Chemometric studies of polycyclic aromatic hydrocarbon shape selectivity in reversed-phase liquid chromatography

Abstract

The molecular shape recognition differences between monomeric and polymeric C18 stationary phases in the reversed-phase liquid chromatography (RPLC) separation of unsubstituted polycyclic aromatic hydrocarbons (PAHs) and methyl-substituted polycyclic aromatic hydrocarbons (MPAHs) are examined through the use of partial least squares (PLS) analysis techniques. The resulting PLS models are able to describe the enhanced shape selectivity of the polymeric phase for recognizing subtle structural differences among planar and nonplanar isomers. PLS component analyses of these models reveal that spatial and topological descriptors are primarily used to rank structural differences among the PAHs (i.e., fused-ring patterns, molecular length and breadth) that control such shape-selective chromatographic processes. This is consistent with the view that polymeric alkyl chain stationary phases contain size- and shape-specific "slots" that promote the separation of structurally-related solutes. In contrast, the monomeric phase model is limited in resolving both the isomer classes and the nonplanarity shape differences among the PAHs. However, an improvement of shape recognition on the monomeric phase was elucidated by the PLS model for two PAHs (phenanthro[3,4-c]phenanthrene and dibenzo[g,p]chrysene) exhibiting the most extreme nonplanarity. These results suggest that a limited amount of space between alkyl chains may exist within the higher-density polymeric phase to recognize shape differences among the bulkier and nonplanar solutes.

Hybrid-Lipid Bilayers Induce n-Alkyl-Chain Order in Reversed-Phase Chromatographic Surfaces, Impacting their Shape Selectivity for Aromatic Hydrocarbon Partitioning

“…The selectivities that these phases exhibited for partitioning of a planar versus a nonplanar PAH increase proportionally with the ordering of the n -alkyl chain; see Figure . This result agrees with the previous predictions of the effect of hydrocarbon chain ordering of reversed-phase materials on their shape selectivity. − , In the present experiments, in situ Raman spectroscopy provides direct evidence of the hydrocarbon chain order based on intensity ratios trans-to-gauche C–C conformers in the spectra.…”

“…This result agrees with the previous predictions of the effect of hydrocarbon chain ordering of reversed-phase materials on their shape selectivity. [11][12][13]53 In the present experiments, in situ Raman spectroscopy provides direct evidence of the hydrocarbon chain order based on intensity ratios trans-to-gauche C−C conformers in the spectra.…”

“…Reversed-phase liquid chromatographic studies concluded that greater shape-selectivity could be produced by stationary phases with greater n-alkyl-chain density, which was presumed to confer greater chain order. [11][12][13]53 In this work, we investigate structural evidence for this conclusion by carrying out in situ measurements of Raman scattering from within individual reversed-phase chromatographic particle whose n-alkyl chain order is modified by the deposition of phospholipids forming hybrid-supported lipid bilayers. 39 The Raman spectra of C 18 particles and four hybrid supported phospholipid bilayers prepared by deposition of DOPC, POPC, DMPC, and DPPC on the interior surfaces of porous C 18 -modified chromatographic particles are plotted in Figure 1; see Figure S-2 for the lipid structures.…”

“…Numerous liquid chromatographic studies have been carried out using reversed-phase alkyl-chain modified columns to understand how the shape of a molecule impacts its retention. Sander, Wise, and their co-workers noted that reducing temperature, increasing grafting density, increasing alkyl chain length, and using polymeric bonded phases results in greater shape selectivity for retention of PAH isomers in reversed-phase columns. − They attributed these trends to the greater ordering of the stationary phase, although chromatographic retention provides no direct evidence of how these variables impact alkyl-chain order. Spectroscopic techniques, including FTIR, − solid-state NMR, , Raman scattering, − and fluorescence spectroscopy, − have been used to investigate the structure of covalently bonded alkyl-chain-modified silica.…”

Three-Dimensional Nanoporous Covalent Organic Framework-Incorporated Monolithic Columns for High-Performance Liquid Chromatography

“…40,41 Hence, the larger L/B of anthracene (1.57) gave it easier access to the 3D-IL-COF-1 monolith and so stronger retention than phenanthrene (L/B, 1.46). 42,43 The good linearity of the Van't Hoff plots for isomers on the 3D-IL-COF-1 monolith implies that the separation mechanism has little change within the studied temperature range 15 (Figure S14). The thermodynamic parameters for the separation of isomers on the 3D-IL-COF-1 monolithic column were further calculated from Van't Hoff plots (Table.…”