ABSTRACT

II. PRESSURE-DRIVEN CHROMATOGRAPHY The use of miniaturized separation techniques, such as capillary electrophoresis, capillary electrochromatography, and capillary liquid chromatography, in analytical chemistry continues to attract much interest [1,2]. The major limitations in capillary liquid chromatography are the lack of reliable instrumentation for the delivery of micro flow rates and the excessive column pressure drop linked to the use of small particles in combination with small column dimensions. The applicability of high flow rates is limited by the backpressure that different parts of the chromatographic system (pump, injector, and column) can withstand. The pressure drop across a packed column can be approximated by Eq. (1) [3]:

AP = (|)Lr|u/dp2 (1) where ÁP is the pressure drop, (j) is the flow resistance factor, L is the column length, r\ is the mobile phase viscosity, and dp is the particle diameter. The maximum pressure of the pump in routine work is between 150 and 500 bar, although the use of higher pressures (up to 1400 bar) has been described [4]. The equipment capable of sustaining such high pressures is sophisticated and expensive.