Eluent/solvent mixture choice

In the choice of eluent/solvent mixtures, and in the adjustment of retention, the previously mentioned eluotropic series is valuable together with information on the solubilities of the solutes. To avoid loss of stationary phase it is important that the eluent is fully saturated with stationary phase and, furthermore, that the temperature of the system is well controlled. Equilibrium establishment for in situ coated columns often requires several hours of elution, for which reason the elaboration of separation methods by the use of this technique is time consuming. Typical examples of separations are shown in Figs. 4.4.4 and 5. Gradient elution for the separation of complex mixtures is very difficult to perform due to the necessity of saturating the eluent with stationary phase, and thus for this purpose a bonded phase system should be used. 

It is possible to obtain a desired elution strength by mixing two solvents. A choice of binary mixtures is shown in Fig. 9.4. If, for example, a solvent strength of 8 = 0.4 is needed, this can be obtained by the following binary eluents ... 

Depending upon the type of adsorbent and the final destination of the extract, various solvents or solvent mixtures are used to reextract pesticides from adsorbent cartridges. With Ci8, N-vinyl-pyrrolidone polymer, and PS-DVB materials, methanol, or acetonitrile is the eluent of choice when analyzing via LC instrumentation. With Cig cartridges and... 

SPE has several advantages over LLE no emulsions, less solvent use, improved recoveries, and chemically cleaner extracts. The choice of sorbents and selectivity of the organic solvent mixtures used as eluents give SPE considerably more versatility than LLE for the extraction of THC and its metabolites. 

From the theoretical viewpoint, acetonitrile is the most suitable solvent to study the correlation of retention times and log P values of analytes, since the dipole moment (2.44) is nearly equal to that of water (2.55) (Figure 4.4). The electron donor effect can therefore be eliminated, and the elution order is not changed on modification of the acetonitrile-water mixture ratio. The first choice of an eluent should therefore be an acetonitrile-water mixture for non-ionic compounds in reversed-phase liquid chromatography. Methanol, acetone, THF, or DMF can then be added to improve the resolution. 

If methanol is not an appropriate solvent, generally the next choice is acetonitrile. In Table 5-1, various concentrations of acetonitrile in water result in improved selectivity for the key pair mentioned above. The best mobile phase in this table is the 25/75 mixture of acetonitrile/water in which the selectivity for the key pair is a = 2.8/2.0 = 1.40. The retention for the methyltestosterone is long (16.5), but perhaps not too long considering the favorable selectivity in this eluent. Thus, by changing to a different organic solvent, a mobile phase could be found which gave enhanced selectivity. 

Chromofiltration methods are powerful and rapid but often require study for optimization. We have found, for example, that choice of the appropriate solvent for solution-based multiple parallel synthesis can occasionally result in reaction mixtures from which the desired product can be isolated in pure form by suitable choice of absorbent and concentration or evaporation from the eluent (139). However this is generally exceptional. One can sometimes doctor silica gel, for example, to enhance its use for these purposes. After amide formation, when the acid component is used in excess, adding 1% of sodium bicarbonate to silica gel and mixing thoroughly provides a convenient way to remove reaction debris so that filtration produces pure amide on evaporation (139). 

If the chromatographic eluent is a mixture of organic modifier and water and contains no additives such as involatile buffers/acids, then product recovery can be effected simply by removing solvent, e.g., by rotary evaporation. Alternatively, if the solvent volumes are large, the organic modifier can be removed by rotary evaporation and the product recovered by adsorption/elu-tion from a suitable adsorbent such as a reverse phase silica or an organic copolymer resin. This is also a method of choice where additives such as involatile buffers (phosphates, acetates) and acids (orthophosphoric) are present in the fraction and need to be removed. 

The choice of solvent is governed primarily by the polarity of the substance mixture to be separated the more polar the mixture, the more polar the solvent must be, so that it is a more powerful eluent. Table 166 contains a selection of the most commonly used and satisfactory solvents. Many others are mentioned in the text and in Table 170. 

The resin is extracted with 200 mL of refluxing solvent for at least 12 h. A 1 1 mixture of methanol and dichloromethane is an excellent eluent. However, the high concentration of organically bound chlorine makes it a poor choice, if the resin is to be used for analyses of chlorinated hydrocarbons. In that case the acetonitrile-water azeotrope (16.3% water) should be used despite lower recoveries of aliphatic hydrocarbons measured with surrogate standard additions. After cooling, extraction is resumed with fresh solvent for another 12 h period. 

Table 6.8 shows some adsorbents used to prepare the stationary phase in the chromatographic separation of carotenoids by TLC. The choice between them depends on the solvent or mixture of solvents to be used as eluent phase. The adsorbent layer is placed on the glass plate (normally 20 X 20 cm) as a slurry, with a thickness that is variable but small (0.2-0.7 mm). The adsorbent is allowed to air-dry and is activated in the oven at 110°C. The pigment extract is applied to the base of the plate, and the plate is put into a tank containing the eluent. Development is usually carried out upwards, and when complete, the band or bands of interest are selected, scraped off, and eluted from the silica with either diethyl ether (in the case of polar carotenoids) or acetone or ethanol (if the polarity is medium), and filtered to remove the sihca. 

---