Influence of Analyte Overloading on Retention in Gas−Liquid Chromatography:  A Molecular Simulation View

Abstract

In an attempt to elucidate the molecular basis for concentration (isotherm) effects on retention in gas-liquid chromatography, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to investigate changes in analyte partitioning caused by overloading a model chromatographic system with either an alkane or an alcohol. Squalane was used as the stationary-phase material, and the analytes included n-pentane, n-hexane, n-heptane, 1 -butanol, and 1-pentanol. Three systems were studied that differed in the mobile-phase composition: (i) a helium vapor, (ii) a n-hexane vapor, and (iii) a 1-pentanol-saturated helium vapor. While the amount of helium that partitions into the stationary phase is very small, both n-hexane and 1-pentanol partition strongly into and thereby swell the stationary phase. Although the swelling of the stationary phase leads to a reduction in the partition coefficients for the alkane solutes for both the n-hexane- and 1-pentanol-swollen stationary phases, the effects on the alcohol solutes differ markedly. Whereas saturation by n-hexane causes a decrease of the alcohol partition contants (to an extent similar to that for the alkane solutes), the saturation by 1-pentanol causes a dramatic increase of the alcohol partition coefficients; e.g., the Kovats index of 1-butanol increases by more than 150 Kovats units. The formation of hydrogen-bonded alcohol aggregates in the liquid phase is the microscopic origin for the dramatic effect of 1-pentanol saturation on the retention of alcohols.

Microscopic Structure and Solvation in Dry and Wet Octanol

“…As a result, the properties of the coexisting phases (e.g., structures, mutual solubilities, and solute partitioning) can be determined directly from a single simulation. 21,[28][29][30] Two different systems were used here to investigate the effect of water on the solvent characteristic and structural organization of the dry and wet 1-octanol phases: system G/O (vapor/dry 1-octanol) and system G/O*/W (vapor/wet 1-octanol/water). In addition to 1-octanol and water solvent molecules, the simulated systems also contained eight types of solute molecules: methane to n-butane and methanol to 1-butanol.…”

“…Configurational-bias Monte Carlo (CBMC) simulations , in the Gibbs ensemble (GE) are ideally suited for an investigation of the octanol−water partitioning system because these simulations employ two or three separate simulation boxes which are in thermodynamic contact (analogous to the experimental setup). As a result, the properties of the coexisting phases (e.g., structures, mutual solubilities, and solute partitioning) can be determined directly from a single simulation. ,− Two different systems were used here to investigate the effect of water on the solvent characteristic and structural organization of the dry and wet 1-octanol phases: system G/O (vapor/dry 1-octanol) and system G/O*/W (vapor/wet 1-octanol/water). In addition to 1-octanol and water solvent molecules, the simulated systems also contained eight types of solute molecules: methane to n -butane and methanol to 1-butanol.…”

Transferable Potentials for Phase Equilibria. 7. Primary, Secondary, and Tertiary Amines, Nitroalkanes and Nitrobenzene, Nitriles, Amides, Pyridine, and Pyrimidine

“…Called TraPPE (transferable potentials for phase equilibria), this force field has been extended to branched alkanes, alkenes, and arenes; , to primary, secondary, and tertiary alkanols; and to molecules containing ether, glycol, diol, ketone, or aldehyde functionalities . The TraPPE force field has already found many applications, including retention in chromatographic systems, , octanol−water and hexadecane−water partitioning, , dynamics in biophysical systems, design of biomimetic and CO 2 -philic polymers, , miscibility in polymer systems, adsorption in pharmaceutical solids, and transport properties of lubricants. − …”

Dependence of chromatographic retention indices on a ratio of amounts of target and reference compounds

“…On the other side, there are a lot of publications devoted to the description and modeling the distortions of the shapes of chromatographic peaks influenced by different factors, including sample size and overloading the column (see, e.g. [30][31][32][33][34][35][36]). However, such distortions have never been considered in relation to the dependence of retention indices of analytes vs. the ratio of their amounts to the quantities of reference compounds.…”