Knox-Saleem kinetic performance limits in liquid chromatography—A contemporary tutorial

Abstract

The kinetic performance limits evaluated by Knox and Saleem in 1969 are reevaluated herein. Published 55 years ago, the original study did not address several key features of contemporary chromatography. The following features of chromatographic analyses were assumed in the source:

• •
  The static operations (isothermal isobaric GC, isocratic isothermal isobaric LC).
• •
  The columns packed with discrete particles.
• •
  The columns were optimized to deliver the smallest plate height.

Additionally, currently obsolete parameters and notations were used complicating comprehension of the original study.

In this tutorial focusing mostly on LC, the original Knox-Saleem study is extended to gradient elution LC, to the columns with arbitrary structure (open, packed, pillar array, monolithic, etc.) and to suboptimal operations – all expressed in contemporary notations. The study is based on previously published basic structure-independent equations of column kinetic performance.

Some conclusions of this tutorial are different from previous ones. It has been concluded herein that at any pressure (no matter how low) any separation performance (no matter how high) can be achieved as long as the analysis time is acceptable. This seems to contradict with Knox-Saleem statement suggesting that there is “the critical pressure below which a separation number S cannot be achieved however much time is available.” Similar statement was also previously known from Giddings (1962): “critical pressure, p_c, the inlet pressure below which a [required, LB] separation can never be obtained”.