Characterization of 1,3-alternate calix[4]arene-silica bonded stationary phases and their comparison to selected commercial columns by using principal component analysis

Twelve calix[4]arene stationary phases in 1,3-alternate conformation, synthesized in the authors’ laboratory, were characterized in terms of their surface coverage, hydrophobic selectivity, aromatic selectivity, shape selectivity, hydrogen bonding capacity and ion-exchange capacity. The set of tests commonly used for evaluation of commercially available stationary phases was applied to assess fundamental chromatographic properties of the calixarene phases. The new calixarene phases were compared to each other, to Caltrex and LiChrosorb C-18 columns. Principal component analysis has been used to provide comparison between 1,3-alternate calix[4]arene phases and commercially available phenyl, fluorophenyl and fluoroalkyl columns.     

Chromatographic classification and comparison of commercially available reversed-phase liquid chromatographic columns using principal component analysis

A total of 135 commercially available alkyl, cyano, phenyl, perfluorinated, polar embedded, enhanced polar selectivity (i.e., polar/hydrophilic endcapped), "Aqua type" and a variety of novel phases including some non-silica based stationary phases have been characterised in terms of their surface coverage, hydrophobic selectivity, shape selectivity, hydrogen bonding capacity and ion-exchange capacity at pH 2.7 and 7.6. Principal component analysis has been used to provide a simple graphical comparison of the differences/similarities between columns in the entire database and differing subsets such as "Aqua type"/enhanced polar selectivity phases. The PCA has been correlated to the phase's ability to analyse a range of hydrophilic bases.     

π-Selective stationary phases: (III) Influence of the propyl phenyl ligand density on the aromatic and methylene selectivity of aromatic compounds in reversed phase liquid chromatography

The retention characteristics of phenyl type stationary phases for reversed phase high performance liquid chromatography are still largely unknown. This paper explores the retention process of these types of stationary phases by examining the retention behaviour of linear PAHs and n-alkylbenzenes on a series of propyl phenyl stationary phases that have changes in their ligand density (1.23, 1.31, 1.97, 2.50 μmol m⁻²). The aromatic and methylene selectivities increased with increasing ligand density until a point where a plateau was observed, overall the propyl phenyl phases had a higher degree of aromatic selectivity than methylene selectivity indicating that these columns are suitable for separations involving aromatic compounds. Also, retention characteristics relating to the size of the solute molecule were observed to be influenced by the ligand density. It is likely that the changing retention characteristics are caused by the different topologies of the stationary phases at different ligand densities. At high ligand densities, the partition coefficient became constant.     

Chromatographic retention behaviour, modelling and separation optimisation of the quaternary ammonium salt isometamidium chloride and related compounds on a range of reversed-phase liquid chromatographic stationary phases

This paper describes the reversed-phase liquid chromatographic behaviour of the trypanocidal quaternary ammonium salt isometamidium chloride and its related compounds on a range of liquid chromatographic phases possessing alkyl and phenyl ligands on the same inert silica. In a parallel study with various extended polar selectivity phases which possessed different hydrophobic/silanophilic (hydrogen bonding) activity ratios, the chromatographic retention/selectivities of the quaternary ammonium salts was shown to be due to a co-operative mechanism between hydrophobic and silanophilic interactions. The highly aromatic and planar isometamidium compounds were found to be substantially retained on stationary phases containing aromatic functionality via strong π-π interactions. The chemometric approach of principal component analysis was used to characterise the chromatographic behaviour of the isometamidium compounds on the differing phases and to help identify the dominant retention mechanism(s). Two-dimensional (temperature/gradient) retention modelling was employed to develop and optimise a rapid liquid chromatography method for the separation of the six quaternary ammonium salts within 2.5 min which would be suitable for bioanalysis using liquid chromatography-mass spectrometry. This is the first reported systematic study of the relationship between stationary phase chemistries and retention/selectivity for a group of quaternary ammonium salts.     

Liquid chromatographic behavior of two alanine‐substituted calix[4]arene‐bonded silica gel stationary phases

Two new kinds of alanine‐substituted calix[4]arene stationary phases of 5,11,17,23‐p‐tert‐butyl‐25,27‐bis(l ‐alanine‐methylester‐N‐carbonyl‐methoxy)‐26,28‐dihyroxycalix[4]arene‐bonded silica gel stationary phase (BABS4) and 5, 11, 17, 23‐p‐tert‐butyl‐25,26,27,28‐tetra(l ‐alanine‐methylester‐N‐carbonyl‐methoxy)‐calix[4]arene‐bonded silica gel stationary phase (TABS4) were prepared and characterized in the present study. They were compared with each other and investigated in terms of their chromatographic performance by using polycyclic aromatic hydrocarbons, disubstituted benzene isomers, and mono‐substituted benzenes as solute probes. The results indicated that both BABS4 and TABS4 exhibited multiple interactions with analytes. In addition, the commonly used Tanaka characterization protocol for the evaluation of commercially available stationary phases was applied to evaluate the properties of these two new functionalized calixarene stationary phases. The Tanaka test results were compared with Zorbax Eclipse XDB C₁₈ and Kromasil phenyl columns, respectively. BABS4 has stronger hydrogen‐bonding capacity and ion‐exchange capacity than TABS4, and features weaker hydrophobicity and hydrophobic selectivity. Both of them behave similarly in stereoselectivity. Both BABS4 and TABS4 are weaker than C₁₈ and phenyl stationary phases in hydrophobicity and hydrophobic selectivity.     

Additional studies on shape selectivity by using the carotenoid test to classify C18 bonded silica

Numerous chromatographic tests are applied to study the C18 bonded phases, either to classify these phases, or to better determine their properties or their chromatographic behaviours. Because the carotenoid test is developed in supercritical fluid chromatography (SFC), many correlations with high performance liquid chromatography (HPLC) results have been necessary to ensure the trueness of the classification reached in these analytical conditions. Consequently, the analytical conditions of the carotenoid test were chosen to fit with the TbN/BaP values from the NIST 869a test, which describe the shape selectivity of the alkyl bonded phases. Additional studies performed in this paper by using well standardized silica (J'Sphere, YMC Pack ODS A, Wakosil II), which varied from their surface coverage (J'Sphere), their pore shape (YMC), or their bonding type and carbon content (Wakosil II). The use of these C18 stationary phases allows to reach more accurate conclusion on the comparison of the shape selectivity values provided either by the cis/trans β-carotene selectivity or by the TRI/oTER or TbN/BaP ones. The extension of the studies to many other C18 stationary phases allows clarifying the relationships between the carotenoid test and the tests based on the use of PAH, as well as the usage limit of TRI/oTER and TBN/BaP selectivity in regards of the bonding density of the stationary phases for both monomeric and polymeric phases. By checking other aromatic compounds, another selectivity (anthracene/oTER) display greater correlation with the carotenoid test, that suggest an improvement of the relevance of this new compound couple for the steric selectivity study.     

聚4-甲基-5-乙烯基噻唑键合硅胶柱的选择性研究

 较为系统地研究了聚4甲基5乙烯基噻唑键合硅胶固定相(PMVCAphase)与C18,C8及苯基柱在反相色谱中甲醇水体系下的选择性差别。结果表明,该固定相与常用反相色谱固定相有相似性,显示了一般反相色谱固定相填料的特性;另一方面,又由于其特殊结构,更多地显示了其在反相色谱上的特殊选择性。     

气相色谱研究杯[4]芳烃衍生物

选取两种不同结构的怀「４」芳烃做固定相，研究了烃基取代苯，不同数目的甲基取代苯，稠环芳烃和醇系列化合物的保留值变化规律；讨论了杯「４」芳烃固定相与溶质分子间的弥散力作用，氢键作用及空间适应性；指出杯「４」芳烃对芳香烃等良好选择性；对乙腈和甲醇等小分子似有包结。     

Influence of silica gel pretreatment and bonding technique on PAH selectivity of octadecyl bonded phases

Summary The effect of silica gel pretreatment and of different bonding techniques, on the chromatographic properties of octadecyl bonded phases is described. The experiments show that a monomeric octadecyl bonded phase, prepared from silica gel that has not been treated before bonding, has a poor selectivity towards polycyclic aromatic hydrocarbons (PAHs) and shows high adsorption of basic components. Silica gel treatment before the bonding of the monofunctional silane does not give a substantial improvement. Polymeric phases on the other hand show a much better PAH selectivity and a lower adsorptivity towards basic components. Further improvement can be achieved with the polymeric modification if silica gel treatment is carried out before bonding.     

Phenyl‐bonded stationary phases—The influence of polar functional groups on retention and selectivity in reversed‐phase liquid chromatography

The chromatographic properties of four phenyl‐bonded phases with different structures were studied. The columns used were packed with a stationary phase containing a phenyl ring attached to the silica surface using different types of linkage molecules. As a basic characteristic of the bonded phases, the hydrophobicity and silanol activity (polarity) were investigated. The presence of the polar amino and amide groups in the structure of the bonded ligand strongly influences the polarity of the bonded phase. Columns were compared according to methylene selectivity using a series of benzene homologues and according to their shape and size selectivity using polycyclic aromatic hydrocarbons. The measurements were done using methanol/water and acetonitrile/water mobile phases. The presented results show that the presence of polar functional groups in the ligand structure strongly influences the chromatographic properties of the bonded phase.     

Investigating retention characteristics of a mixed‐mode stationary phase and the enhancement of monolith selectivity for high‐performance liquid chromatography

The synthesis and chromatographic behavior of an analytical size mixed‐mode bonded silica monolith was investigated. The monolith was functionalized by an in situ modification process of a bare silica rod with chloro(3‐cyanopropyl)dimethyl silane and chlorodimethyl propyl phenyl silane solutions. These ligands were selected in order to combine both resonance and nonresonance π‐type bonding within a single separation environment. Selectivity studies were undertaken using n‐alkyl benzenes and polycyclic aromatic hydrocarbons in aqueous methanol and acetonitrile mobile phases to assess the methylene and aromatic selectivities of the column. The results fit with the linear solvent strength theory suggesting excellent selectivity of the column was achieved. Comparison studies were performed on monolithic columns that were functionalized separately with cyano and phenyl ligands, suggesting highly conjugated molecules were able to successfully exploit both of the π‐type selectivities afforded by the two different ligands on the mixed‐mode column.     

液晶性芳香酰胺化合物的合成

合成了一系列炖粹以酰胺基为中心桥键的刚性芳香酰胺小分子化合物，并对其作了表征，发现其中有些化合物具有液晶性。酰胺键之间能形成很强的分子间氢键，使芳香酰胺小分子化合物的熔点很高，难于形成液成液晶态。研究发现，如果在这类化合物的中心苯环上引入合适的取代基以减弱分子间氢键，同时引入合适的末端基时，则可使芳香酰胺化合物生成液晶相的能力增强。     

π-Selective stationary phases: (I) Influence of the spacer chain length of phenyl type phases on the aromatic and methylene selectivity of aromatic compounds in reversed phase high performance liquid chromatography

Phenyl type stationary phases of increasing spacer chain length (phenyl, methyl phenyl, ethyl phenyl, propyl phenyl and butyl phenyl, with 0–4 carbon atoms in the spacer chain, respectively) were synthesised and packed in house to determine the impact that the spacer chain length has on the retention process. Two trends in the aromatic selectivity, q_aromatic, were observed, depending on whether the number of carbon atoms in the spacer chain is even or odd. Linear log k′ vs ? plots were obtained for each stationary phase and the S coefficient was determined from the gradient of these plots. For the phenyl type phases, the S vs n_c plots of the retention factors of linear polycyclic aromatic hydrocarbons vs the number of rings exhibit a distinct discontinuity that between 3 and 4 rings, which increases with increasing spacer chain length for even phases but decreases for odd phases. Accordingly, we suggest that the retention factors depend differently on the number of carbon atoms in the spacer chain depending on whether this number is even or odd and that this effect is caused by different orientations of the aromatic ring relative to the silica surface.     

Retention properties of novel β-CD bonded stationary phases in reversed-phase HPLC mode

With the given special structures, the CD bonded stationary phases are expected to have complementary retention properties with conventional C18 stationary phase, which will be helpful to enhance the polar selectivity in RP mode separation. In this work, two β-cyclodextrin (β-CD) bonded stationary phases for reversed-phase HPLC, including 1, 12-dodecyldiol linked β-CD stationary phase (CD1) and olio (ethylene glycol) (OEG) linked β-CD stationary phase (CD2), have been synthesized via click chemistry. The resulting materials were characterized with FT-IR and elemental analysis, which proved the successful immobilization of ligands. The similarities and differences in retention characteristics between the CD and C18 stationary phases have been elucidated by using comparative linear solvation energy relationships (LSERs). The force related to solute McGowan volume has no significant difference, while the hydrogen bonding and dipolar interactions between solutes and CD stationary phases are stronger than between solutes and C18, which is attributed to the special structures (CD and triazole groups) of CD stationary phases. Chemical origins are interpreted by comparison between CD1 and CD2. Similar dispersive interactions of CD1 and CD2 are attributed to their similar length of spacer arms. CD2 which contains OEG spacer arm has relative weaker HBD acidity but stronger HBA basicity. CD stationary phases display no serious different methylene selectivity and higher polar selectivity than in the case of C18. Higher acid selectivity and lower basic selectivity are observed on CD2 than on CD1. Distinctive retention properties and good complementary separation selectivity to C18 make the novel CD bonded stationary phases available for more application in RPLC.     

杯[4]芳烃衍生物气相色谱固定相分离芳烃异构体的热力学/超热力学研究

用热力学／超热力学方法对杯[4]芳烃衍生物气相色谱固定相上芳烃位置异构体的分离进行了研究。考察了一系列芳烃位置异构体在杯芳色谱柱上分离过程的热力学参数及其选择性之间的关系,并对杯[4]芳烃衍生物气相色 谱固定相分离芳烃位置异构体的保留机理进行了探讨。     

Polarizable biphenyl polysiloxane stationary phase for capillary column gas chromatography

A crosslinkable biphenylmethylpolysiloxane stationary phase was synthesized for capillary column gas chromatography and compared with methyl, phenyl, and cyanopropyl polysiloxane stationary phases for the separation of isomeric polycyclic aromatic compounds. While the new phase gave similar separations of nonpolar isomers when compared to the nonpolar phases, separations of polar isomers were greatly improved because of the induced polarity of the biphenyl group of the stationary phase by the solute molecules. This polarizable stationary phase offers a unique selectivity which is not available in other stationary phases.     

Propylamido-Modified C18 Reversed-Phase for High-Performance Liquid Chromatography

The synthesis and chromatographic properties of a propylamido-modified C18 stationary phase are described. The propylamido-modified C18 phase was prepared by bonding stearic acid to 3-aminopropyl-silica gel. The synthesis was simple and reproducible through amide formation by using N-hydroxysuccinimide and dicyclohexylcarbodiimide. This prepared stationary phase exhibited excellent abilities to separate polycyclic aromatic hydrocarbons (PAHs), polysiloxanes and phenols by HPLC. The chromatographic results show that both the C3 moiety and the amido-group in this phase contribute significantly to the HPLC separation process. The chromatographic behavior of the prepared phase and commercial C18 phases was also compared in analyses of PAHs and phenols. Although the commercial C18 phases have been extensively used, both the unique selectivity and the highly reproducible synthesis of our prepared phase make it an excellent complement to ordinary C18 phases.     

Reversed-phase HPLC of peptides: Assessing column and solvent selectivity on standard, polar-embedded and polar endcapped columns

We desired to evaluate the chromatographic selectivity for peptides of silica-based RP high-performance liquid chromatography stationary phases with various modifications (polar embedding and polar endcapping on C(18) columns; ether-linked phenyl column with polar endcapping) compared with n-alkyl (C(18), C(8)) and aromatic phenylhexyl columns. Thus, we have designed and synthesized two series of synthetic peptide standards with the sequence Gly-Gly-Leu-Gly-Gly-Ala-Leu-Gly-X-Leu-Lys-Lys-amide, where the N-terminal either contains a free α-amino group (AmC series) or is N(α)-acetylated (AcC series) and where position X is substituted by Gly, Ala, Val, Ile, Phe or Tyr. These represent series of peptides with single substitutions of n-alkyl (Gly相似文献   

Development of three end-capped para-benzoyl calix[4,6, or 8]arene bonded stationary phases for HPLC

Three end-capped para-benzoyl calixarene bonded silica gel stationary phases are prepared and characterized by elemental analysis, infrared spectroscopy, and thermal analysis. The comparison and selectivity of these phases are investigated by using PAHs, disubstituted benezene, and naphthalene positional isomers as probes. Possible separation mechanism based on the different interactions between calixarenes and analytes are discussed. The results indicate that the separation for those analytes are influenced by the supramolecular interaction including π-π interaction, π-electron transfer interactions, space steric hindrance, and hydrogen bonding interaction on the calixarene columns. Importantly, the aromatic probes with polar groups such as -OH, -NO(2), and -NH(2) could regulate the selectivity of calixarene-bonded stationary phases.