Effect of aliphatic alcohols as mobile phase modifiers on separation of phenylurea and phenoxyalkyl acid herbicides by micellar electrokinetic capillary chromatography

The rapid and single separation of a mixture of neutral and ionic herbicides, five phenylurea (diuron, fenuron, linuron, monolinuron and monuron) and four phenoxyalkyl acids (dichlorprop, 2,4-D, 2,4,5-T and 2,4-DB) was achieved by micellar electrokinetic capillary chromatography (MECC) after different aliphatic alcohols (1-heptanol, 1-hexanol, 1-pentanol, 1-butanol and methanol) were added to a phosphate buffer (0.02 M) containing 0.05 M sodium dodecyl sulphate (SDS) as surfactant. The influence of the alcohol chain length and concentration on some important parameters related with the resolution, such as the capacity factor (k') and the elution range (t(eo)/t(mc)) has been studied. For k' calculation, Terabe's equation has been used for the neutral compounds, while for the ionic herbicides a new expression has been derived. Experimental results show an improvement on resolution for anionic herbicides when aliphatic alcohols are present, while separation of neutral compounds seems to be independent of the mobile phase modifiers. Furthermore, it has been experimentally found that the optimal alcohol concentration decreases as chain length increases. These results are explained in terms of the variation of the aqueous phase polarity (P), the micelle surface charge density (rho) and the micellar volume (V-mc).