Capillary electrophoresis in N,N‐dimethylformamide

Abstract

N,N-Dimethylformamide (DMF) is a dipolar protophilic solvent with physicochemical properties that makes it suitable as solvent for capillary electrophoresis (CE). It is prerequisite for the proper application of CE to adjust and to change the pH of the background electrolyte (BGE) in a defined manner. This was done in the present work using benzoic acid-benzoate by selecting different concentration ratios of acid and salt, and calculating the theoretical pH from the activity-corrected Henderson-Hasselbalch equation. The mobilities of the analytes (chloro- and nitro-substituted phenolates) were found to follow reasonably well the typical sigmoid mobility versus pH curve as predicted by theory. The actual mobilities and pK(a) values (at 25 degrees C) of the analytes were derived from these curves. pK(a) values were in the range of 11.1-11.7, being thus 3-4.4 units higher than in water. This pK(a) shift is caused by the destabilization of the analyte anion and the better stability (solubility) of the molecular analyte acid in DMF, which overcome the higher basicity of DMF compared to water. Absolute mobilities were calculated from the actual mobilities; they were between 32x10(-9) and 42x10(-9) m(2)/Vxs. Slight deviations of the measured mobilities from the theoretical mobility versus pH curve were discussed on the bases of ion pairing and heteroconjugation and homoconjugation of either buffer components or buffer components and analytes. Heteroconjugation was used as a mechanism for the electrically driven separation of neutral analyte molecules in a BGE where salicylate acted as complex forming ion. Rough estimation of the complexation constants for the phenolic analytes gave values in the range of 100-200 L/mol. Addition of water to the solvent decreased the effect of heteroconjugation, but it was still present up to the surprisingly high concentration of 20% water. Electrophoretically relevant parameters like ionic mobilities and pK(a) values, and conjugation and ion pairing are dependent on the water content of the solvent. The water uptake of DMF was measured when exposed to humidity of ambient air. The resulted behavior of the water uptake was found rather similar to that for acetonitrile and methanol.

Are the asserted advantages of organic solvents in capillary electrophoresis real? A critical discussion

“…Note that the discussion about selectivity does not include the effect of additional separation mechanisms, like heteroconjugation [24] and ion pair formation, on the selectivity coefficient. Heteroconjugation, e.g., can pronouncedly affect selectivity in organic solvents like ACN [49,[55][56][57][58], DMF [10], NM [13], and propylene carbonate [58].…”

“…Thus, the relative differences of the mobilities in a given solvent do not change so much, and differences in the actual mobilities are comparable to those of the absolute mobilities. Note, however, that the absolute variation of the mobility as function of the ionic strength can differ much stronger among the solvents (see, e.g., [10,13,14]). An example is given in Fig.…”

“…In some solvents, the low solubility of the ionic species (especially anionic ones) might even be a problem while selecting suitable BGE components. We were confronted with these limitations especially in ACN [53], DMF [10], and NM [13]. Accordingly, even if the increase in ionic strength would be advantageous in some occasions (see the discussion about the electromigration dispersion in Section 2.2.2.2), it is often not possible to further increase the ionic strength of the BGE in the organic solvents due to these solubility reasons.…”

“…Obviously, such data can be beneficial when planning the experimental setup, and is also valuable for the interpretation of the results obtained. In addition to MeOH and ACN other solvents, like ethanol (EtOH) [7], 1-propanol [8], propylene carbonate [9], N,N-dimethylformamide (DMF) [10], formamide (FA) [11], N,N-dimethylacetamide (DMA) [12], and nitromethane (NM) [13], have been applied to CE.…”

Electromigration of a heteroconjugated imidazole–acetate complex in ACN

“…Subsequent work in ACN has included heteroconjugation reactions of phenols with carboxylates [18], phenols and benzoic acids with acetates, bis(triflyl)amide and hexafluorophosphate [19], methyl-substituted phenols with various anions (including acetate, chloride and bromide) [20], and phenols with acetate and trifluoroacetate [21]. In addition, heteroconjugation of phenolic compounds with acetate, chloride, heptafluorobutanoate and trifluoroacetate has been investigated in propylene carbonate [21], benzoic acids with chloride in NM [22] and phenols with salicylate in DMF [23]. In most of these works heteroconjugation constants have been determined, too.…”