Final solvent strength

Key Gradient Parameters (Initial and Final Solvent Strength, Gradient Time (tG), Flow Rate)... 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

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. 

The choice of solvent directly influences the retention of the analyte on the sorbent and its subsequent elution, whereas the solvent polarity determines the solvent strength (or ability to elute the analyte from the sorbent in a smaller volume than a weaker solvent). Dean [272] gave solvent strengths for normal- and reversed-phase sorbents. The elution solvent should be one in which the analytes are soluble and should ideally be compatible with the final analysis technique. For example, for HPLC analysis, a solvent similar to the mobile phase is a good choice of elution solvent. For the elution step it is also important to consider the volume of the solvent. A minimum volume of elution solvent (typically 250 xL per 100 mg of sorbent) allows maximum concentration of the analytes. 

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. 

The point at which the sample is spotted can be regarded as the origin of a coordinate system (9). The process of development is performed in two steps the first in the direction of the x-axis to a distance Lz. After evaporation of the solvents used, the second development will be performed in the direction of the y-axis to a distance Ly. The positions of the compounds after development in the x-direction depend on the ST and Sy values of the first mobile phase being applied. Similarly, the migration distances of the individual compounds also depend on the total solvent strength and total selectivity of the second mobile phase. After development in the x-direction, the ordinates of all compounds are zero. After development in the y-direction, their abscissa values follow from their positions on the x-axis after the first development. The final positions of the spots are thus determined by the coordinates x(i) and y(t ), which can be expressed as follows ... 

As in analytical liquid chromatography (LC), analyte retention depends on sample concentration, solvent strength, and sorbent characteristics. An empirical approach to methods development initially involves screening the available sorbents. The first step is to determine which sorbents best retain the analyte. The second consideration is to evaluate the solvents needed to elute the compound and the compatibility of those sorbents to the chromatographic testing procedure. The third step is to test the blank sample matrix to evaluate the presence of possible interferents. Finally, recoveries of known quantities of analyte added to the sample matrix must be determined. 

The final set of solvatochromic data are shown in Figure 6 for phenol blue in SF C02 doped with various amounts of the co-solvent or entrainer, methanol. Consider a pressure of 100 bar where the Et of phenol blue in C02 is 54 kcal/mol. The red shift is increased more by the addition of 3.5 mole percent methanol at constant pressure than by an increase in the pressure of pure C02 of over 200 bar. The large specific "solvent strength" of methanol causes this behavior. The red shift caused by the co-solvent is in... 

Finally, we will summarize here all the correlational equations and experimental solvent parameters required for predictions of solvent strength and selectivity in LSC, and discuss their significance in terms of mobile-phase optimization strategies. 

When the sum of the concentrations of the two polar solvents 1 and 2, (py, is constant but their ratio is variable, the final elution strength in ternary normal-phase mobile phases is affected much less than in mobile phases where [Pg.60]

The final application of solvatochromic solvent strength scales is the correlation of reaction rate and equilibrium constants in SCF solvents. Solvatochromic scales are often quantitative indicators of the solvent effect on rate constants for a variety of reaction mechanismsU) In a SCF, this solvent effect may be achieved conveniently with a single solvent using pressure. Based on solvatochromic data, it was predicted that an activation volume can reach thousands of mL/mol in a SCF(8). This prediction was confirmed for various types of reactionsClSbZl). For example, the solvatochromic parameter Ex for phenol blue... 

This step is used for the final optimization of solvent strength. It corresponds to a vertical shift in the regular part of the prism, starting from the optimal selectivity point (Ps) established in step 2. If all products of interest are sufficiently separated, the solvent strength can be increased or decreased to reach the desired goal. 

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