The retention factor (k) has a great effect on the efficiency in open-tubular chromatography.
This is illustrated for a 250-µm i.d. open-tubular GC column below. The figure shows actual H vs. u (“van Deemter”) curves, calculated using the Golay equation (Section 1.7.7) with df = 0.3 and Dm = 5×10-6 m2/s. The legend indicates the value of the retention factor.
Two factors are mainly responsible for the width of a peak as it appears in the chromatogram, i.e. the plate count and the speed at which is elutes from the column. If the plate count is constant and identical for each peak, the peak width in an isothermal gas chromatogram increases linearly with the retention time (tR) or with 1+k. If at the same time the plate height increases with k, so that the plate count decreases, this effect is aggravated. As a result, isothermal open-tubular GC (or isocratic open-tubular liquid chromatography) is quite unattractive.
The common remedy is to use programmed analysis. Almost all open-tubular GC separations are conducted in temperature-programmed mode. This has the result that analytes stay at the top of the column when their volatility is very low and start moving more and more rapidly as the temperature increases. They travel through most of the column with a low k value and the speed at which they elute is determined by the k value at the moment of elution (ke), which is similar for all analytes in a linear temperature program.
For more on band broadening in open-tubular columns see section 1.7.7.
For more on temperature programming in GC see Chapter 2.
