Extra-column tailing

Extra-column volumes should be kept as low as possible because they affect the separation. They are one cause of band broadening and of tailing. Their contribution to band broadening is additive. If each part adds 5% to peak width the decrease in separation performance is not negligible at all. Whereas it can be difficult to modify the instrument with regard to injector and detector, it is possible to use adequate tubing to connect the parts. Adequate means capillaries with an inner diameter of 0.17 or... 

Tailing may also arise even if the unavoidable extra-column volumes are kept to a minimum and the sample size is below l J,gg In this instance, the chosen chromatographic system of mobile and stationary phase is unsuitable and does not fit the sample this is known as chemical or thermodynamic tailing. For examples see also Figures 9.9 and 10.10. 

Low programming temperature rates of the column can result in a widening of the chromatographic peak, which in addition can show a typical asymmetry (tailing peak) derived from extra column effects and from active points in the column. On the other hand, peak front tailing can originate from overloading or from too fast carrier gas velocities. 

For some applications mixed-bed columns are combined with indivi-dual-pore-size columns for optimum utility. Using two mixed-bed columns plus a 500 A provides extra pore volume at the low-molecular-weight tail of the distribution and ensures that all polymer chains will be well resolved from any low-molecular-weight additive or impurity peaks that may appear at the end of the chromatogram. 

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