Extraction phase processes are well covered in literature, but lack of attention to the mechanism of these systems deprives researchers of information needed to evaluate their current and future use.
A new paper written by Colin F. Poole in the Department of Chemistry at Wayne State University in Detroit, Michigan, USA, eschews a discussion of extraction processes in sample preparation in favor of examining extraction phase selectivity, hoping to shed more light on mechanistic details (1).
Poole’s work, published in the Journal of Chromatography A, suggests that other review articles focusing on specific properties of the extraction phase have covered processes extensively, but that the lack of attention given to mechanism does not give interested parties enough information to consider the phase’s current and future applications. He proposes the solvation parameter model, which generally characterizes biphasic extraction systems relative to their capability for solute-phase intermolecular interactions, including dispersion, dipole-type, and hydrogen bonding, and within phase solvent-solvent interactions for cavity formation, such as cohesion.
This approach, he said, compares liquid and solid extraction phases on equal footing, and explains the selectivity of target compounds using solvent extraction, liquid-liquid extraction (LLE), and solid-phase extraction (SPE) in a gas, solid, or liquid phase.
Defined generally, extraction refers to the isolation of target compounds as they transfer from one phase to another for further evaluation. Selectivity, in this case, is determined by the concentration of the target compounds in the extraction phase compared to that of the matrix, or compounds that do not contribute to the information being sought. Discussions of selectivity routinely tend toward the qualitative, which Poole said serves to provide an assessment, but not a ranking. He theorizes that several factors should be able to be explained: which methods are unlikely to be successful, comparing the extraction approaches of different techniques, optimizing working conditions, understanding sorption properties, and again, offering mechanistic details.
Using the solvation parameter model, Poole found that some problems in characterization of classic adsorbents, like carbon and inorganic oxides, were identified, while other extraction phases were able to be successfully described. In aqueous samples, selective extraction was controlled by the properties of water, limiting the modification of intermolecular interactions while non-aqueous samples provided a larger selectivity range. Water, Poole found, was more cohesive and hydrogen-bond acidic than some other commonly used extraction phases, strongly competitive in dipole-type interactions, and dispersion was the main extraction mechanism for as phase samples, for which low selectivity extraction phases are typically used, then optimized.
While still paling in comparison to the amount of literature available on extraction processes, this article at least starts the conversation about selectivity and the ways in which it can be evaluated.
(1) Poole, C.F. Selective activity of extraction systems. J. Chromatogr. A 2023, 1695, 463939. DOI: 10.1016/j.chroma.2023.463939