficient yt. However, in reality, the influence of is usually negligibly small compared to that of yt. 

Doubled peaks or altered retention times (Figure 25-16) sometimes occur if the solvent in which sample is dissolved has much greater eluent strength than the mobile phase. Try to dissolve the sample in a solvent of lower eluent strength or in the mobile phase itself. 

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 E s of the nonpolar solvents, CF3CI and C2H4, become equal to tnat of n-hexane at a pressure in the range of 1-2 kilobar. Notice that the Hildebrand solubility parameters of these three solvents are roughly equivalent at this condition of constant E. The same result is also observed for the polarizabilities/ volume of these solvents. Again, the molar densities of these supercritical fluids are considerably higher than that of n-hexane at this equivalence point in solvent strength, since the polarizabilities/molecule are lower. 

According to eqn.(3.72) a higher value of ° will result in a lower value for the capacity factor. It can be concluded from table 3.4 that the solvent strength on silica and alumina stationary phases roughly increases with increasing polarity (6) of the solvent, but that there is no quantitative correlation between these two solvent properties. For example, ethers are much stronger solvents (especially on silica) than can be anticipated on the basis of their solubility parameters. 

Conversely, at lower temperatnres the hydrophobic effect entropically leads the adsorption of the solutes [12]. Actually the solvent strengths of aU mixtures change with temperature [13] and this influences selectivity. Also, the non-constancy of with changing temperature may also be due to the difference of the heat capacities of the analytes in the mobile and stationary phases according to the Kirkhoff equation... 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

Ideally, samples should be dissolved in the eluent prior to injection. However, if the sample has low solubility, it is sometimes necessary to use a more powerful solvent and in such cases only small injection volumes should be used otherwise broad peaks with shoulders can arise. Injection solvents of lower elution strength than the eluent can be useful for dilute samples, as the material concentrates on the top of the column during the injection. Neverftieless, lai e injections of any liquid other Hum eluent can cause serious detector distmlxmces. 

---