Retention Mechanism In Liquid Chromatography Research Articles

Understanding mixed-mode retention mechanisms in liquid chromatography with hydrophobic stationary phases.

The chromatographic retention mechanisms of two hydrophobic bonded phases, octadecyl ethyl-bridged organic/inorganic (BEH-C18) and straight-chain perfluorohexylpropyl silica (C6F13), have been investigated by using a homologous series of alkyl-benzenes and perfluoroalkyl acids as test compounds in a variety of acetonitrile/water mobile phases and at different temperatures. On both columns, polar compounds exhibited a characteristic U-shape retention behavior in function of acetonitrile amount in the eluent, whereas retention of neutral molecules decreased continuously, following an increase of organic modifier, over the entire mobile phase range. The dependence of perfluoromethylene selectivity upon eluent composition explains the typical reversed-phase behavior (decreasing in retention following an increase of acetonitrile in mobile phase) initially exhibited by perfluoroalkyl acids, but alone it cannot justify their increasing of retention at organic-rich mobile phases (approximately >90% v/v for acetonitrile with the C6F13 column and acetonitrile >80% v/v for the BEH-C18 one). It actually predicts an opposite trend, indicating thus the presence of mixed-mode retention mechanisms. Indeed it was found that, at organic-rich mobile phases, the transfer from the mobile to the stationary phase of the polar moiety of molecules drives retention. This finding has been correlated to the excess adsorption isotherm of acetonitrile/water binary mixtures and thus to the composition of the stationary phase. At organic-rich mobile phases, in fact, stationary phases are characterized by a positive excess of adsorbed water that creates an "environment" suitable to the transfer herein of polar groups.

Thermodynamic description of retention mechanism in liquid chromatography

The chemical and physical properties of the stationary and the mobile phase determine the retention and selectivity of the separated molecules in chromatographic process. It should explain the specific and non-specific interactions in the chromatographic system. Special emphasis should be placed on column selection through the adequate choice of column dimensions (diameter and length) and the type of stationary phase with required parameters, which contributes the system effectiveness. However, column effectiveness is unable to guarantee a satisfactory level of substance separation. For this reason, the interactions between the analyte, the stationary and the mobile phase need to be investigated to determine the retention factors of the separated substances and the optimal parameters of the chromatographic system. According to the principles of thermodynamics, in particular molecular interactions, the chromatographic process has to be carried out with a high level of control. This work can be treated a brief tutorial devoted to thermodynamics of liquid chromatography process with a special emphasis on molecular interactions between analyte species and the components of the mobile and stationary phases.

Two-dimensional liquid chromatography of synthetic polymers

Two-dimensional liquid chromatography, 2D-LC of synthetic polymers is critically assessed. Similarities and differences of 2D-LC of low-molecular-mass and polymeric substances are reviewed. The rationale of application of 2D-LC to macromolecular substances is discussed. Basic information on retention mechanisms in liquid chromatography of synthetic polymers is furnished. The principles, reasons, and significance of coupling of retention mechanisms are explained. The resulting separation processes are elucidated, and the technical concepts of the corresponding experimental arrangements are described. The benefits of 2D-LC are demonstrated together with numerous problems and shortcomings of the method.

Investigating the Retention Mechanisms of Liquid Chromatography Using Solid-Phase Extraction Cartridges

Liquid chromatography (LC) experiments for the undergraduate analytical laboratory course often illustrate the application of reversed-phase LC to solve a separation problem, but rarely compare LC retention mechanisms. In addition, a high-performance liquid chromatography instrument may be beyond what some small colleges can purchase. Solid-phase extraction (SPE) cartridges offer an inexpensive alternative, while still providing effective model columns for investigating LC retention mechanisms. We have shown that the four major retention mechanisms (normal-phase, reversed-phase, ion-exchange, and size-exclusion) can be investigated experimentally using two types of SPE cartridges (silica and C18) as columns. The C18 cartridges can be pre-treated with anionic or cationic polymers to produce ion-exchange columns. The concept of post-column detection is also introduced. This experiment is suitable for a first-year general chemistry course or a third-year analytical course to illustrate the importance of solvent polarity and the types of retention mechanisms employed in separation science

Adsorption and Enthalpic Partition Retention Mechanisms in Liquid Chromatography of Noncharged Synthetic Polymers. Two Critical Ranges

Two independent liquid chromatographic critical ranges were identified in systems composed of the non-end-capped silica gel C-18 column packing, poly(methyl methacrylate)s (PMMAs), and mixed eluents containing acetonitrile (ACN) and toluene at 35 °C. ACN suppressed surface adsorption of PMMA on free silanols and promoted enthalpic partition of polymer species in favor of a solvated C-18 bonded phase. On the contrary, toluene promoted adsorption of PMMA on silanols and suppressed its enthalpic partition in favor of the C-18 phase. Changing the eluent composition, it was possible to separately control both the above enthalpic retention mechanisms. Consequently, a critical adsorption range was reached at one eluent composition with a negligible or only small effect of enthalpic partition, and vice versa, compensation of enthalpic partition and exclusion retention mechanisms was attained in eluent-suppressing adsorption. In this way, the existence was indirectly proved of surface adsorption and enthalpic partition as two similar but independent enthalpic retention mechanisms operative in high-performance liquid chromatography of polymers. In many cases adsorption and enthalpic partition act concurrently in a synergetic or an antagonistic way, and this may strongly affect the selectivity of polymer separation.

Adsorption and enthalpic partition retention mechanisms in liquid chromatography of non-charged synthetic polymers

Adsorption and enthalpic partition processes in high performance liquid chromatography (HPLC) of macromolecules are qualitatively described and compared. These two similar but not identical HPLC retention mechanisms result from enthalpic interactions between polymer sample and column packing, which are in different ways affected by interactions between eluent and column packing, and between eluent and sample. The understanding of basic principles of adsorption and enthalpic partition retention mechanisms in polymer HPLC may facilitate the suppression of their unwanted effects, as well as their appropriate coupling with the exclusion retention mechanism. The goal is an easy selection of suitable column packing, mobile phase, and temperature of system to attain desired separation and molecular characterization of particular complex polymer sample. The tailored coupling of enthalpic and entropic retention mechanisms with exclusion allows to either reduce or enhance separation selectivity of polymer species according to their molar masses. In the former case, macromolecules can be separated (practically) irrespectively of their molar mass in terms of other molecular characteristics, namely their chemical structure or physical architecture. On the contrary, increased selectivity of separation in terms of sample molar mass enables improving both the accuracy and speed of analysis. “Flower-like” enthalpic interactions of macromolecules in narrow pores may substantially affect the sample retention.

The effects of molecular width on permeation of organic solute through cellulose acetate reverse osmosis membranes

In order to examine the effects of molecular shape on the permeation of organic solute through a cellulose acetate reverse osmosis membrane the molecular length and width parameters were computed by modification of the sterimol parameters, where the orientation of the solute at the surface of cellulose acetate was considered. The orientation of the organic compound was estimated based on the interpretation of the retention mechanism in liquid chromatography with cellulose acetate as the stationary phase. The molecular width was recognized to be a useful descriptor of the effective molecular size from the following facts: the molecular width estimated in this study was correlated linearly with the Stokes radius in an aqueous solution system and corresponded very well to the distance parameter ( D) developed empirically as the apparent molecular size in the pores of a cellulose acetate membrane by Matsuura et al. [Ind. Eng. Chem., Process Des. Dev., 23 (1984) 673]. In the case of bulky compounds (molecular width ⪢ ca. 3x10 −10 m), however, the values of the parameter D were significantly larger than the molecular width. The results suggested the influence of the microcavities in the pores of cellulose acetate membranes.

逆浸透法（ＲＯ）と液体クロマトグラフィー（ＬＣ）

It was possible to measure the interaction between membrane polymer and solutes by liquid chromatography pached with membrane materials. The results obtained from liquid chromatograhy are useful to consider the mechanism of membrane separation and membrane materials. The retention mechanism in liquid chromatography in these systems, however, are not made clear sufficiently.In this paper, we described the retention characteristics of inorganic salts and of organic compounds in cellulose acetate-water system, and discussed the retention mechanism and the impacts upon membrane process. Moreover, the other systems of liquid chromatography were also mentioned and the availabillity of liquid chromatography for the study of membrane separation was discussed.