Nonaqueous Capillary Electrophoresis

Table 1 Classification of organic solvents according to their Br+nsted acid-base behaviour

Solvent designation

Relative acidity

Relative basicity

Examples

Neutral

+

+

MeOH, glycerol, phenol, tert, butyl alcohol

Amphiprotic

Protogenic

#

-

Sulfonic acid, formic acid, acetic acid

Protophilic

-

+

Liquid ammonia, FA, NMF

Dipolar protophilic

-

+

DMSO, DMF, tetrahydrofurane, 1,4-dioxan,

pyridine

Aprotic

Dipolar protophobic

-

-

MeCN, acetone, nitrobenzene, sulfolane, PC

Inert

-

-

Aliphatic hydrocarbons, benzene,

1,2-dichlorethane, tetrachloromethane

- indicates weaker and # indicates stronger acid or base than water. DMF, A/,A/-dimethylformamide; DMSO, dimethyl sulfoxide: FA, formamide; MeCN, acetonitrile; MeOH, methanol; NMF, N-methylformamide; PC, propylene carbonate. Solvents in italicare the ones that are preferred for NACE. Reproduced with permission from Tjcrnelund J and Hansen SH (1999) Journal of Biochemistry and Biophysical Methods 38: 139-153.

- indicates weaker and # indicates stronger acid or base than water. DMF, A/,A/-dimethylformamide; DMSO, dimethyl sulfoxide: FA, formamide; MeCN, acetonitrile; MeOH, methanol; NMF, N-methylformamide; PC, propylene carbonate. Solvents in italicare the ones that are preferred for NACE. Reproduced with permission from Tjcrnelund J and Hansen SH (1999) Journal of Biochemistry and Biophysical Methods 38: 139-153.

strong acids or bases are dissolved in water, they all show up with about the same acid or base strength. If the same acids or bases are dissolved in organic solvents they will exhibit very different protolytic behaviour depending on the degree of dissociation, which again depends on the solvent in question.

Important factors influencing the choice of organic solvent or solvent mixture for a given separation are volatility, the dissolving power towards suitable electrolytes, viscosity and dielectric constant, UV transparency and, last but not least, the effect on the separation selectivity of the system. Information on the viscosity and volatility, the auto protolysis constant, the dielectric constant at standard conditions and the UV transparency of the neat solvents may be found in the literature. In contrast, data on solvent mixtures and systematic studies of how to choose solvents and electrolytes in order to control the selectivity of the electrophoretic system are limited and thus the choice of separation media is still a matter of trial and error. Solvents may be classified according to their Br+nsted acid-base behaviour; a simplified version of this classification is shown in Table 1.

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