Extra Column Dispersion - Separations on Small Bore Columns 2m Long > Page 66

The column was 10 m long, 1 mm I.D. packed with Partisil Silica Gel 20 mm particle diameter. At the optimum flow rate (i.e., 10 ml/min.) the column gave a quarter of a million theoretical plates. However, the chromatogram shown in figure 34 was obtained at a flow rate of 38 ml/min. and, thus, as it was operated well above its optimum velocity, the column only gave an efficiency of 160,000 theoretical plates. As the chromatographic data was acquired and processed by a computer portions of the chromatogram could be expanded and these are shown as inserts in the figure. It is seen that the apparently confused peaks at the start of the chromatogram are, in fact, well resolved into individual small peaks. It is also seen that the late small peak has retained its symmetry and is almost perfectly Gaussian in shape.

Those familiar with cinnamon bark oil separated on GC capillary columns may wonder at the relatively few peaks that appear on the chromatogram. It should be pointed out that a UV detector was employed to monitor the separation and, thus, only, those substances that adsorb in the UV would be disclosed. As the majority of the substances in essential oils are UV transparent, only a limited number of the components will be detected. The example is given to illustrate the wide range of solutes that can be separated and that, providing adequate efficiency is available together with suitable apparatus, multi component mixtures can be separated by LC as well as GC.

Separations on Small Bore Columns 2m Long

Small bore column can have a fairly wide range of useful lengths and need not be 10 meters or more in length and have inordinately long elution times (usually many hours). Incidentally, it is shown (see Book 22) that high resolution in LC must always entail very long analysis times. High resolution and high column efficiencies are paid for in two currencies available inlet pressure and analysis time. This is because high efficiencies and high numbers of theoretical plates require both long columns and very small particles, both of which result in high flow impedance and, thus, demand the use of high inlet pressures. Unfortunately, the maximum available inlet pressure is limited by the design of the apparatus, in particular the sample valve that must be leak proof at the operating pressure of the chromatograph. This limits the pressure in contemporary chromatographs to an absolute maximum of about 10,000 p.s.i. and more often (due to the pressure limitations of the sample valve) to about 6000 p.s.i. Thus, if the pressure is limited, then to utilize longer columns the particle diameter must be increased to reduce the flow impedance and allow the longer column to be operated at the optimum mobile phase velocity. The use of larger particles to reduce flow impedance and thus permit the use longer column is possible because, at the optimum velocity, the inlet pressure decreases as the square of the particle diameter but the efficiency is only reduced approximately linearly with the particle diameter (thids is true for packed columns only). Thus, doubling the particle diameter allows the column length to be increased by a factor of four and as the plate height will be increased by a factor 2 the net result will be to double the number of theoretical plates.