VOLUME EXPANSION AND LOSS OF SAMPLE DUE TO INITIAL SELF-HEATING IN CAPILLARY ELECTROSEPARATION (CES) SYSTEMS

The application of a high voltage, V, in capillary electroseparations following sample injection, can cause loss of analyte if the rate of t hermal expansion of the liquid in the capillary (due to ohmic heating) is more rapid than the rate of electro-migration of the slowest movin g analyte into the column. We show that the limiting condition for avo idance of this undesirable effect requires that the ramp-up rate for t he applied voltage is below a critical value. This critical (maximum) value is given to a good approximation by a simple formula (Eq. (30)). Limiting values of dV/dt are in the region of 1000 V s-1 when the pow er loss in the capillary is around 3 W m-1 (e.g. with a field of 30,00 0 V m-1 and a current of 100 muA). A detailed mathematical analysis wh ich takes full account of the thermal dependence of key variables, ind icates that thermal explosion will occur at fields above a critical va lue (Eq. (21)). We recommend that commercial CES instrumentation incor porates manual or software led ramp-up voltage control.