Tetrazole-Functionalized Silica for Hydrophilic Interaction Chromatography of Polar Solutes

In this work, tetrazole-functionalized stationary phase was prepared with nitrile-modified silica by an ammonium-catalyzed (3 + 2) azide-nitrile cycloaddition reaction. The prepared stationary phase was used for separation of nucleobases and nucleosides by hydrophilic interaction chromatography (HILIC) mode. A typical HILIC mechanism was observed at higher content of acetonitrile (>85%, v/v) in the mobile phase. The retention mechanism of the column was investigated by the models used for describing partitioning and surface adsorption through adjustment ratio of water in the mobile phase, and by the influence of salt concentration, buffer pH, and temperature on the retention of solutes. The results illustrated that the surface adsorption through hydrogen bonding dominated the retention behavior of nucleobases/nucleosides under HILIC mode. From the separation ability, the tetrazole-functionalized stationary phase could become a valuable alternative for the separation of the compounds concerned.     

糖类化合物亲水作用色谱保留行为评价

以糖类化合物为研究对象,系统评价了其在亲水模式下的色谱保留行为。分别考察了流动相、固定相和缓冲盐等对糖类化合物保留的影响,建立了糖类化合物在亲水模式下的保留方程。结果表明,糖类化合物随着流动相中乙腈比例的降低,保留时间减小;随着缓冲盐浓度的增加,保留时间增加;同时,糖类化合物的保留行为还会受到有机溶剂种类和固定相类型的影响;其保留行为可使用顶替吸附-液相相互作用模型定量描述。将该模型进一步用于实际样品中糖类化合物保留行为的预测,获得了较好的实验结果,预测保留时间与实测保留时间的相对误差小于0.3%。对糖类化合物亲水模式下的保留行为进行了系统的评价和定量描述,该研究结果将有助于糖类化合物亲水作用色谱分离方法的发展。     

Hydrophilic interaction liquid chromatography with alcohol as a weak eluent

There has been a significant increase of interest in polar compound separation by hydrophilic interaction liquid chromatography (HILIC), in which acetonitrile is mostly used as a weak eluent. Although replacing acetonitrile with alcohols as organic modifiers has been previously reported, the separation mechanism was poorly understood. In this paper we explored the separation mechanism through the method development for the analysis of the trace amounts of polar and basic hydrazines, which were genotoxic in nature. Separation parameters such as the type and concentration of alcohol, acid modifier, and buffer in mobile phase as well as the choice of stationary phase and column temperature were studied. The data indicated that both electrostatic and hydrophilic interactions contributed to the retention and separation of the hydrazines. The results presented here provide insight into the adjustment of the retention and separation of analytes in HILIC mode with alcohol as a weak eluent. The optimized HILIC method coupled with chemiluminescent nitrogen detection (CLND) is simple and sensitive (reporting limit at 0.02%) and was applied to simultaneous analysis of hydrazine and 1,1-dimethylhydrazine in a pharmaceutical intermediate.     

Investigating the Effect of Chromatographic Conditions on Retention of Organic Acids in Hydrophilic Interaction Chromatography Using a Design of Experiment

Small organic acids have shown significant retention on various stationary phases, such as amide, amino, aspartamide, silica and sulfobetaine phase commonly used in hydrophilic interaction chromatography (HILIC). This study investigated the effect of chromatographic conditions on the retention behavior of organic acids in HILIC using the tool of design of experiment (DOE). The results of the DOE study indicated that both the content of organic solvent (i.e., acetonitrile) and salt concentration in the mobile phase had significant effects on the retention of organic acids. Higher content of organic solvent in the mobile phase led to a significant increase in retention on all types of stationary phases. Increasing salt concentration also resulted in a moderate increase in retention; however, the effect of salt concentration varied with the type of stationary phase. The study also revealed that column temperature had less impact on retention than organic solvent content and salt concentration in HILIC.     

Synthesis,chromatographic evaluation and hydrophilic interaction/reversed-phase mixed-mode behavior of a “Click β-cyclodextrin” stationary phase

A native β-cyclodextrin (β-CD) stationary phase was prepared by covalently bonding β-CD on silica particles via Huisgen [3 + 2] dipolar cycloaddition between the organic azide and terminal alkyne, the so-called Click chemistry. The resulting β-CD bonded silica (Click β-CD) was characterized by FT-IR, solid state ¹³C cross polarization/magic-angle spinning (CP/MAS) NMR and elemental analyses, which proved the successful immobilization of β-CD on the silica support with Click chemistry. The retentive properties of Click β-CD were investigated under hydrophilic interaction liquid chromatography (HILIC) mode in different mobile phase conditions with a set of polar compounds including nucleosides, organic acids and alkaloids. The effects of water content, concentration of the salt and pH of the buffer solution on the retention time were studied and the results demonstrated the typical retention behavior of HILIC on Click β-CD. Separation of very polar components, such as nucleosides and oligosaccharides, and chiral separation under HILIC mode were successfully achieved. In addition, Click β-CD was chromatographically evaluated with a set of flavone glycosides. The retention curves depending on the mobile phase of acetonitrile content were “U” curves, which is an indication of HILIC/RPLC mixed-mode retention behavior. The difference of the separation selectivity between HILIC and RPLC was described as orthogonality by using geometric approach and the orthogonality reached 69.4%. The mixed-mode HPLC properties and excellent orthogonality demonstrated the flexibility in HPLC methods development and great potential in two-dimensional liquid chromatography separation.     

UPLC a Powerful Tool for the Separation of Imidazolium Ionic Liquid Cations

Ultra-performance liquid chromatography (UPLC) in reversed-phase (RP), ion pair (IP) and hydrophilic interaction chromatography (HILIC) has been investigated for the separation of imidazolium-based ionic liquid (IL) cations. Among the three stationary phases (i.e., C18, C8 and phenyl) studied under RP conditions the phenyl phase provided much stronger retention for the IL cations. Four acids (hydrochloric, methanesulfonic, perchloric and trifluoroacetic) as mobile phase additives were compared in light of their effects on the retention of IL cations. It was shown that the retention of all IL cations decreased upon acidification of the mobile phase, possibly due to suppression of residual silanol ionization. Very fast (~3 min) and efficient RP-UPLC separation of six cations was achieved by gradient elution with acetonitrile?Cwater mobile phase containing 2.5 mmol L^?1 perchloric acid. In IP-UPLC all solutes were well resolved in about 4 min by gradient elution with acetonitrile?Cwater mobile phase containing 1 mmol L^?1 sodium 1-octanesulfonate as ion pairing reagent. Finally, under HILIC conditions by using isocratic elution with acetonitrile?Cwater (85:15, v/v) mobile phase containing 5 mmol L^?1 ammonium formate (pH 3.2) the separation time was reduced to less than 2 min while maintaining excellent peak shapes and sufficient resolution. Compared to current LC systems UPLC allowed considerably faster separations with better peak shapes.     

Preparation and Evaluation of Poly-l-Lysine Stationary Phase for Hydrophilic Interaction/Reversed-Phase Mixed-Mode Chromatography

In this work, a poly-l-lysine-grafted stationary phase was synthesized by polymerization of N-carboxyanhydride of l-lysine initiated by 3-aminopropylated silica. The resulting material was characterized by FT-IR spectra, elemental analysis and thermogravimetric analysis, which clearly indicated that the new phase had been prepared successfully. The retention of polar solutes depending on acetonitrile content in mobile phase exhibited ??U-shaped?? curves, which was an indication of hydrophilic interaction liquid chromatography (HILIC)/reversed-phase liquid chromatography (RPLC) mixed-mode retention behavior. The retention mechanisms in HILIC and RPLC modes also were investigated. Phenol compounds, aniline compounds and hydrophilic compounds were separated in RPLC or HILIC mode on the new stationary phase, respectively. This result shows that the new phase could be used for both RPLC and HILIC applications, providing greater flexibility for real sample analysis.     

A New Definition of the Stationary Phase Volume in Mixed-Mode Chromatographic Columns in Hydrophilic Liquid Chromatography

Polar columns used in the HILIC (Hydrophilic Interaction Liquid Chromatography) systems take up water from the mixed aqueous–organic mobile phases in excess of the water concentration in the bulk mobile phase. The adsorbed water forms a diffuse layer, which becomes a part of the HILIC stationary phase and plays dominant role in the retention of polar compounds. It is difficult to fix the exact boundary between the diffuse stationary and the bulk mobile phase, hence determining the column hold-up volume is subject to errors. Adopting a convention that presumes that the volume of the adsorbed water can be understood as the column stationary phase volume enables unambiguous determination of the volumes of the stationary and of the mobile phases in the column, which is necessary for obtaining thermodynamically correct chromatographic data in HILIC systems. The volume of the aqueous stationary phase, Vex, can be determined experimentally by frontal analysis combined with Karl Fischer titration method, yielding isotherms of water adsorbed on polar columns, which allow direct prediction of the effects of the composition of aqueous–organic mobile phase on the retention in HILIC systems, and more accurate determination of phase volumes in columns and consistent retention data for any mobile phase composition. The n phase volume ratios of 18 columns calculated according to the new phase convention strongly depend on the type of the polar column. Zwitterionic and TSK gel amide and amine columns show especially strong water adsorption.     

硅胶色谱柱的亲水作用保留机理及其影响因素

亲水作用色谱（HILIC）是替代反相色谱（RPLC）分离强极性及亲水性化合物的另一色谱模式,其分离机理与RPLC有很大不同,具有和RPLC互补的选择性。在HILIC模式中,采用正相色谱（NPLC）中的极性固定相及含高浓度有机溶剂（通常为乙腈）的水溶液为流动相。硅胶是开发最早、研究最为深入及应用最为广泛的HILIC固定相,本文介绍了硅胶色谱柱的HILIC保留机理,详细概述了操作条件如硅胶柱类型、流动相组成及柱温对HILIC分离的影响,并对硅胶填料色谱柱的HILIC模式的发展方向与应用前景进行了展望。     

Underivatized amylose and cellulose as new stationary phases for hydrophilic interaction chromatography

Two polysaccharide stationary phases have been newly suggested for application in hydrophilic interaction chromatography (HILIC). Both columns (amylose‐silica, 250 × 4.6 mm, 5 μm and cellulose‐silica, 250 × 4.6 mm, 5 μm) demonstrated a satisfactory retention of polar compounds. The influence of the mobile‐phase composition (acetonitrile content, pH, salt concentration) on the retention was in agreement with the HILIC concept. The phases showed a very similar behavior, typical efficiency of about 50 000 plates/m, cellulose retained test compounds somewhat more strongly. Under the experimental conditions, electrostatic (non‐HILIC‐type) interactions due to the dissociation of silanol groups on the silica surface did not influence the retention, noticeably. The applicability of polysaccharide stationary phases for the chromatography of polar compounds was proven by the separation of mixtures of sugars (fructose, glucose, saccharose, maltose, trehalose) or vitamins (nicotinamide, pyridoxine, riboflavin, thiamine, nicotinic acid, ascorbic acid).     

Determination of Atenolol in Human Plasma by Pseudo Reversed Phase Liquid Chromatography-Tandem Mass Spectrometry

Previous HPLC determination of atenolol on reversed-phase packings has often required a mobile phase containing three components: organic modifier, buffer and ion-pairing reagent or organic amine. In addition to the complexity of the eluents employed, alkyl sulphonates and organic amines in the mobile phase can reduce the life of silica-based bonded columns. A new simple, rapid and sensitive method—pseudo reversed-phase liquid chromatography/tandem mass spectrometry has been developed for the analysis of atenolol in human plasma using bare silica as the stationary phase coupled with a simple mobile phase consisted of 5% acetonitrile and 95% formate buffer. The optimization of separation is fast and easy. The assay was validated for the concentration range 1–100 ng mL^?1 with a detection limit of 1 ng mL^?1. Moreover, the silica column was durable with the mainly aqueous eluents. No obvious loss in performance was observed for 30,000 column volumes of eluent.     

L-异亮氨酸键合色谱固定相的制备及其性能

利用异氰酸丙基三乙氧基硅烷与L-异亮氨酸反应合成了一种新型的硅烷偶联剂,并进一步将其与硅胶反应制得键合有L-异亮氨酸的亲水色谱固定相。 通过核磁共振氢谱表明亮氨酸功能化硅烷偶联剂的成功合成、元素分析表征证明亮氨酸已成功键合到硅胶表面。 将其作为亲水模式下的固定相填料填装在150 mm×4.6 mm不锈钢色谱柱中,以一系列经典的极性小分子作为探针,考察了这些探针分子在固定相上的色谱行为。 极性化合物的保留时间随着流动相中有机溶剂含量提高而逐渐增大,表现出典型的亲水保留特征。 进一步研究了流动相中乙腈含量、缓冲盐pH值及缓冲盐浓度等因素对分析物在固定相上的保留的影响。 在优化了相关参数后,将固定相应用于碱性化合物、水溶性维生素以及核苷类极性物质的分离当中。 在等度洗脱下,5种碱性化合物、6种水溶性维生素和8种核苷类物质分别在8、18及25 min内被成功分离。 分离结果表明了合成的L-异亮氨酸键合亲水色谱固定相具有较好的色谱性能,在极性化合物的分离上具有良好的应用前景。     

多羟基化合物键合亲水色谱固定相的制备及其分离性能

用硅胶与氨丙基三甲氧基硅烷反应,再与δ-葡萄糖酸内酯反应,制备了一种多羟基化合物键合的新型亲水色谱固定相。以水-有机溶剂(乙醇、乙腈、四氢呋喃)为流动相,通过改变流动相中有机溶剂的种类及浓度、缓冲盐浓度和柱温,考察了该固定相对6种强极性中药组分的保留行为和保留机理。当水的比例在0～40%(v/v)范围时,溶质的保留随着流动相中水的比例的增大而减小,属于典型的亲水色谱分离模式;而当流动相中水的比例在0～100%(v/v)范围内变化时,溶质的保留随着水的比例变化呈“U”形曲线,属于亲水色谱和反相色谱的混合保留机理。缓冲盐的浓度和pH效应说明,所选用的中药组分与所制备的固定相间还存在弱的静电作用。该固定相对6种中药组分以及丹参注射液具有良好的分离性能,表明其在强极性中药有效成分的分离及其他强极性物质的分离分析中具有一定的应用前景。     

亲水作用色谱固定相及其在中药分离中的应用

亲水作用色谱(HILIC)作为一种分离极性化合物的液相色谱模式,近年来越来越受到关注和重视。一方面是因为强极性化合物的分离问题引起了各个研究领域的重视,如药物分析、代谢组学、蛋白质组学等研究领域都不同程度地涉及强极性化合物的分离问题;另一方面是由于HILIC具有流动相组成简单、分离效率较高、与质谱兼容以及反压较低等优势。固定相是HILIC发展和应用的基础,本文主要从固定相分子结构的角度对HILIC固定相的结构特征、保留特性以及应用概况等进行了综述。对传统正相色谱固定相用于HILIC以及专门设计的HILIC固定相进行了介绍,评述了各自的优缺点和应用概况;对近年来HILIC固定相在中药分离中的应用进行了介绍;并对HILIC固定相的发展进行了展望。     

低共熔溶剂——新型亲水作用色谱流动相改性剂

低共熔溶剂被用作亲水作用色谱流动相的新型改性剂。选用硅胶柱(150 mm×4.6 mm, 3 μm),以乙腈与低共熔溶剂(氯化胆碱-乙二醇(摩尔比为1:3))的混合溶液为流动相,考察了6个碱基与核苷的色谱分离效果,并讨论了低共熔溶剂在流动相中的比例及温度条件对分离的影响。结果表明,与传统的水相流动相条件相比,在加入低共熔溶剂改性后的流动相条件下,碱基与核苷分离效果得到明显的改善,尤其是胞嘧啶与胞苷能达到完全分离;同时,随着低共熔溶剂在乙腈中浓度的增加,6个碱基与核苷在色谱柱上的保留均有不同程度的减小,其中胞苷的保留减小最为显著;随着柱温的升高,碱基与核苷的保留同样有所减小。本文验证了低共熔溶剂作为亲水作用色谱流动相改性剂的可行性,可在一定程度上解决传统亲水作用色谱分离的困难。     

Hydrophilic interaction chromatography using amino and silica columns for the determination of polar pharmaceuticals and impurities

Hydrophilic interaction chromatography (HILIC) is described as a useful alternative to reversed-phase chromatography for applications involving polar compounds. In the HILIC mode, an aqueous-organic mobile phase is used with a polar stationary phase to provide normal-phase retention behavior. Silica and amino columns with aqueous-acetonitrile mobile phases offer potential for use in the HILIC mode. An examination of the retention and separation of several pyrimidines, purines, and amides on silica and amino columns from three manufacturers revealed that mobile phases should contain a buffer or acid for pH control to achieve similar and reproducible results among columns from different sources. Amino columns may also be used in an anion-exchange mode, which provides an advantage for some applications. In some cases, silica can provide different selectivity and better separation than an amino column. Example applications include: low-molecular-mass organic acids and amides as impurities in non-polar drug substances, 5-fluorouracil in 5-fluorocytosine, guanine in acyclovir, and different selectivity for polar basic compounds compared to an ion-pairing system.     

Efficiency of the same neat silica column in hydrophilic interaction chromatography and per aqueous liquid chromatography

The dependencies on the mobile phase flow velocity of the efficiency of a column packed with shell particles of neat porous silica (Halo) was measured under two different sets of experimental conditions. These conditions corresponded to the retention mechanisms of per   aqueous liquid chromatography (PALC) at low acetonitrile concentrations and of hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. The results are compared. Small amounts of a diluted solution of caffeine were injected in order to record the chromatograms under strictly linear conditions. These efficiencies were measured in both water-rich (PALC retention mechanism) and acetonitrile-rich (HILIC mechanism) mobile phases for the same retention factors, between 0.25 and 2.5. The mobile phases were mixtures of acetonitrile and water containing neither supporting salt nor buffer component. At low retention factors, the efficiency of caffeine is better in the PALC than in the HILIC mode. For k^′=0.5$k^{\prime }=0.5$, the minimum reduced height equivalent to a theoretical plate (HETP) is close to 2.5 in PALC while it exceeds 5 in HILIC. The converse is true for high retention factors. For k^′>2.5$k^{\prime }>2.5$, the HETP is lower in HILIC than in PALC, because the major contribution to band broadening and peak tailing in this latter mode originates from the heterogeneous thermodynamics of retention and eventually restricts column performance in PALC. Most interestingly, the reduced HETP measured in HILIC for caffeine never falls below 4. This suggests that the mass transfer of caffeine between the multilayer adsorbed phase (due to the interactions of the strong solvent and the silanol groups) and the acetonitrile-rich bulk eluent is slow.     

木糖醇和麦芽糖醇型亲水作用色谱固定相的制备及其分离性能的评价

利用-NCO和-OH的加成反应,通过简单的两步反应将木糖醇和麦芽糖醇成功地键合于硅胶表面,制备了两种新型糖醇类亲水作用色谱固定相。流动相中乙腈含量对保留的影响曲线表明,这两种糖醇固定相具有典型的亲水作用色谱固定相性质,对极性和亲水性化合物有很强的保留作用。利用这两种固定相成功分离了水溶性维生素、水杨酸及其类似物、碱基及其相应的核苷和淫羊藿苷类似物等模型混合物,同时糖醇固定相展现了新颖的选择性,特别是相对于线形的木糖醇键合固定相,非线形的麦芽糖醇键合固定相表现出了对糖基的独特保留能力。此外,缓冲盐的pH和浓度对保留的影响表明静电作用在这两种糖醇固定相的保留机理中也发挥着一定的作用。本文所发展的糖醇类固定相具有良好的分离性能,有望在亲水作用色谱分离领域发挥潜在的应用价值。     

Stationary and mobile phases in hydrophilic interaction chromatography: a review

Hydrophilic interaction chromatography (HILIC) is valuable alternative to reversed-phase liquid chromatography separations of polar, weakly acidic or basic samples. In principle, this separation mode can be characterized as normal-phase chromatography on polar columns in aqueous-organic mobile phases rich in organic solvents (usually acetonitrile). Highly organic HILIC mobile phases usually enhance ionization in the electrospray ion source of a mass spectrometer, in comparison to mobile phases with higher concentrations of water generally used in reversed-phase (RP) LC separations of polar or ionic compounds, which is another reason for increasing popularity of this technique. Various columns can be used in the HILIC mode for separations of peptides, proteins, oligosaccharides, drugs, metabolites and various natural compounds: bare silica gel, silica-based amino-, amido-, cyano-, carbamate-, diol-, polyol-, zwitterionic sulfobetaine, or poly(2-sulphoethyl aspartamide) and other polar stationary phases chemically bonded on silica gel support, but also ion exchangers or zwitterionic materials showing combined HILIC-ion interaction retention mechanism. Some stationary phases are designed to enhance the mixed-mode retention character. Many polar columns show some contributions of reversed phase (hydrophobic) separation mechanism, depending on the composition of the mobile phase, which can be tuned to suit specific separation problems. Because the separation selectivity in the HILIC mode is complementary to that in reversed-phase and other modes, combinations of the HILIC, RP and other systems are attractive for two-dimensional applications. This review deals with recent advances in the development of HILIC phase separation systems with special attention to the properties of stationary phases. The effects of the mobile phase, of sample structure and of temperature on separation are addressed, too.     

万古霉素及其杂质的亲水作用色谱分析

目前,万古霉素色谱分析方法主要为反相色谱法,该法分离万古霉素及其杂质时,存在极性选择性不足以及所使用的流动相体系与质谱兼容性差等问题。基于亲水作用色谱(HILIC)对糖肽类物质的色谱保留和极性选择性,本文选取万古霉素及其常见杂质为对象,考察了HILIC固定相、流动相组成比例、缓冲盐添加剂浓度和pH值等色谱条件,对万古霉素及其主要杂质进行了HILIC分离方法研究。确立了以Click XIon色谱柱为固定相,以甲酸铵为流动相添加剂的亲水作用色谱条件,实现了万古霉素及主要杂质的分离,为万古霉素类化合物的分离提供了新的途径。