A New Equation for Solute Retention in Liquid Chromatography

In consideration of the adsorption of solvent, diluent and solute molecules on the surface of a stationaryphase, a new equation for solute retention in liquid chromatography is presented. This equation includesthree parameters: the displacement equilibrium constant (Ksd) between the solvent and diluent molecules onthe surface of the stationary phase, the total number(N) of the solvent and diluent molecules released fromthe stationary phase after one solute molecule being adsorbed, and the parameter (I) related to the thermody-namic equilibrium constant for the solute adsorption on the stationary phase. Over the whole concentrationrange of the solvent in the mobile phase, the experimental retention data can be well described by this equa-tion, parameters K~, N and I can be obtained by the regression analysis of the experimental retention data,and consequently the number of the solvent and the diluent molecules displaced by one solute molecule fromthe stationary phase can also be derived at different solvent concentrations in the mobile phase,     

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. V. Study of retention mechanism using linear solvation energy relationships

A linear solvation energy relationship model was used to characterize the retention behavior of a stationary phase based upon a nematic side-on liquid crystalline polymer (SOLCP) in reversed-phase liquid chromatography. The set of solutes was constituted of a high variety of compounds whose molecular sizes were considerably smaller than the mesogenic unit size. The results showed good statistical fits for these retention data in 65:35, 75:25 and 85:15 (v/v) methanol-water mobile phases. Both the cavity term and excess molar refraction are the most important favorable retention-governing parameters, whereas the solute hydrogen bond acceptor basicity is the most unfavorable retention parameter. Hydrophobicity and pi-pi interactions decrease strongly when the percentage of methanol increases, leading to an important retention decrease despite the fact that the hydrogen bond interaction weakens as the organic solvent is added. The shape recognition ability of this side-on liquid crystalline stationary phase on polycyclic aromatic hydrocarbon solutes is partly explained by the solutes' high polarizability due to the presence of pi-electrons. However, the solute polarizability is not sufficient and a stationary phase's "structure effect" must to be taken into account for the shape discrimination observed. The strong interaction between liquid crystal molecules caused likely a adsorption retention mechanism rather than a partition mechanism.     

Effect of the organic modifier concentration on the retention in reversed-phase liquid chromatography I. General semi-thermodynamic treatment for adsorption and partition mechanisms

A semi-thermodynamic treatment is adopted to account for adsorption or partition of solute molecules from aqueous mobile phases on/in reversed-phase liquid chromatography stationary phases. The theoretical expressions of ln k' versus organic modifier content are tested against 10 data sets covering a variety of solute molecules. It is shown that the mean field approximation, adopted widely in ptevious studies, is marginally valid in aqueous mobile phases, especially in the presence of solute molecules, and the lattice model approximation, which is also used in relevant studies, is a poor approximation. Clear conclusions about the validity of either the adsorption or the partition model for the retention mechanism could not be drawn. The equations of the adsorption model describe all data sets absolutely satisfactorily and yield a physically reasonable picture about the behavior of modifier and solvent at the adsorbed layer. However, the high applicability of the adsorption model may not safely entail the validity of the adsorption mechanism at a molecular level, especially in the case of solutes with small and non-polar molecules, where our analysis gives strong indications about the validity of the partition mechanism. The next steps needed for the final elucidation of the retention mechanism in reversed-phase chromatographic columns are indicated.     

Determination of liquid-liquid partition coefficients by separation methods

By essence, all kinds of chromatographic methods use the partitioning of solutes between a stationary and a mobile phase to separate them. Not surprisingly, separation methods are useful to determine accurately the liquid-liquid distribution constants, commonly called partition coefficient. After briefly recalling the thermodynamics of the partitioning of solutes between two liquid phases, the review lists the different methods of measurement in which chromatography is involved. The shake-flask method is described. The ease of the HPLC method is pointed out with its drawback: the correlation is very sensitive to congeneric effect. Microemulsion electrokinetic capillary electrophoresis has become a fast and reliable method commonly used in industry. Counter-current chromatography (CCC) is a liquid chromatography method that uses a liquid stationary phase. Since the CCC solute retention volumes are only depending on their partition coefficients, it is the method of choice for partition coefficient determination with any liquid system. It is shown that Ko/w, the octanol-water partition coefficients, are obtained by CCC within the -1 < log Ko/w < 4 range, without any correlation or standardization using octanol as the stationary phase. Examples of applications of the knowledge of liquid-liquid partition coefficient in the vast world of solvent extraction and hydrophobicity estimation are presented.     

反相液相色谱中溶剂强度规律的研究

 以溶质计量置换保留模型 (SDM R)为依据 ,通过研究 3种正链醇同系物溶剂置换剂对 14种正链醇同系物溶质色谱保留行为的影响 ,发现计量置换参数Z(对应 1mol溶质被吸附时从溶质与固定相接触处释放出的溶剂的总摩尔数 )、lgI(与 1mol溶质对固定相亲和势有关的常数 )和 j(与 1mol溶剂对固定相亲和势有关的常数 )均随着同系物置换剂相对分子质量的增大而减小 ,并呈现出线性变化 ,表明溶剂强度与溶剂分子的大小有关 ,分子愈大 ,溶剂洗脱能力愈强 ,并遵循同系物规律。     

Hydrophobicity of ionisable compounds studied by countercurrent chromatography

Countercurrent chromatography (CCC) is a liquid chromatography technique in which the stationary phase is also a liquid. The main chemical process involved in solute separation is partitioning between the two immiscible liquid phases: the mobile phase and the support-free liquid stationary phase. The octanol-water partition coefficients (P(o/w)) is the accepted parameter measuring the hydrophobicity of molecules. It is considered to estimate active principle partitioning over a biomembrane. It was related to the substance biological activity. CCC is able to work with an octanol stationary phase and an aqueous mobile phase. In this configuration, CCC is a useful and easy alternative to measure directly the P(o/w) of the molecules compared to other methods including the classical and tedious shake-flask method. Three ketones are used as model compounds to illustrate the CCC protocol of P(o/w) measurement. The focus of this work is put on ionisable molecules whose apparent P(o/w) is completely changed by ionization. β-Blockers, diuretics and sulfonamides are compound classes that were studied. Some of the experimentally determined P(o/w) coefficients of the molecular forms disagreed with calculated and experimental values available in the literature. The P(o/w) coefficients of the ionic forms and the acidity constants were also calculated using a theoretical model. Relationships between biological properties and hydrophobicity are also discussed.     

大分子键合型手性固定相的研究进展

高效液相色谱（HPLC）被广泛认为是分离制备光学纯单一对映体最有效的方法。在高效液相色谱手性拆分中,手性固定相的性能直接影响到色谱柱的手性分离能力。在众多手性固定相中,键合型手性固定相具有溶剂耐受性好,分离模式灵活等优点,是很重要的一大类手性固定相。本文主要针对大分子键合型手性固定相,包括多糖衍生物键合型手性固定相、蛋...     

在脂肪酸-水流动相中小分子在反相高效液相色谱中的保留表征

在反相液相色谱(RPLC)中用Snyder经验议程和计量置换保留模型(SDM-R)中的参数对深质为脂肪醇同系物,流动相为脂肪酸同系物时深质的保留行为进行了研究,结果表明用SDM-R参数具有明显的优越性,另外,由于用Snyder经验公式中二参数之间的作园无法准确求得斜率,且其不具有明确的物理意义,而由SDM-R二参数作图,不但能准确求得斜率j(与1mol溶剂和固相结合能有关的常数),而且j具有明确的物理意义并符合碳数规律,所以,参数j有可能用于RPLC中表征深剂强度,由此得出,在RPLC中,对同一置换剂面言,随同系物溶质的Z(1mol深剂化深质被深剂化固定相吸附时,从二接解面释放出的置换剂分子数)和logI(与1mol深质和固定相亲和势有关的常数)值增大,它们的保留时间也增大,对同一深质而言,随着在同系物置换剂中碳链的增长,Z和logI值的减小,它的保留时间也缩短,同时还可得出,随着同系物置换剂j值的减小,它们的洗脱能力也增强.     

Retention of C60 and C70 fullerenes on reversed-phase high-performance liquid chromatographic stationary phases.

The separation of C60 and C70 fullerenes on four different polysiloxane stationary phases was examined. It was determined that polar solvents can be used as mobile phases effectively for the separation of fullerene molecules. Unlike previously published work, a polymeric octadecyl siloxane (ODS) stationary phase provided higher separation factors for C70/C60 than did monomeric ODS stationary phases or phenyl substituted stationary phases. For example, for a methanol-diethyl ether (50:50, v/v) mobile phase and C60, k' approximately 5.0 separation factors, alpha = 3.3, were achieved with polymeric ODS compared to alpha = 2.2, with a monomeric ODS stationary phase. A linear solvation energy relationship (LSER) was used to model the importance of solvent interactions and stationary phase interaction to solute retention.     

碱性药物在硫醚嵌入苯磺酸硅胶固定相上的色谱行为

混合模式色谱（MMC）在复杂样品的分离分析方面具有独到的优势,相比于单一模式色谱,MMC受到多种作用控制,保留机理更为复杂。利用巯基-烯点击化学方法分别制备了单配体和双配体两种硫醚嵌入苯磺酸硅胶固定相,通过改变pH、离子强度和有机溶剂强度等流动相条件,以4种碱性药物为模型,对其保留机理进行了探讨。结果表明,两种固定相都具有反相和离子交换的混合保留机理。通过改变流动相中盐浓度、考察溶质保留因子与盐浓度倒数的关系,证明了反相、单纯离子交换和反相协同离子交换三种作用形式的保留模型更为合理。定量研究表明,在两个固定相上,由单纯离子交换和反相协同离子交换构成的总离子交换作用占主导,各作用占比与溶质、流动相组成、固定相配体的类型及其比例等密切相关,并且协同作用对溶质的保留和分离选择性影响很大。混合模式色谱保留机理的研究对于新型固定相设计和复杂体系的分离优化具有重要理论指导意义。     

Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation II. Effects on solute retention

Particle-based Monte Carlo simulations were employed to examine the molecular-level effects of bonding density on the retention of alkane and alcohol solutes in reversed-phase liquid chromatography. The simulations utilized octadecylsilane stationary phases with various bonding densities (1.6, 2.3, 2.9, 3.5, and 4.2mumol/m(2)) in contact with a water/methanol mobile phase. In agreement with experiment, the distribution coefficient for solute transfer from mobile to stationary phase initially increases then reaches a maximum with increasing bonding density. A molecular-level analysis of the solute positional and orientational distributions shows that the stationary phase contains heterogeneous regions and the heterogeneity increases with increasing bonding density.     

线性参数logI对反相液相色谱中的非极性小分子溶质体系的表征

从理论上阐明了计量置换平衡常数的对数ｌｏｇＫａ对液相色谱中计量置换保留模型的线性参数ｌｏｇＩ起着主导作用。ｌｏｇＩ表示溶质对固定相的亲合势，且具有热力学平衡常数的性质。     

Dual‐mode hydrophilic interaction normal phase and reversed phase liquid chromatography of polar compounds on a single column

Adopting a stationary phase convention circumvents problematic definition of the boundary between the stationary and the mobile phase in the liquid chromatography, resulting in thermodynamically consistent and reproducible chromatographic data. Three stationary phase definition conventions provide different retention data, but equal selectivity: (i) the complete solid phase moiety; (ii) the solid porous part carrying the active interaction centers; (iii) the volume of the inner column pores. The selective uptake of water from the bulk aqueous‐organic mobile phase significantly affects the volume and the properties of polar stationary phases. Some polar stationary phases provide dual‐mode retention mechanism in aqueous‐organic mobile phases, reversed‐phase in the water‐rich range, and normal‐phase at high concentrations of the organic solvent in water. The linear solvation energy relationship model characterizes the structural contributions of the non‐selective and selective polar interactions both in the water‐rich and organic solvent‐rich mobile phases. The inner‐pore convention provides a single hold‐up volume value for the retention prediction on the dual‐mode columns over the full mobile phase range. Using the dual‐mode monolithic polymethacrylate zwitterionic micro‐columns alternatively in each mode in the first dimension of two‐dimensional liquid chromatography, in combination with a short reversed‐phase column in the second dimension, provides enhanced sample information.     

一个描述液相色谱体系中溶质进样量与保留值关系的方程

通过考虑溶质和溶剂在液相色谱体系中的相互作用,从理论上给出了一个描述溶质在液相色谱体系中进样量与保留值之间关系的方程。由方程可以证明,当进样量趋于零时,溶质的保留值为一定值,当进样量无限大时,溶质的保留值趋于零,且随着进样量的增加,溶质的保留值必然减小。通过方程的线性形式,可以获得两个描述色谱体系特征的重要参数：一个是溶质与固定相相互作用的平衡常数Ｋ,另一个是假想的分布在固定相表面上的活性点总数。     

Experimental Study of Model Bonded Stationary Phases for Liquid Chromatography I. Silica/Polyethyleneoxide

Abstract

Model bonded phases have been prepared by reaction of polyethyleneoxide of various molecular weights (200 < M < 5.10⁶) on silica.

The retention behavior of solutes on these bonded stationary phases for liquid chromatography depends on silica loadings, grafted molecule length, solute size and solvent nature. Different mechanisms such as dissolution effect in the grafted phase, adsorption on mineral support and steric exclusion due to the residual porosity of silica, are involved in the observed separations. An expression of elution volumes in relation with these mechanisms is proposed.

Chromatographic data and thermodynamic predictions are in good agreement.     

Incorporation of ionic liquid into porous polymer monoliths to enhance the separation of small molecules in reversed‐phase high‐performance liquid chromatography

An ionic liquid was incorporated into the porous polymer monoliths to afford stationary phases with enhanced chromatographic performance for small molecules in reversed‐phase high‐performance liquid chromatography. The effect of the ionic liquid in the polymerization mixture on the performance of the monoliths was studied in detail. While monoliths without ionic liquid exhibited poor resolution and low efficiency, the addition of ionic liquid to the polymerization mixture provides highly increased resolution and high efficiency. The chromatographic performances of the monoliths were demonstrated by the separations of various small molecules including aromatic hydrocarbons, isomers, and homologues using a binary polar mobile phase. The present column efficiency reached 27 000 plates/m, which showed that the ionic liquid monoliths are alternative stationary phases in the separation of small molecules by high‐performance liquid chromatography     

Calculation of the heats of solutions of alkylbenzenes in a nonpolar stationary phase

The heats of solution of 32 alkylbenzenes in nonpolar stationary phases (pentadecane, squalane, and Apiezon L) have been determined by gas-liquid chromatography. It is shown that the heats of solution of these substances may be calculated a priori as the sum of three quantities: a) the calculated dispersion force taking into account intramolecular screening; b) the energy of vacancy formation; and c) the interaction of the solvent molecules and the methyl groups of the solute. The presence of a double bond in the alkyl substituent increases the heat of solution of an alkylbenzene in the nonpolar stationary phase.