Plate Theory and Extensions - Vacancy Chromatography > Page 70
Vacancy chromatography is a unique method of chromatographic development that can provide both negative and positive peaks in the chromatogram. It is not in common use, despite it having certain characteristics that make it especially applicable to process monitoring. If a mobile phase that contains a solute at a given concentration is continually passed through a column until equilibrium is achieved, then the concentration of solute in the column eluent will be the same as that at the inlet.
Let a sample of pure mobile phase be placed on the first plate of a column. This will cause a fall in the solute concentration in the first plate which, mathematically, will represent the injection of a sample having a negative concentration. A negative concentration profile will pass through a column in the same way as a positive concentration profile and at the end of the column will be recorded as a negative peak. The negative concentration profile will be described by exactly the same elution equation and its retention volume will be identical to that for a positive concentration profile. The generation of negative peaks by injecting a sample of pure mobile phase into an equilibrated mobile phase containing solutes at constant and known concentrations that is understood by the term vacancy chromatography. This term was first introduced by Zhukhovitski and Turkel'taub (19), who used the technique for process monitoring with GC. Later, Reilley et al. (20) also explored its use in process monitoring using LC.