KINETIC OPTIMIZATION OF STRAIGHT OPEN-TUBULAR LIQUID-CHROMATOGRAPHY

A theoretical analysis of liquid chromatography in open-tubular columns along the lines of the treatment given by Knox and Saleem for packed columns indicates that for a given pressure drop, plate number, solute and eluent, the optimal diameter of the open tubular column is about one fifth of the diameter of the particles in the optimal packed column and the analysis time is about 100 times less. However, the calculated bores of capillaries are impracticably small for operation oat or near the minimum in h unless N, the number of theoretical plates required, exceeds several million. The practical limitation to capillary liquid chromatography undoubtedly arises from the dispersion produced by the detector. Accordingly, optimal conditions are derived for capillary LC in which the standard deviation of the unretained solute, σ°v, and pressure drop are taken as limiting conditions. It is shown that with these constraints the optimal column bore is independent of N while the analysis time increases directly with N. Comparison of the performance of capillary columns limited in this way, with packed high-performance liquid chromatographic (HPLC) columns operated under conditions of minimum h, shows that if σ°v 0.1 mm3 then capillaries of 30 μm bore are required and will give better performance than packed columns only if N > 300,000. However, if σ°v 0.001 mm3 then capillaries of 10 μm bore are required and are faster than packed columns whe N > 30,000. For example, the time, tm, for a peak of N 106 is 2 h with the capillary compared with 55 h with the packed column. Published experimental data are examined in the light of the Golay equation and the agreement is shown to be excellent for both unretained and retained solutes. It is concluded that if detector volumes can indeed be reduced to around 0.001 mm3 there are excellent prospects for capillary HPLC. © 1979.