Use of isopycnic plots in designing operations of supercritical fluid chromatography. III: Reason for the low column efficiency in the critical region

This paper discusses the origins of efficiency loss in supercritical fluid chromatography (SFC) when analyses are carried out in the low pressure supercritical region of carbon-dioxide, close to its critical point. Recent publications [1], [2] and [3] have shown strong evidence of radial thermal heterogeneity inside an SFC column and suggested that it leads to peak-shape distortion and greatly reduces the column efficiency. We demonstrate that the physico-chemical properties of CO2 close to the critical point are such that formation of thermal heterogeneity inside the column is highly probable and could cause the observed efficiency loss. Consideration of isopycnic plots of CO2 permits clear identification of the problematic region and explains why these properties of CO2 are primarily responsible for the often perplexing efficiency losses taking place during the SFC operations.

► We discuss the issue of efficiency loss in SFC in the near-critical region.
► Recent studies implicate the role of thermal heterogeneity inside the column for the efficiency loss.
► We show that at near-critcal region CO2 properties lead to sharper temperature drop per unit drop in density.
► Under the same condition the CO2 thermal diffusivity drops strongly as well.
► This condition, of higher cooling and lesser ability to equilibrate temperature, facilitates thermal heterogeneity, hence efficiency loss.