The effect of column characteristics on the minimum analyte concentration and the minimum detectable amount in capillary gas chromatography. Part II: the stationary phase film thickness

For a typical narrow bore (50 m) and wide bore (320 m) capillary column the effects of increased stationary phase film thickness (df) on the minimum detectable amount, Qo, as well as on the minimum analyte concentration, Co, 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 Qo and Co for both mass flow and concentration sensitive detectors. At constant temperature, Co always increases with the film thickness for mass flow sensitive detectors (e.g. FID). Wide bore thin film columns offer the lowest value of Co attainable. For concentration sensitive detectors (e.g. TCD), Co is affected neither by column diameter nor by film thickness. The Qo-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 Co. For constant capacity ratio Qo always increases with the film thickness for both types of detectors. As indicated above, in some situations the lowest values of Co and Qo 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 Co and Qo, the use of thin film columns is to be preferred.