Dimensionless plate height is unsuitable for comparing chromatographic columns

Abstract

Dimensionless plate height (h) is differently defined for the columns with different type of internal filling (internal support structure). It is h = H/d for open columns (d is the internal diameter), h = H/d_p for packed columns (d_p is the particle size), h = H/d_dom (d_dom is the domain size) for columns with other filling (e.g., monolithic, pillar array). The plate height (H) and the column kinetic performance in general depend on the flow channels through the column filling - on their cross-sectional dimensions (d_chan), shape, and wall porosity. Other than through their effect on the flow channels, the dimensions of skeleton of the filling and the d_dom do not affect H. Parameters d, d_p and d_dom cannot be found from external measurements of a column as a "black box" separation device and, therefore, are unmeasurable subjective metrics, and, therefore, so is h. Moreover, in some fillings (e.g., monolithic, pillar array) d_dom can be chosen independently of d_chan. By increasing d_dom without changing d_chan, one can make h as small as desired with no effect on H and on the column kinetic performance in general. This implies that h does not represent column performance and cannot be used for comparing differently structured column. In this report, previously introduced measurable (objective) performance metrics - the kinetic performance factor (q) and the quality factor (q_max) are described and evaluated. The latter is suitable for comparing differently structured columns. A column practically achievable performance depends not only on q_max, but on the largest pressure that a column can tolerate, the narrowest flow channels that can be manufactured and other practical factors. Practically achievable performance of several column types is compared in this report.

Keywords: Dimensionless plate height; Kinetic performance factor; Quality factor; Transport diameter; Transport efficiency.