Effect of an open tube in series with a packed capillary column on liquid chromatographic performance. The influence of particle diameter, temperature, and system pressure.

A postcolumn reactor or a simple open tube connecting a capillary column to, for example, a mass spectrometer affects the performance of a capillary liquid chromatography system in two ways: stealing pressure from the column and adding band-spreading. This effect is especially intolerable in fast separations. Our calculations show that in the presence of a 25 microm radius postcolumn reactor, column (50 microm radius) efficiency (number of theoretical plates) is severely reduced by more than 75% with a t(0) of 10s and a particle diameter from 1 to 5 microm for unretained solutes at room temperature. Therefore, it is necessary to minimize the reactor's effect and to improve the column efficiency by optimizing postcolumn conditions. We derived an equation that defines the observed number of theoretical plates (N(obs)) taking into account the two effects stated above, which is a function of the maximum pressure P(m), the particle diameter d(p), the reactor radius a(r), the column radius a(c), the desired dead time t(0), the column temperature T and zone capacity factor k''. Poppe plots were obtained by calculations using this equation. The results show that for a t(0) shorter than 18s, a P(m) of 4000 psi, and a d(p) of 1.7 microm, a 5 microm radius reactor has to be used. Such a small reactor is difficult to fabricate. Fortunately, high temperature helps to minimize the reactor effect so that reactors with manageable radius (larger than 12.5 microm) can be used in many practical conditions. Furthermore, solute retention diminishes the influence of a postcolumn reactor. Thus, a 12.5 microm reactor supersedes a 5 microm reactor for retained solutes even at a t(0) of 5s (k''>3.8, or k'>2.0).