Tubing, extracolumn effects

Considering a chromatographic process controlled by a partition equilibrium and neglecting extracolumn effects (i.e., band broadening caused by factors outside the column, e.g., tubings, detector etc.), several factors can contribute to the overall solute band broadening eddy diffusion, longitudinal diffusion, and resistance to mass transfer in mobile and stationary phase. 

It is important, first, to realize that efficiency is not a function solely of the column. Bad extracolumn parameters, such as detector cell volume or tubing diameters, can make the best column in the world look terrible. Second, efficiency measurements are very poor ways of comparing or purchasing columns unless all other parameters are constant. Many columns are bought and sold because they have a higher plate count than someone else s column. The efficiency calculations could have been made with different equations, on different compounds, on different machines, at different flow rates, all of which will have a profound effect on efficiency. The only valid use of plate counts that I have found is in column comparisons where all other variables are equal, or in following column aging over a period of days or months. 

In some injector types, worn seals or partially clogged tubing can cause excessive extracolumn band spreading. In other injectors, the material of construction can adsorb the sam[ffe. A measurement of the extracolumn band spreading and a comparison to previous results should help in the diagnosis of this cause. Consult tte manual of the manufacturer of the injector for specific effects and troubleshooting recommendations. 

HPLC detectors Like GC, HPLC has a wide variety of detectors, universal or specific, destructive or nondestructive, mass flow or concentration responsive, and with even more challenging requirements for interfacing to spectrometers in hyphenated techniques. Tubing of even smaller bore than described earlier in item 5 is necessary to connect the effluent end of the column to the detector to avoid extracolumn band-broadening effects. Pressures are lower, and high strength and density PEEK plastic may be used in place of stainless steel. 

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