Description of Adsorption in Liquid Chromatography under Nonideal Conditions

Abstract

A thermodynamically consistent description of binary adsorption in reversed-phase chromatography is presented, accounting for thermodynamic nonidealities in the liquid and adsorbed phases. The investigated system involves the adsorbent Zorbax 300SB-C18, as well as phenetole and 4- tert-butylphenol as solutes and methanol and water as inert components forming the eluent. The description is based on adsorption isotherms, which are a function of the liquid-phase activities, to account for nonidealities in the liquid phase. Liquid-phase activities are calculated with a UNIQUAC model established in this work, based on experimental phase equilibrium data. The binary interaction in the adsorbed phase is described by the adsorbed solution theory, assuming an ideal (ideal adsorbed solution theory) or real (real adsorbed solution theory) adsorbed phase. Implementation of the established adsorption model in a chromatographic code achieves a quantitative description of experimental elution profiles, with feed compositions exploiting the entire miscible region, and involving a broad range of different eluent compositions (methanol/water). The quantitative agreement of the model and experimental data serves as a confirmation of the underlying physical (thermodynamic) concepts and of their applicability to a broad range of operating conditions.

Signal convolution indicates chromatographic pulse flow and open-end flow

“…Recent developments in chromatography include analysis of chiral compounds [2], high-speed analysis [3][4][5], microanalysis [6][7][8], nano analysis [9] and lab-on-a chip [10]. The optimization and understanding of chromatographic systems rely on two theories or combinations thereof: adsorption theory [11,12] and plate theory [13,14]. The retention and separation of chemical species is understood in terms of adsorption and partitioning [13].…”

“…The optimum conditions of peak shape with respect to chromatographic separation are described by the Van Deemter equation [23], which is widely used for purposes of optimization [35]. Very large injection volumes produce long segments when injected into narrow pipes, and the resulting 'square distribution' of the concentration of solute molecules is pulse-like [11,25,36] rather than Gaussian shaped [11,37]. The chromatographic peak is observed not as a square signal, but as a signal that is smoothed by the response function of the detector, extra-column band broadening [38,39] and column-only band broadening [40].…”

Evaluation of the van Deemter equation in terms of open‐ended flow to chromatography

“…Although the theory is close to the experimental results in Figure 7, it should be emphasized that small changes in the parameters lead to relatively large changes in the curve owing to the exponential function that enters the expression in Eq. (11). Parameters A = 7.3 × 10 −6 m, B = 9.1 × 10 −7 m 2 /s, and C = 7.5 × 10 −6 s in the present description (Eq.…”

“…Thus, by considering Eqs. (11)(12)(13), where the second-order term in Eq. 11is ignored, the constants A, B, and C may be given an alternative meaning compared to conventional interpretations.…”

“…During flow, both mobile phase [8] and solute molecules [9] may be trapped within pores of the stationary phase [10]. It is still debated whether adsorption theory [11,12], partition theory [1,3,13,14], or combinations thereof [15] should be used to model chromatographic peaks; nevertheless, all of these theories [16][17][18] give rise to Gaussian peak shapes. However, this is rarely (or never) observed in practice [19][20][21].…”