Effects of Solvent Composition and Temperature on the Normal Phase Liquid Chromatographic Separation of Substituted Phenols

Abstract

Temperature-dependent measurements of the capacity factors for the liquid chromatographic separations of position isomers of cresol and chlorophenol have been made at various concentrations of 2-propanol in n-heptane using an amino bonded phase column. Extra-thermodynamic relationships, i.e. linear free energy relationship and linear ΔH-pKa relationships are applied to demonstrate that the basic interaction mode does not vary with the solvent composition, consistent with an earlier report by Scott and Kucera. Hydrogen bonding interaction at the molecular level is proposed.     

CEC for Studying the Retention and Separation of Pesticides on a Humic Acid Stationary Phase

A humic acid stationary phase is evaluated for studying by capillary electrochromatography (CEC) the binding of four pesticides with humic acid (HA), the main organic component in soil. It was demonstrated that the humic acid capillary was stable in terms of efficiency and retention during a long period of time. It appeared that HA has a lower affinity for neutral than for charged pesticides. The influence of the mobile phase constitution on the pesticide retention was also investigated in order to provide valuable information about the binding mechanism. The results suggest that the interactions of the pesticides during CEC are solely based on chromatographic mechanisms and that electrophoresis only plays a minor role. As well, it was shown that the HA stationary phase was able to separate these pesticides by CEC with a low analysis time. This HA capillary column could soon become attractive to determining the risk assessment of pesticides.     

Effect of Temperature and Mobile Phase Composition on RP-HPLC Separation of Cephalosporins

Abstract

The effects of mobile phase composition and column temperature on resolution in reversed-phase high performance liquid chromatography (RP-HPLC) were used to separate the following cephalosporins: cefonicid, cefaclor, cephazolin, cefodizime, cephaloridine, cephamandole and cephalotin, from a single sample. The capacity factor (k') was described as a function of temperature and mobile phase composition. Semi-empirically estimated values of k' were determined using a small number of experimental data for different temperatures between 20°C and 60°C, and mobile phase compositions (acetate buffer/isopropanol). The capacity factor of each cephalosporin was observed to decrease with increasing temperature and the volume fraction (V) of isopropanol in the mobile phase; linear relationships were obtained for plots of In k' versus 1/T and log k' versus log V. The method developed, while simple, reveals the optimal isocratic elution conditions for column temperature and eluent concentration for the complete separation and rapid analysis of the cephalosporins studied.     

Extractant Effect on the Retention of the Stationary Phase in a Rotating Coiled Column

The effects of physicochemical properties of two-phase liquid systems (interfacial tension and differences in density and viscosity) on the retention of the stationary phase in the column were examined. These effects mainly determine the separation parameters of compounds. Extractant/decane–aqueous phase systems were used; their physicochemical properties changed both as a result of adding an extractant (di-(2-ethylhexyl) phosphoric acid, trioctylamine, or tributyl phosphate) to the organic solvent and because of a change in the composition of the aqueous phase. Aqueous ammonium sulfate of varying concentration was used as a mobile phase. It was shown that interfacial tension substantially affects the behavior of the systems under consideration. An increase in the ammonium sulfate concentration only slightly affects the retention factor of the stationary phase in the column. With a proper choice of the stationary phase, countercurrent chromatography can be used for the extraction of components from salt solutions of various concentrations.     

Temperature and Solute Molecular Size Effects on the Retention and Enantioselectivity of a Series of D, L Dansyl Amino Acids on a Vancomycin-Based Chiral Stationary Phase

Summary The influence of column temperature (0–28 °C) and solute molecular size on the retention and enantioselectivity of a series of D, L dansyl amino acids with a non-polar side chain (valine, leucine, phenylalanine and tryptophan) were investigated using a vancomycin-based chiral stationary phase (CSP). The enthalpic and entropic terms for the solute-CSP association were determined from the linear vant Hoff plots. Two solute groups were distinguished in relation to these thermodynamic quantities: the solute group I (dansyl valine, dansyl leucine, dansyl phenylalanine) for which large negative values of enthalpic terms were obtained and the solute group II (dansyl tryptophan) for which H value was much less negative. The enthalpy-entropy compensation study revealed that the interaction mechanism was identical for the group I solute enantiomers but changed for D, L dansyl tryptophan. This was further exemplified as the group I compound enantiomers were resolved over the temperature range while the enantiomers of dansyl tryptophan were not separated in the operating conditions. Relationships between both the solute retention factors and apparent enantioselectivity, and the accessible surface area of the amino acid side chain indicated that when the solute molecular size increased (i) the retention was enhanced by the hydrophobic effect and (ii) the chiral discrimination decreased dependent, at least in part, on a steric hindrance phenomenon at the vancomycin aglycone pocket.     

Separation of Pyrimidine Bases on a HPLC Stationary RP-18 Phase Coated with Calix[4]resorcinarene

Lipophilic calix[4]resorcinarenes, derived from lauryl aldehyde and resorcinol, are strongly adsorbed on the modified silica gel RP-18 for HPLC chromatography, while their solutions are passed through the column. The calix[4]resorcinarene-coated RP-18 phases were found to be useful beds for HPLC separation of uracil, thymine and cytosine.     

Effect of the Solvent Composition on the Content of the Crystalline Phase and Melting Temperature of Paraffin Waxes

Russian Journal of Applied Chemistry - The results of a study of melting the commercial waxes in the composition of asphaltene-resin-paraffin deposits and for comparison of petroleum waxes in...     

流动相组成及温度对4种1,2,4-三唑类农药对映体手性拆分的影响

流动相组成及温度对4种1;2;4-三唑类农药对映体手性拆分的影响;纤维素-三(二甲基苯基氨基甲酸酯);手性固定相;HPLC     

柱温对胆固醇键合固定相分离黄酮苷的影响及其热力学分离机理研究

柱温属于高效液相色谱(HPLC)的可调参数之一,但在实际操作过程中,柱温对溶质保留行为的影响通常被忽略,不作为色谱条件优化参数.本研究分别以甲醇-0.02 mol/L乙酸(30∶70,V/V)及甲醇-0.02 mol/L乙酸(25∶75,V/V)为流动相,讨论了柱温对6种黄酮苷在一种新型色谱固定相-胆固醇键合固定相以及C18键合固定相上分离的影响.结果表明,随着柱温升高,不同于十八烷基键合固定相,黄酮苷在胆固醇键合固定相上的分离效果得到改善,且峰形变好.同时,拟合了25℃～55℃温度范围内的van't Hoff方程,从热力学角度比较了黄酮苷在胆固醇键合固定相和十八烷基键合固定相上的保留机理.结果表明,黄酮苷在两种色谱柱上的van't Hoff方程均具有良好的线性关系(R2 ＞0.99),且拟合参数相近(△H0＜0,△S0＜0),表明这些物质在胆固醇键合固定相上的保留机理与在十八烷基键合固定相上相似,以单一的疏水性保留机理为主导,均属于焓驱动过程.本实验证实,在使用胆固醇键合固定相时,柱温可作为一项重要调节参数,参与色谱优化过程,从而使液相色谱从两变量调节方式(流动相种类和流动相比例)变为三变量调节方式(流动相种类、比例和柱温),从而为色谱条件优化提供了更多选择.     

The Effect of Stationary Phase Type and Mobile Phase pH on the Separation of Some Catecholamines

JPC – Journal of Planar Chromatography – Modern TLC - The catecholamines are very important in the control and regulation of a variety of functions acting as neurotransmitters in both...     

Phase Separation,Wetting and Dewetting in PS/PVME Blend Thin Films: Dependence on Film Thickness and Composition Ratio

The effects of film thickness and composition ratio on the morphology evolution of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blend thin films were investigated. Diverse morphology evolutions including droplet-matrix structure, hole emergence, bicontinuous structure formation, percolation-to-droplet transition could be observed under annealing in two-phase region, depending on film thickness and composition ratio. The mechanism for these morphology variations was related to the complex effects of phase separation, dewetting and preferential wetting. The comparison between the thickness of bottom PVME layer and the twice of gyration radius 2R_g(PVME) played a dominant role in morphology control. Only when the PS/PVME film had specific film thickness and compositional symmetry, phase separation and dewetting could happen in sequence.     

套索冠醚固定相的性能和分离机理研究

用３种新合成的套索冠和为固定相,制成石英毛细管柱,测其柱效、惰性、热稳定性、平均极性及选择等性质,实验表明,它们具有良好的色谱性能和分离选择性,适用于对醇、卤代烃、芳香烃等及其异构体的分离。     

Influence of Mobile Phase Composition on the Apparent Thermodynamic Characteristics in Liquid Chromatographic Enantioseparation on a Tartardiamide-Based Stationary Phase

Four amino acid derivatives and two binaphthyl compounds were enantioseparated successfully by HPLC with a chiral stationary phase commercially named Kromasil CHI-DMB, which contains an immobilized network polymer derived from L-tartaric acid. Chromatographic experiments were performed under normal phase conditions. n-Hexane/2-propanol was used as mobile phase. The effects of the content of 2-propanol and the column temperature on the retention and enantioseparation were studied systematically. Associated apparent thermodynamic parameters were calculated from the van’t Hoff plots. It was found that the enantioseparations of these amino acid derivatives were typical enthalpy driven, but for binaphthyl compounds the entropy term was also favorable to enantioselectivity in most cases. From the changes of the thermodynamic parameters and the isoenantioselective temperatures with the variety of mobile phase polarity, it was deduced that the CSP underwent remarkable conformational change when the content of 2-propanol was below 3%.     

反应时间、温度和溶剂对缬氨酸自组装成聚合肽反应的影响

寡肽类化合物一般具有较强的生物活性,多数可作为药物或药物前体,因此成为人们的研究热点之一．目前寡肽的合成方法主要有液相法、固相法及酶促合成法,这些合成方法往往需要对氨基酸的活性基团进行选择性保护,步骤繁琐．近期研究发现,在水-醇体系中α-氨基酸被磷酰化为N-磷酰-α-氨基酸后,可发生自组装聚合肽反应;在无水条件下,N,O-二(三甲基硅基)-α-氨基酸可与磷酰化试剂如O,O-亚苯基磷酰氯反应生成N-磷酰-α-氨基酸,后者也可通过自组装反应得到二～八肽,但这些N-磷酰-α-氨基酸的制备均需使用有机磷试剂．我们发现,在无机磷试剂如PCl5等辅助下,α-氨基酸同样可以发生自组装反应,得到一系列寡聚肽,这既可为寡聚肽的合成提供一种新的方法,同时,因无机磷试剂在原始地球条件下更可能存在,因而对于揭示多肽及蛋白质的起源具有重要意义．氨基酸的自组装反应具有链锁反应的特点,因此必须控制反应条件实现对反应的控制。     

柱温和流速对反相离子对色谱法分析寡核苷酸的影响

通过不同长度n/(n-1)系列和同长n系列寡核苷酸样品在柱温25℃~45℃和流速0.3~0.7 mL/min条件下分离度的变化趋势,研究了离子对色谱法中温度、流速对不同性质寡核苷酸模拟样品保留和分离的影响特点。结果显示:n/(n-1)系列样品的分离度随柱温增加逐渐增大;n系列3组样品则呈现增加、减小和基本不变3种趋势,其中长度相同、碱基组成相同的寡核苷酸样品随温度增加,分离度几乎线性下降。温度并非仅通过影响寡核苷酸分子的传质来改变分离,还可能通过影响不同序列的空间结构改变碱基的空间容量。两系列样品均在流速0.4 mL/min获得最大分离,相对较低的流速可提高寡核苷酸样品的传质,有利于分离,在优化流速的同时应考虑流速改变对梯度洗脱中有机溶剂比例变化率的影响。     

Chiral Separation of Nateglinide and its L Enantiomer on a Molecularly Imprinted Polymer-Based Stationary Phase

A new chiral stationary phase for nateglinide (N-(trans-4-isopropylcyclohexylcarbonyl)-D-phenylalanine) based on a molecularly imprinted polymer has been prepared by non-covalent imprinting. For chromatographic analysis the effects on the separation of mobile phase composition, flow rate, and temperature were investigated, and the optimum conditions for HPLC were shown to be: mobile phase, acetonitrile; flow rate, 0.5 mL min^?1; temperature, 25 °C. It was shown that the nateglinide-imprinted polymer was capable of recognizing the enantiomeric difference between nateglinide and its L enantiomer, whereas the non-imprinted polymer had no such ability. Scatchard analysis was used to investigate the binding characteristics of the nateglinide-imprinted stationary phase; this indicated that two classes of binding site were present in the imprinted polymer. The equilibrium dissociation constant (K _D) and the apparent maximum number (Q _max) of high- and low-affinity binding sites were 3.7 × 10^?4 mol L^?1 and 11.38 μmol g^?1, and 1.81 × 10^?3 mol L^?1 and 27.73 μmol g^?1, respectively.     

The Convergence Coordinates of the Affinity of a Component to the Stationary Phase vs. the Composition of Mobile Phase in HPLC

By plotting of the affinity of component to adsorbent (_a) vs. the logarithm of the solvent activity in the mobile phase in a reversed phase HPLC system, it is shown that when the concentration of organic solvent in the mobile phase increases, _a of a series of aromatic alcohol homologues tends to converge to a point. A theoretical model for evaluating the convergent coordinates was presented by using the linear relationship between _a and log a_D and the stoichiometric displacement theory for retention (SDT-R). The physical meaning of the coordinates of convergence point was also elucidated. Convergence occurs when the activity of the displacing solvent approaches the value when in a pure state. The experimental results fit the equations and the coordinate of the convergence point well. At the same time, a unified equation to express the quantitative relation between parameter _a describing the adsorption process in a liquid-solid system and k describing the retention process in a liquid chromatographic system is also derived.     

Separation of Free Amiimo Acids on Reverse Stationary Phases Using an Alkyl Sulfonate Salt as a Mobile Phase Additive

Abstract

Alkylsulfonate (RSO₃ ^?) salts were evaluated as mobile phase additives for the separation of free amino acids on reverse stationary phases using an acidic mobile phase where the amino acids are cations. The enhanced amino acid retention is the result of two major interactions, one being retention of the RSO₃ ^? salt on the stationary phase and the other an ion exchange selectivity between the amino acid analyte cation and the RSO₃ ^? countercation, or other countercations in the mobile phase. Major mobile phase variables are: type and concentration of RSO₃ ^? salt (the studies focused on C₈SO₃ ^? salts), presence of organic modifier, type of countercation present, and mobile phase pH and ionic strength. Alkyl modified silica and polystyrenedivinyl-benzene copolymeric reverse stationary phases were compared. A mobile phase gradient, increasing per cent organic modifier was shown to be best, is necessary for separating complex mixtures of polar and nonpolar or basic amino acids. The procedure is applicable to the identification and/or determination of amino acids in mixtures or in peptides after hydrolysis.     

Separation of Bacterial Menaquinones by HPLC Using Reverse Phase (RP18) and a Silver Loaded Ion Exchanger as Stationary Phases

Abstract

Bacterial menaquinones were separated isocratically on a reverse phase Li Chrosorb RP18 5 μm and a silver loaded ion exchanger.

On octyldecylsilica support the separation of the menaquinones depends on their lipophilic character, on the silver column mainly on the number of double bonds in the isoprenyl chain. Comparing the runs of both columns the menaquinones were easily differentiated.