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 thermal expansion of the liquid in the capillary (due to ohmic heating) is more rapid than the rate of electro-migration of the slowest moving analyte into the column. We show that the limiting condition for avoidance of this undesirable effect requires that the ramp-up rate for the 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 power loss in the capillary is around 3 W m-1 (e.g. with a field of 30,000 V m-1 and a current of 100 muA). A detailed mathematical analysis which takes full account of the thermal dependence of key variables, indicates that thermal explosion will occur at fields above a critical value (Eq. (21)). We recommend that commercial CES instrumentation incorporates manual or software led ramp-up voltage control.