For a typical narrow bore (50 pm) and wide bore (320 Fm) capillary column the effects of increased stationary phase film thickness (df) on the minimum detectable amount, Q, , as well as on the minimum analyte concentration, C , , are described. In treating the effect of an increased film thickness, two approaches can be followed: either the separation temperature is kept constant, resulting in larger values of the capacity ratio, k, or the column temperature is increased such as to keep k constant. For normalized chromatographic conditions the effects of both approaches on the minimum plate height, optimum carrier gas velocity, and required plate number are described, finally yielding expressions for Q, and C, for both mass flow and concentration sensitive detectors. At constant temperature, C , always increaseswith thefilm thickness for mass flow sensitive detectors (e.g. FID). Wide bore thin film columns offer the lowest value of C, attainable. For concentration sensitive detectors (e-g. TCD), C, is affected neither by column diameter nor by film thickness. The Q,-df plot for constant temperature shows a minimum, suggesting an optimum film thickness for mass flow sensitive as well as concentration sensitive detectors. The corresponding capacity ratio has a value between 0.5 and 1.5. At elevated temperatures (k constant) in combination with mass flow sensitive detectors, again an optimum film thickness exists, corresponding to a minimum value of C, .
For constant capacity ratio 0, always increases with the film thickness for both types of detectors. As indicated above, in some situations the lowest values of C , and Q, are obtained at an increased film thickness, the effect being marginal. As an initial guideline, for the daily practice of capillary gas chromatography with respect to minimum values of C , and Q, , the use of thin film columns is to be preferred. 'I For part I, describing the effect of the column diameter, see reference [9].