Is the general conclusion justified that higher applicable field strength results in shorter analysis time with organic solvents in CE?
In this paper, a widespread opinion in CE with organic solvents for the background electrolyte is critically questioned, namely that in general a shorter analysis time can be achieved due to the higher field strength applicable compared with aqueous electrolyte systems. This view, common in the literature, is based on the supposition that the conductance in organic solvents is lower than in water. Indeed in many organic solvents with higher viscosity than water lower ion mobility is observed, and higher fields can be applied in these cases. However, in this paper the problem is sharper defined and treated two-fold: (i) in all solvents conditions are such that either the same electric power is generated, or (ii) the same temperature increase is taken into account. It was shown that for the same electric power the field strength in the organic solvent can be changed to a less extent than the ionic mobility changes. As a result, the migration velocity of the analytes is lower and the analysis time is longer in most organic solvents compared with water; acetonitrile (MeCN) is an exception (in this solvent the mobilities are higher than in water). The more stringent treatment of the problem takes an equal temperature increase due to Joule heating into account rather than equal electric power. The temperature increase in the capillary depends on the thermal conductivity of the solvent, which is only about one-third of that of water for organic liquids. The consequence is that in none of the organic solvent systems a shorter analysis time can be achieved compared with water (given that the experimental conditions are comparable, e.g. zero EOF). The theoretical predictions were confirmed by measurements with water, methanol, propylenecarbonate, and MeCN as solvents.
Téllez
Adolfo
A
Departament de Química Analítica, Universitat de Barcelona, Spain.
Kenndler
Ernst
E
D000097:Acetonitriles; D001557:Benzenesulfonates / Q000302:isolation & purification; D004553:Electric Conductivity; D019075:Electrophoresis, Capillary / Q000379:methods*; D012997:Solvents; D014783:Viscosity; D014867:Water
D000097:Acetonitriles; D001557:Benzenesulfonates; D012997:Solvents; D014867:Water; C032159:acetonitrile