Chromatography, general solvent selection

Solvent strength and selectivity are the properties commonly used to classify liquid stationary phases as selection tools for method development in gas chromatography [29,102-104]. Solvent strength and polarity are often used interchangeably and can cause confusion. Polarity is sometimes considered to be the capacity of a stationary phase for dipole-type interactions alone, while more generally solvent strength is defined as the capacity of a stationary phase for all possible intermolecular interactions. The latter definition is quite sensible but unworkable because there is no substance that is uniquely polar that might be used to probe the polarity of other substances. Indirect measurements of polarity, such as those scales related one way or another to the... 

Some broad generalizations can be made about the selection of preferred solvents for liquid chromatography. Suitable solvents have a low viscosity, are compatible with the detection system, are available in a pure form, are relatively inexpensive, and if possible, have low flammability and toxicity. In practice, any useful solvent must be able to dissolve the sample without reacting with it chemically. An abbreviated list of suitable solvents for liquid cbromatography and their physical properties is given in Table 4.13 [565-568]. Special grades of solvents are available for HPLC most are carefully purified... 

Several mobile-phase optimization strategies in TLC are based on the use of isoeluotropic solvents (i.e., solvent mixmres of identical strengths but different selectivities). Selecting mobile phase will be achieved based on the eluotropic series (Figs. 1 and 2). These considerations are very general. The selection and optimization of one system of eluent is a more complex problem and must be discussed for each particular system. For example, the separation of 13 phenylurea and s-triazine herbicides was performed by overpressured layer chromatography (OPLC) with a binary... 

Column chromatography is the most generally used method, and the use of several different solvents, or graded-concentration mixed solvents affords a considerable selectivity in many cases. The difficulty always remains, however, that an unidentified and unexpected species may accompany one of the carriers and give false results. This can be particularly misleading in cases where the various eluted fractions are not specifically identified, but are classified only by the polarity of the solvent. 

---