Preparation of small diameter thick film capillary columns for microcolumn separations

Open tubular columns of 25 and 50 μm i.d. have been coated with up to 2 μm thick layers of SE‐54 (lowest β‐value = 6.25). Filling of the columns and evaporation of the solvent were studied as a function of column length and time. Since the viscosity of the stationary phase solution increases exponentially with concentration, the filling process becomes time consuming and high filling pressures are required. The evaporation or coating speed is shown to be controlled by the resistance to mass transfer of the solvent vapor through the coated sections of the column. For 25 and 50 μm i.d. columns, experimental evaporation times deviate little from theoretical predictions. Almost no influence of the stationary phase on the coating speed was observed. The transition from mass transfer limited coating speed to heat transfer limited coating speed is illustrated using literature data.