Evaluation of the Effect of Citronellol Group on Functionalized Mesogenic Materials by Capillary GC

In this paper, the effects of functionalization with terpenes on two new liquid-crystalline stationary phases for gas chromatography (GC) are described. Citronellol was used as the terminal group in the first material, and tetrahydrogeraniol was used with a second material. Inverse GC showed that the new materials have wide liquid-crystalline ranges (mesophases), 371–500 and 395–501 K, respectively. Moreover, they show good thermal stability up to 523 K and good potential as stationary phases for capillary GC. To clarify the effects of the liquid crystal structures and functional groups on retention and separation, the chromatographic behaviors of the two stationary phases were compared by eluting alkylbenzenes, polycyclic aromatic hydrocarbons, aromatic compounds, and terpenoids. The selectivities for a wide range of analytes achieved using the citronellol column were significantly better than those obtained using the tetrahydrogeraniol column. The columns showed different retention behaviors and fine resolutions for some of the main constituents of essential oils. Introduction of the double bond of citronellol greatly improved the polarization interactions involved in the shape recognition of the liquid-crystalline state for isomers. The new citronellol liquid-crystalline stationary phase, therefore, has a high affinity for natural compounds.

     

硅藻土载负葫芦[7]脲气相色谱固定相的制备及其性能评价

在酸性条件下,将自制的葫芦[7]脲均匀地涂覆到102白色硅藻土担体上,制得葫芦[7]脲气相色谱固定相。 采用红外光谱、质谱、元素分析和热重分析表征了葫芦[7]脲在载体表面的结构。 利用相关探针测定了新固定相的麦氏常数,表征了其基本色谱性能。 考察了葫芦[7]脲气相色谱固定相填充柱对芳香烃、卤代烃、醇、酮、酯及硅氧烷的分离能力。 结果表明,葫芦[7]脲固定相热稳定性高,柱色谱性能稳定,对较广泛的化合物尤其对高沸点的酯类及硅氧烷类化合物显示出良好的色谱分离能力(7 min内分离),作为气相色谱固定相有较好的应用前景。 初步讨论了葫芦[7]脲固定相对上述化合物的分离机理。     

Preparation of Cyclodextrin Type Stationary Phase Based on Graphene Oxide and Its Application in Enantioseparation and Hydrophilic Interaction Chromatography

Graphene oxide (GO) was covalently coupled to the surface of amino silica gel by amide bond. β-cyclodextrin (β-CD) was further chemically bonded with GO to prepare a novel chiral stationary phase. The resulting material was characterized by Fourier transform-infrared (FT-IR) spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analysis and thermogravimetric analysis (TGA). The separation of seven enantiomers was improved in varying degrees. Meanwhile, the stationary phase showed typical characteristics of hydrophilic interaction chromatography (HILIC), and four small nucleoside molecules were separated with the mobile phase of methanol-acetonitrile-water (45:45:10, V/V) in the HILIC mode. In addition, the separation mechanism of the stationary phase was further explored by studying the effects of mobile phase composition, temperature and pH value on the analyte retention. The low temperature was conducive to the separation of analytes at 20–60 °C. The addition of protonated solvent methanol significantly decreased the retention time of the four analytes. The change of pH affected the degree of protonation of the analyte, the interaction between analytes and the stationary phase, and retention time of analytes. The results showed that GO and β-CD played a synergistic effect in the chiral resolution of the chromatographic stationary phase. The retention of analytes in HILIC was attributed to their mixed-mode retention mechanisms including hydrophilic interaction, electrostatic interaction, hydrogen bonding, π-π stacking and so on.     

基于金属有机骨架材料固定相的气相色谱分离应用

金属有机骨架材料(MOFs)是一类由有机配体和金属离子(或金属簇)自组装形成的新型多功能材料。MOFs具有孔隙度高、比表面积大、孔径可调、化学和热稳定性高等特点,被广泛应用于吸附、分离、催化等多个领域。近年来,MOFs作为新型气相色谱固定相用于分离异构体受到了广泛关注。与传统无机多孔材料相比,MOFs在结构和功能上展现出高度的可调性,通过合理地选择配体和金属中心,可以设计合成具有不同孔道大小和孔道环境的MOFs,从而分别从热力学和动力学角度优化色谱分离效果,有效提高分离选择性。该文结合MOFs的结构,讨论了MOFs气相色谱固定相分离不同类型分析物的分离机理。分离机理主要包括MOFs孔道的分子筛效应或形状选择性,MOFs不饱和的金属位点与分析物中不同的官能团之间产生的相互作用,分析物与MOFs孔道之间产生的不同范德华力、π-π相互作用和氢键相互作用。此外,MOFs的手性分离可能主要依赖于外消旋体与手性MOFs中手性活性位点之间的相互作用。该文也对不同分析目标物进行了归类,综述了多种MOFs气相色谱固定相对烷烃、二甲苯异构体和乙基甲苯、外消旋体、含氧有机物、环境有机污染物的气相色谱分离效果。最后,该文还对MOFs在该领域的应用进行了总结与展望,旨在为MOFs气相色谱高效分离的研究提供参考。     

Perhydroxylcucurbit[6]uril as a highly selective gas chromatographic stationary phase for analytes of wide‐ranging polarity

This work describes the separation performance of a perhydroxylcucurbit[6]uril stationary phase for capillary gas chromatography. The perhydroxylcucurbit[6]uril stationary phase exhibits higher resolving capabilities for critical analytes with better peak shapes than cucurbit[6]uril and conventional stationary phases. The advantageous separation feature of the perhydroxylcucurbit[6]uril stationary phase may originate from its unique structure and favorably balanced interactions with the analytes. Also, the perhydroxylcucurbit[6]uril column shows good separation repeatability with relative standard deviations in the range of 0.01–0.13% for intraday, 0.37–0.82% for interday, and 1.0–4.7% for column‐to‐column repeatability.     

Impact of column temperature and mobile phase components on selectivity of hydrophilic interaction chromatography (HILIC)

The retention mechanism and chromatographic behavior for different polar analytes under hydrophilic interaction chromatography (HILIC) conditions have been studied by application of different mobile phases and stationary phases to various analytes at different temperatures. In addition to the commonly accepted mechanism of analyte liquid-liquid partitioning between mobile phase and water-enriched solvent layer which is partially immobilized onto the surface of the stationary phase, hydrogen-bonding, hydrophobic interaction, and ion-exchange interactions may also be involved. The predominant retention mechanism in HILIC separation is not always easily predictable. It can depend not only on the characteristics of the analytes but also on the selection of mobile and stationary phase compositions. The objective of this review is to evaluate the potential application of column temperature and mobile phase composition toward improving HILIC selectivity. The functional groups from analyte structures, stationary phase materials and organic mobile phase solvents will be highlighted.     

HPLC retention behaviors of poly-aromatic-hydrocarbones on Cu(II)-octabromotetrakis(4-carboxyphenyl)porphine derivatives-immobilized aminopropyl silica gels in polar and non-polar eluents

As one of approaches of developing novel HPLC stationary phases, we prepared Cu-octabromotetrakis(4-carboxyphenyl)porphine derivative-immobilized silica gels (Cu-OBTCPP(D)), and evaluated the availability of the resultant Cu-OBTCPP(D) as a stationary phase for separation of poly-aromatic-hydrocarbons (PAHs) and their related compounds. A Cu-OBTCPP(D) column was revealed to have an ability to separate simple PAHs and be useful as a stationary phase in both polar and non-polar eluents. The retention property of the Cu-OBTCPP(D) column was evaluated in various comparative experiments using commercially available columns. In comparison with 2-(1-pyrenyl)ethyl dimetylsilyl silica gel column (PYE column) regarding the retention behavior for PAHs etc., the Cu-OBTCPP(D) column showed stronger interactions involving pi electron in non-polar eluent than PYE column. In comparison with a pentabromobenzyloxy propylsilyl silica gel column (PBB column) regarding the influence of bromination, the Cu-OBTCPP(D) column was affected differently from the PBB column. In comparison with nitrophenylethyl silica gel column (NPE column) regarding the retention behavior for compounds having a dipole in a non-polar eluent, the Cu-OBTCPP(D) column showed electrostatic interactions such as dipole-dipole interaction equivalent to or larger than the NPE column.     

Exploitation of a microporous organic polymer as a stationary phase for capillary gas chromatography

Microporous organic polymers (MOPs) have emerged as a new class of functional porous materials with unique characteristics and potential uses in diverse areas. However, the field of MOPs for gas chromatographic (GC) separations has not been well explored. Herein, a MOP namely KAPs-1 was dynamic coated onto a capillary column for the first time. The fabricated column exhibited a nonpolar nature and the column efficiency for n-dodecane was up to 7769 plates m⁻¹. The KAPs-1 coated column showed high GC separation performance for a series of volatile organic compounds (VOCs) including the challenging ethylbenzene and xylene isomers, which could not be resolved at baseline on the commercial 5% phenyl polysiloxane stationary phase. Moreover, the relative standard deviations for five replicate determinations of the studied analytes were 0.0–0.6%, 0.9–3.2%, 1.1–5.9%, 0.8–3.7% for retention time, peak area, peak height and peak width, respectively. To investigate the interaction between some analytes and the stationary phase, thermodynamic and kinetic parameters were also evaluated. The results of this study show it is very promising to utilize MOPs as stationary phases for capillary GC.     

Separation of dialkyl sulfides by metallo-mesogenic stationary phases for complexation gas chromatography

A copper mesogenic side-chain polymer (P-C(15)CuC(18)) was cross-linked onto the capillary wall as a stationary film for gas chromatography (GC) separation of alkyl sulfides. These organic sulfides are of interest for their large health impact because of their wide range of volatiles and high reactivities toward metals. Different GC parameters for optimal separation efficiency are discussed for use with a mesogenic polymer column along with flame photometric detection (FPD). Both the carrier gas flow-rate and column temperature were studied to determine the relationship of plate height to the chemical structure of the solutes, as well as to determine the morphology of the mesogenic polymer. Van 't Hoff plots show phase transitions of the stationary mesophase as the column temperature was varied. The results reveal that the separation mechanism might be based on ligand exchange and polarity interaction between the analytes and the stationary phase, with the vapor pressure of the analytes also being important. The former interaction dominates in the lamellar crystalline phase and the latter interaction dominates in the hexagonal columnar-discotic phase. With high reproducibility for retention time (RSD< or =0.37%) and for peak area (RSD< or =5.16%), the GC-FPD system produced linear calibration graphs (r> or =0.9918) for the determination of 13 sulfides with a detection limit below 2.5 ng.     

3,4-二(3-苯氧基-4-氟苯基)-2,5-二苯基苯基接枝聚硅氧烷气相色谱固定相的合成与表征（英文）

合成了一种耐高温的3,4-二(3-苯氧基-4-氟苯基)-2,5-二苯基苯基接枝聚硅氧烷(DPFP)固定相,使用静态涂渍法将其涂渍到毛细管柱内壁上,制成气相色谱柱。分离裂解乙烯的色谱图显示DPFP固定相在360℃时仍具有良好的分离能力。DPFP固定相的柱效为3 324块/米(保留因子(k)4.24,萘,0.25 mm i.d.)。麦克雷诺常数计算结果显示DPFP固定相属中等极性。溶剂化参数模型结果显示DPFP固定相与溶质之间的主要作用力为偶极-诱导偶极作用力、氢键碱性作用力。Grob试剂分离结果显示DPFP色谱柱具有良好的选择性与惰性。另外,芳香族同分异构体、苯取代物、多环芳烃、脂肪酸酯及脂肪醇都得到了良好的分离,表明DPFP固定相在应用方面有巨大的潜力。     

甘脲用作气相色谱固定相的色谱性能研究

甘脲具有双环双脲结构,既是质子给体,又是质子受体,能与溶质产生氢键作用等多种作用力。本文制备了以甘脲作为固定相的填充柱,并对它的色谱性能进行了研究。结果表明:甘脲固定相热稳定性高、柱性能稳定,是一种良好的气相色谱固定相。该固定相对烷烃、卤代烃、芳烃、醇、酯、酮、酸、胺等类物质具有良好的分离能力,尤其是对位置异构体(如二取代苯位置异构体)有较好的分离选择性。本文还初步探讨了甘脲固定相的分离机理。     

Retention Mechanism Studies of Selected Amino Acids and Vitamin B6 on HILIC Columns with Evaporative Light Scattering Detection

The goal of the study was to investigate separation mechanism of selected “essential” amino acids (leucine, isoleucine, threonine, tryptophan, proline, and glycine) and vitamin B6 in hydrophilic interaction liquid chromatography (HILIC) with the evaporative light scattering detection. Chromatographic measurements were made on three different HILIC columns: amide-silica (TSK-gel Amide-80), amino-silica (TSK-gel NH₂-100), and cross-linked diol (Luna HILIC). The retention behaviour of the analytes was investigated as a function of different binary hydro-organic mobile phases containing 10–90 % (v/v) acetonitrile. The compounds studied were separated under isocratic and gradient conditions. The best results of tested biologically active compounds separation were obtained on the TSK-gel NH₂-100 column. TSK-gel NH₂ column showed mixed HILIC–ion-exchange mechanism, the highest separation efficiency and better selectivity and resolution for tested analytes than the other studied column, especially at concentration of water in mobile phase lower than 30 % (v/v). Special attention was dedicated to the study of interactions among the stationary phase, mobile phase and the analytes.     

Evaluation and comparison of a methylated teicoplanin aglycone to teicoplanin aglycone and natural teicoplanin chiral stationary phases

HPLC enantiomeric separations of a wide variety of racemic analytes was evaluated using chiral stationary phases (CSPs) based on the macrocyclic glycopeptides teicoplanin (T), teicoplanin aglycone (TAG), and methylated teicoplanin aglycone (Me-TAG) in two different mobile phase modes, i.e., the RP mode and the polar organic (PO) mode. Comparison of the enantiomeric separations using Chirobiotic T, Chirobiotic TAG, and the methylated form of TAG were conducted in order to gain a better understanding of the roles of the polar functional groups on the CSP. Substantial effects due to the cleavage of saccharides and/or methylation on chiral separations were observed in both separation modes. Improved separation efficiencies for many acidic analytes were obtained by methylating the H-bonding groups of TAG. These groups were believed to be a contributing factor to band broadening on TAG due to their negative effect on mass transfer between the stationary phase and mobile phase. Ionic/dipolar interactions between the carboxylate group of the analytes and the amine groups on T, TAG, or Me-TAG are important for chiral discrimination. Therefore, analytes possessing a carboxyl group are good candidates for successful separations on these CSPs. Hydrophobic interactions are important for enantiomeric separations in the RP mode where the H-bonding interactions between analytes and the chiral selectors are relatively weak. Me-TAG offers higher hydrophobicity, which can accentuate the interactions of analytes with hydrophobic moieties, but these interactions are not necessarily stereoselective. In the PO mobile phase, electrostatic/dipolar interactions between polar functional groups are the dominating interactions in chiral recognition. Another important factor is steric fit, which could be changed with every modification of the T structure. Therefore, substantial changes of enantioseparations were obtained within this studied group of CSPs. The PO mode was shown to be the most powerful mobile phase mode for enantiomeric separations on T-based stationary phases, mainly due to the improved efficiency. Methylation of the TAG proved to be a very useful tool for investigating the chiral recognition mechanism for this group of chiral selectors.     

Retention Mechanism Studies of Selected Amino Acids and Vitamin B6 on HILIC Columns with Evaporative Light Scattering Detection

The goal of the study was to investigate separation mechanism of selected “essential” amino acids (leucine, isoleucine, threonine, tryptophan, proline, and glycine) and vitamin B6 in hydrophilic interaction liquid chromatography (HILIC) with the evaporative light scattering detection. Chromatographic measurements were made on three different HILIC columns: amide-silica (TSK-gel Amide-80), amino-silica (TSK-gel NH₂-100), and cross-linked diol (Luna HILIC). The retention behaviour of the analytes was investigated as a function of different binary hydro-organic mobile phases containing 10–90 % (v/v) acetonitrile. The compounds studied were separated under isocratic and gradient conditions. The best results of tested biologically active compounds separation were obtained on the TSK-gel NH₂-100 column. TSK-gel NH₂ column showed mixed HILIC–ion-exchange mechanism, the highest separation efficiency and better selectivity and resolution for tested analytes than the other studied column, especially at concentration of water in mobile phase lower than 30 % (v/v). Special attention was dedicated to the study of interactions among the stationary phase, mobile phase and the analytes.

     

Ionic liquid‐bonded polysiloxane as stationary phase for capillary gas chromatography

Ionic liquid (IL) stationary phase is especially suitable for separation of complex samples, owing to the “dual nature” of IL. In this study, a synthetic method of ionic liquid‐bonded polysiloxane (PSOIL) as stationary phase of GC was proposed. Then, the PSOIL was used to prepare an 8 m capillary column by static method. The column efficiency was measured to be about 4000 plates/m (k=3.55, naphthalene) after the column had been conditioned at 210°C. The durability of PSOIL column was better than that of the mixed stationary phase of IL and OV‐1. Moreover, the Abraham solvation parameter model was employed to characterize the PSOIL. The result revealed that the PSOIL had stronger dispersion force (l) than neat IL and stronger hydrogen bond basicity (a) than DB‐1. That meant the PSOIL might offer good selectivity for both polar and non‐polar analytes. The column exhibited unique selectivity for various organic substances, such as the homologous compounds of alkanes, esters, alcohols and aromatic compounds. It was also found that some aromatic positional isomers could be separated better on the PSOIL column than on the DB‐1 column. Furthermore, the stationary phase was suitable for separation of high‐boiling point compounds such as polycyclic aromatic hydrocarbons, phthalic esters, etc. All of these demonstrated that the PSOIL offered good selectivity and high separation efficiency for a wide range of analytes.     

用强阴离子交换毛细管电色谱分离阴离子化合物

对阴离子性化合物在强阴离子交换毛细管电色谱中的保留行为进行了研究。发现样品中固定相上的吸附使样品的保留因子k^*变小,柱小的分离能力减小;而电压的增大,使酸性样品的k^*也增大,并且电压的改变也能改变分离的选择性;样品保留因子的对数值随着缓冲液离子强度的对数值的增大而线性减小;样品也强阴子交换毛细管电谱和毛细管区带电泳中有不同的保留行为。     

Influence of chromatographic conditions on separation in comprehensive gas chromatography

Comprehensive gas chromatography (GC x GC) is now established as a powerful technique, which offers unprecedented separation power. For complex samples, the distribution of peaks in the two-dimensional (2D) space still may need to be optimised. Since temperature (T) is a critical variable, and compounds can be shifted in relative positions on column 1 arising from temperature program rate (rT) changes, and since retention in the second dimension, D2 (2tR) is likewise affected by the prevailing T (elution temperature; Te), then any factors which alter Te will affect the extent of separation in D2. Since temperature program and carrier gas velocity rate will affect the Te of the solutes, these two factors are considered in this paper. Apart from these two parameters, results of different stationary phase choice for the second dimension column as well as the second dimension column length are reported. rT is found to have the most profound impact on the Te of solutes and will be more likely to cause an inversion of elution order if such behaviour can occur. Peak widths and 2tR increase with a decrease in Te. On the other hand, flow-rate has less impact on peak widths and 2tR although Te is affected by a change in flow-rate. Specific solute-stationary phase interactions will cause the elution order of certain solutes to be altered, and may be observed when a different stationary phase is employed as the second column, depending on the solute-stationary phase separation mechanism. Experiments conducted on different second dimension column length showed that although a longer column will lead to better separations, wrap-around may confound the separation process and may cause the solutes from sequential modulation events to co-elute. Thus a suitable second dimension column phase and length must be employed in order to obtain good separation. The factors investigated in this study will cause different extents of changes in the solute elution order and solute separations, and will affect the 2D contour presentation.     

Supramolecular separation mechanism of pentafluorophenyl column using ibuprofen and omeprazole as markers: LC‐MS and simulation study

The pentafluorophenyl (PFP) column is emerging as a new advancement in separation science to analyze a wide range of analytes and, thus, its separation mechanism at supramolecular level is significant. We developed a mechanism for the separation of ibuprofen and omeprazole using different combinations (ranging from 50:50 to 60:40) of water–acetonitrile containing 0.1% formic acid as the mobile phase. The column used was Waters Acquity UPLC HSS PFP (75 × 2.1 mm, 1.8 μm). The reverse order of elution was observed in different combinations of the mobile phases. The docking study indicated hydrogen bonding between ibuprofen and PFP stationary phase (binding energy was −11.30 kJ/mol). Separation at PFP stationary phase is controlled by hydrogen bonding along with π–π interactions. This stationary phase may be used to analyze both aromatic and aliphatic analytes. The developed mechanism will be useful to separate various analytes by considering the possible interactions, leading to saving of energy, time and money. In addition, this work will be highly useful in preparative chromatography where separation is the major problem at a large scale. Moreover, the developed LC‐MS‐QTOF method may be used to analyze ibuprofen and omeprazole in an unknown sample owing to the low value of detection limits.     

Chromatographic behavior of a new hybrid type RP material containing silica bonded 1,3‐alternate 25,27‐bis‐[cyanopropyloxy]‐26,28‐bis‐[3‐propyloxy]‐calix[4]arene

A novel 1,3‐alternate 25,27‐bis‐[cyanopropyloxy]‐26,28‐bis‐[3‐propyloxy]‐calix[4]arene‐bonded silica gel stationary phase (CalixPrCN) was prepared and its structure was confirmed by ATR‐FTIR spectroscopy and elemental analysis. The CalixPrCN phase was characterized in terms of its surface coverage, hydrophobic selectivity, aromatic selectivity, shape selectivity, hydrogen bonding capacity, residue metal content, and silanol activity based on Tanaka, Lindner, and SMR 870 test protocols. The effect of the acetonitrile content on the retention and selectivity of the selected neutral, basic, and acidic solutes was studied. The neutral and acidic analytes exhibited classical RP behavior, in which retention time decreases with increasing acetonitrile content. In contrast, basic analytes showed an increase in retention at low and high percentages of acetonitrile, forming “U‐shaped” retention profiles. The new calixarene phase was compared with previously reported 1,3‐alternate 25,27‐bis‐[propyloxy]‐26,28‐bis‐[3‐propyloxy]‐calix[4]arene stationary phase and commercial cyanopropyl column. The results indicate that the CalixPrCN stationary phase behaves like RP packing; however, inclusion complex formation, dipole–dipole, and π–π interactions seem to be involved in the separation process. The selectivity of this phase was demonstrated in separation of polynuclear aromatic hydrocarbons, non‐steroidal anti‐inflammatory drugs, and sulfonamides as analytes.     

Polyacrylamide-based monolithic capillary column with coating of cellulose tris(3,5-dimethylphenyl-carbamate) for enantiomer separation in capillary electrochromatography

A hydrophilic chiral capillary monolithic column for enantiomer separation in CEC was prepared by coating cellulose tris(3,5-dimethylphenyl-carbamate) (CDMPC) on porous hydrophilic poly(acrylamide-co-N,N'-methylene-bisacrylamide) (poly(AA-co-MBA)) monolithic matrix with confine of a fused-silica capillary. The coating conditions were optimized to obtain a stable and reproducible chiral stationary phase for CEC. The effect of organic modifier of ACN in aqueous mobile phase for the enantiomer separation by CEC was investigated, and the significant influence of ACN on the enantioresolution and electrochromatographic retention was observed. Twelve pairs of enantiomers including acidic, neutral, and basic analytes were tested and nine pairs of them were baseline-enantioresolved with acidic and basic aqueous mobile phases. A good within-column repeatability in retention time (RSD = 2.4%) and resolution (RSD = 3.2%) was obtained by consecutive injections of a neutral compound, benzoin, on a prepared chiral monolithic column, while the between-column repeatability in retention time (RSD = 6.4%) and resolution (RSD = 9.6%) was observed by column-to-column examination. The prepared monolithic stationary phase showed good stability in either acidic or basic mobile phase.