Solvent compressibility

Reciprocating-piston pumps deliver a constant flow at si fixed backpressure. At high pressures some minor flow variability ziay arise due to the compressibility of the mobile phase. Soms instruments incorporate a flow controller which provides a fixadi backpressure for the pump to work against, independent of the column backpressure. The influence of pressure fluctuations, solvent compressibility, and solvent viscosity on the volumetrie output of the pump are thereby eliminated. Reciprocating-piston pumps can provide continuous solvent delivery, fast solvent change--... 

In high pressure mixing devices each solvent is piunped separately in the proportions required by the gradient into a mixing chamber before being delivered to the column. Solvent compressibility and thermodynamic volume changes on mixing may Influence the accuracy of the composition delivered to the column. 

Figure 6. X-ray fiber diagram of poly(5-methyl-1,4-hexadiene) prepared with a Etj,AlCl/S-TiCl, catalyst at 0°C in pentane solvent. Compression molded sample cold drawn to four times its original length.

SCRF equations governed according to the experimentally measured solvent compressibility, Luo and Tucker (1995) have been able to model these effects efficiently. 

When the mobile phase has a fixed composition (isocratic mode) a single pump is sufficient. However, if the composition of the mobile phase has to vary with time, as in a gradient elution, one of the methods described below can be chosen. The instrument must compensate for differences in solvent compressibility in order to attain the desired composition at a given pressure. 

Our spectroscopic studies of BN in mixture B and in hexane support our contention that ground state conformers are forced by cholesteric mesophases toward extremes of 0 (i.e., closer to 0° or 180° than in hexane solvent). As the two naphthyl groups become more coplanar, their u-overlap increases. Consequently, the 0-0 transitions in absorption (and excitation) occur at longer wavelengths (lower energies) (43). For the same reasons, the cholesteric solvent compresses excited singlets of BN, causing their fluorescence spectra to be red-shifted with respect to those in hexane. 

Local solvent compression. The next application of the solvato-chromic data will be to determine the magnitude of the local compression of a supercritical fluid solvent in the immediate environment of the solute. The of a dye such as phenol blue can be predicted in liquids where no specific interactions are present by treating the solvent as a homogeneous polarizable dielectric (22,29). The intrinsic "solvent strength", E, °, describes dispersion, Induction, and dipole-dipole forces and is given by (22). 

Apparent hydration numbers have been derived from experimental measurements assuming the formation of a hydration complex studied as a chemical reaction. xhe change of volume for the reaction is calculated from an equation of state which includes variation of the dielectric constant based on the solvent isothermal compressibility, while the bare ion and the complex are assumed spherical with crystallographic and Stokes-Einstein radii respectively. The latter radius is obtained from conductance measurements. Due to these assumptions, the apparent hydration numbers increase when temperature increases and diverge near the critical point due to the divergence of the solvent compressibility. Furthermore, negative values are obtained when the Stokes-Einstein radius for the complex is smaller than the crystallographic radius. 

There have been a number of modeling efforts that employ the concept of clustering in supercritical fluid solutions. Debenedetti (22) has used a fluctuation analysis to estimate what might be described as a cluster size or aggregation number from the solute infinite dilution partial molar volumes. These calculations indicate the possible formation of very large clusters in the region of highest solvent compressibility, which is near the critical point. Recently, Lee and coworkers have calculated pair correlation functions of solutes in supercritical fluid solutions ( ). Their results are also consistent with the cluster theory. 

Complex mixtures, i.e. those that contain 20 or more components, in most cases present separation problems. Under isocratic conditions (Figure 18.1)the initial peaks are likely to be poorly resolved and the final peaks will probably be broad and flat and may be swamped by background noise. If a weaker solvent is used, the initial peaks show improved resolution but the final ones are not eluted at all. A stronger solvent compresses the early peaks together more, so that some components can no longer be distinguished. 

Flash chromatography is a quick preparation technique that is, in effect, a hybrid between medium pressure and short column chromatography. It uses a short, fat column (e.g., 1-5 cm i.d. x 45 cm) packed with silica gel and filled with solvent. Compressed air is used to compress and remove the air from the solvent which then elutes quickly. The sample is then added and the column filled again. Pressure is adjusted to achieve a separation in 5-10 minutes. It is a fast and inexpensive method for the preparative separation of mixtures requiring only moderate resolution. Use of 40-63 pm sihca gel and a pressure driven flow rate of 2.0 in/min are essential for successful separation [30]. 

Due to solvent compressibility, [Q] increases with increasing pressure, and the appropriate correction factor usually amounts to a few percent, depending on the solvent, over the range 0.1-300 MPa. 

Other. A calibration for changes in solvent compressibility can be made manually by using a screw-driver. Basically, this system is not very different from those operating at constant pressure, with dual-head, special-drive pumps. The main difference is that in one instance the reference pressure is calculated as a function of the selected flow-rate while in the other the reference pressure itself is selected. 

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