Activity testing and surface characterization of pretreated fused-silica capillaries for gas chromatography : a new modification of existing intermediate column tests

A new modification of existing intermediate capillary column tests for uncoated columns is presented. A short (1 m), thick film (2 µm) precolumn is coupled to the column to be tested. The use of a short precolumn results in a rapid test. This makes it attractive in terms of time to inject the test compounds separately, thus excluding mutual influences, in diluted solutions spiked with n-alkanes as references. Moreover, the short precolumn gives sharp Gaussian injection profiles on the second column. A simple versatile coupling device has been developed, which permits easy control and adjustment of pressures and carrier flows, monitoring of the effluents of the precolumn and introduction of sharp input bands on the second column. The monitor detector accurately shows the shape and time of introduction of the compounds on the second column. This permits the evaluation of peak distortion, peak shift and yield on the second column. Test compounds were selected on the basis of their physico-chemical properties (vapour pressure, acid-base properties, dipole moments, hydrogen bonding, complexation with metals, etc.). A number of differently pretreated capillaries were tested with these compounds under various external conditions. Accurate quantification and qualification of the activity of the capillary present difficulties. The test is a case of nearly ideal non-linear chromatography. When peaks of different sizes are superimposed, the tails lie on a common envelope. The k' of the end of the tail is fixed, irrespective of the amount injected, and forms an important parameter in the evaluation. Reconstruction of the chromatogram with a k' scale instead of a time scale eliminates the influence of carrier velocity and column length. The method is a sensitive tool for intermediate column testing and shows promises for surface characterization.