Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of temperature

This study represents the first time that both the mobile phase composition and the temperature are simultaneously controlled to examine silica-bonded octadecylsilyl (C18) ligands spectroscopically at typical liquid chromatographic (LC) mobile phase flow-rates and back-pressures. Raman spectroscopy is used to characterize the behavior of the C18 bonded ligands equilibrated at temperatures from 45 to 2 degrees C in neat, single-component, mobile phase solvents including: water, acetonitrile, methanol, and chloroform. In addition, the effect of stationary phase ligand bonding density is examined by using two different monomeric reversed-phase liquid chromatographic (RPLC) stationary phases, a 2.34 and a 3.52 micromol m(-2) Microporasil C18 stationary phase, under identical conditions. The direct, on-column, spectroscopic analysis used in this study allows direct evaluation of the temperature-dependent behavior of the bonded C18 ligands. The temperature-dependent ordering of the stationary phase ligands is examined to determine if the ligands undergo a phase transition from a less-ordered "liquid-like" state at higher temperatures to a more-ordered "solid-like" state at lower temperatures. A discrete phase transition was not observed, but rather a continual ordering as temperature was lowered.     

Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of mobile phase composition

Raman spectroscopy is used to examine the effect of mobile phase composition on the orientation of octadecyl-bonded silica-based reversed-phase liquid chromatographic (RPLC) stationary phase ligands. The effect of ligand bonding density is also investigated. The present experimental set-up utilizes a direct, noninvasive, on-column approach to examine the solvent dependent conformational behavior of the bonded ligands under flow-rate and back pressure conditions similar to those used during conventional RPLC measurements. Neat, single-component, mobile phase solvents including water, acetonitrile, methanol and chloroform are used to investigate the hypothesized collapsing and extension of stationary phase ligands with changes in mobile phase composition. No evidence of phase collapse was observed upon changing the mobile phase composition from an organic to an aqueous content. Also, Raman spectroscopic measurements allowed the differentiation between associated and free acetonitrile solvent.     

Reversed-phase liquid chromatographic separation of enantiomers on polysaccharide type chiral stationary phases

The direct chiral separation by chiral stationary phases (CSPs) is one of the most important techniques to analyze enantiomeric purity as well as to get enantiomerically pure material quickly. Among various types of CSPs, polysaccharide type CSPs are well known by their versatility and durability. They are not only effective under normal-phase conditions, but also under reversed-phase conditions. In order to get a good separation under the reversed-phase conditions, it is the key to choose an appropriate mobile phase. For example, a simple mixture of water/acetonitrile or water/methanol are sufficient for a neutral analyte, while it is necessary to use an acidic solution instead of water for an acidic analyte and a solution of a chaotropic salt (or a basic solution) for a basic analyte, respectively. The paper also presents lists of more than 350 separation examples that include 22 validated methods for drug analyses from serum, plasma, or urine samples on polysaccharide type CSPs under reversed-phase conditions.     

Chiralpak AD-H和Chiralcel OJ-H手性固定相拆分扁桃酸系列化合物

对比了商品化的淀粉型手性固定相Chiralcel OJ-H和纤维素型手性固定相Chiralpak AD-H柱在正相条件下对扁桃酸系列8个化合物的拆分,结果表明Chiralcel OJ-H柱对扁桃酸系列化合物具有更强的手性识别能力,8个外消旋扁桃酸化合物在36 min内都得到了基线分离。研究发现,扁桃酸苯环上的取代基对其拆分的难易程度影响很大,其电子诱导效应影响扁桃酸类化合物在固定相上的保留时间,其空间位阻效应是扁桃酸在固定相上被拆分成败的决定因素。通过对比分析扁桃酸和手性柱的结构,探讨了可能的手性拆分机理是基于Chiralpak AD-H(Chiralcel OJ-H)手性固定相和扁桃酸系列化合物之间的氢键-氢键、偶极-偶极、π-π电子相互作用以及空间适应性等诸多因素的综合影响,其中空间适应性起到至关重要的作用。本研究可为一些实际光学活性扁桃酸及其类似物的对映体纯度测定与拆分研究提供参考。     

Structure-function relationships in high-density docosylsilane bonded stationary phases by Raman spectroscopy and comparison to octadecylsilane bonded stationary phases: effects of aromatic compounds

The effects of four aromatic compounds on alkyl chain conformational order for a series of high-density docosylsilane (C22) stationary phases with surface coverage ranging from 3.61 to 6.97 micromol/m2 are investigated using Raman spectroscopy. Aromatic compounds studied include benzene-d6, toluene-d8, aniline-d7 and anisole-d8. In general, these aromatic solvents decrease the conformational order of the C22 phases relative to air suggesting partitioning of the aromatics into the alkyl chains of these stationary phases. Changes in alkyl chain conformational order are linearly dependent on the solvent hydrophobicity parameter, log K ow, and are also dependent on stationary phase properties (i.e. polymerization method and surface coverage). A comparison is made between C22 and C18 bonded phase systems. The conformational order of the alkyl chains in a mixed solute/mobile phase system is also studied using 80% methanol/water as the mobile phase and aniline-d7, anisole-d8 or toluene-d8 as solutes. Collectively, the Raman spectroscopic evidence at the molecular level suggests interaction of these aromatic species with the bonded alkyl chains through partitioning.     

Preparation and evaluation of new Pirkle type chiral stationary phases with long alkyl chains for the separation of amino acid enantiomers derivatized with NBD-F.

In order to improve the high-performance liquid chromatographic separation of alpha-amino acids derivatized with the fluorogenic reagent 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) on commercially available chiral stationary phases (CSPs) such as SUMICHIRAL OA-2500(S) (CSP 1) and OA-4700 (CSP 3), the preparation of two new CSPs (CSP 2 and CSP 4) having 11-aminoundecanoic acid between the aminopropyl silica gel support and the chiral moiety in CSP 1 and CSP 3 is described. CSP 2 and CSP 4 improved both the mutual and enantiomeric separation of NBD-amino acids compared with CSP 1 and CSP 3. Thus, 17 pairs of NBD-amino acid enantiomers and NBD-glycine were separated on CSP 2 except for six NBD-amino acids (D-Asn, D-Ser, D-Gln, L-Pro, L-Ser and Gly). CSP 2 and CSP 4 also showed better enantiomeric separation of NBD-amino acid esters and amides than CSP 1 and CSP 3. It was considered that the achiral long alkyl chains in the CSPs might form a hydrophobic space which assisted the stereoselective interaction of analytes with the chiral moiety by changing the environment around the chiral moiety. On CSP 1 and CSP 2, NBD-beta-amino acid was also enantiomerically separated.     

Examination of covalently bound polymeric stationary phases by luminescence spectroscopy

Luminescnce spectroscopy is used to prove silica-gel surfaces derivatized with [3-(3-pyrenyl)propyl]methyldichlorisilane. Evidence of solution pre-polymerizatiln and subsequent derivatization to acceisibl surface silanols is presented. Conformatonal changes of the bound ligands in polar and non-polar solvents indicate a dynamic surface which minimized its surface area in hostile solvents.     

Stabilization of gas chromatographic stationary phases with nanosized particles

Summary Stabilization of polar and apolar stationary phases with nanosized particles has been demonstrated for gas chromatographic capillary columns. Mixtures of small amounts of (2–10%) of solid particles and liquids form gels, which preserve the stability of the films of stationary phases on the column walls. The stabilization effect of polar and apolar particles has been studied by determining some rheological properties of gels (flow curves, temperature dependence of yield stress values) and validated by gas chromatographic measurements using the most polar and apolar stationary phases. Dedicated to the memory of late Dr. Gábor Alexander, who was the inventor of this film stabilization method, and took active part in the early experiments.     

Surface bonding on silicon surfaces as probed by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been used to obtain the Raman signal of surface species on silicon single crystal surfaces without the necessity for surface enhancement by addition of Ag nanoparticles. By illuminating the hydrogen terminated silicon surface covered with a droplet of 4-vinylpyridine with UV light, a 4-ethylpyridine modified silicon surface can be easily obtained. By bringing a scanning tunneling microscope (STM) Au tip with a nanoscale tip apex to a distance of ca. 1 nm from the m...     

Gas chromatographic retention of alkyl phosphates on ionic liquid stationary phases

Retention behaviors of alkyl phosophates were studied on a series of ionic liquid gas chromatography columns. The selectivity of the IL columns for alkyl phosphates were compared with a 5% phenyl column as a route to evaluating the potential use of IL columns in the analysis of alkyl phosphates in petroleum samples in both one- and multi-dimensional GC. Most interestingly, we demonstrate for the first time the dependence of elution order on separation temperature for members of a homologous series of compounds. At low temperatures it was found that trihexyl phosphate eluted before trioctyl phosphate, while at higher temperatures this pattern was reversed.     

The dynamic chromatographic behavior of tri-o-thymotide on HPLC chiral stationary phases

The interconverting stereoisomers of tri-o-thymotide have been separated by HPLC on chiral stationary phases and the temperature dependence of the dynamic chromatographic patterns has been interpreted in terms of exchange between enantiomeric propeller and helical conformations. Computed low energy structures and CD spectra were used in absolute configuration assignment.     

Polysaccharide-based chiral stationary phases for high-performance liquid chromatographic enantioseparation

Recent developments of polysaccharide-based chiral stationary phases (CSPs) for the direct separation of enantiomers in high-performance liquid chromatography (HPLC) are mainly reviewed together with the results on mechanistic studies by means of chromatography, NMR and mass spectroscopies, and computational methods. Miscellaneous applications of polysaccharide derivatives to the newly developed, chiral dynamic high-performance liquid chromatography (DHPLC) for obtaining a nonracemic compound are also described.     

葫芦脲在色谱固定相中的研究进展

葫芦脲是超分子化学中继冠醚、环糊精、杯芳烃之后发展起来的又一类新型高度对称的桶状大环主体分子,是一种由多个甘脲单元组成的大环穴状配体,被誉为"第四代超分子化合物"。由于其具有特殊的分离选择性和稳定性,在超分子化学和色谱的交叉领域备受关注。本文从葫芦脲的结构特征出发,侧重概述了葫芦脲同系物及其衍生物在色谱固定相中的研究现状和进展。     

Liquid chromatographic separation of metal ions on several silica-bound chelating-agent stationary phases

Summary The separation of metal-ions on several new silica-bound azo-coupled chelating stationary phases is described. Retention on 2-methyl-8-quinolinol and N-(1-naphthyl)-ethylenediamine phases is in the order of known homogeneous-solution stability constants. Very short retention times were obtained on the 2-methyl-8-quinolinol phase compared to an 8-quinolinol phase. A reversed order of retention was observed on -diketone and dithizone stationary phases in tartrate mobile phase. Two different mechanisms of metal-column interaction appear to occur. Large pH and mobile-phase buffer concentration effects were exhibited, especially by the diketone and dithizone phases. An excellent separation of a six-metal mixture is demonstrated on a dithizone column.     

High-performance liquid chromatographic separation of dihydropyrimidine racemates on polysaccharide-derived chiral stationary phases

The chromatographic behavior of a set of racemic dihydropyrimidines (DHPMs) has been evaluated on two polysaccharide-derived chiral stationary phases under normal phase conditions. One of these is coated, the other chemically immobilized. The outstanding solvent compatibility of the immobilized chiral stationary phase (CSP) permits the use of solvents such as ethyl acetate (EtOAc) that are unsuitable for coated supports, for which traditional 2-propanol:hexanes mixtures have been employed. Drastic changes in the chromatographic retention and resolution of DHPMs and, in general, in the separation performances have been observed for the two systems. From a practical point of view, EtOAc has been proven to be a better choice for the separation of this important class of compounds. By comparing molecules different in specific positions of their scaffolds, hypotheses concerning the role of individual chemical groups on retention and selectivity have been done. These effects have been quantified, in some cases, in terms of standard Gibbs energy variations. Even if no chromatographic measurements have been made under nonlinear conditions, clear indications of the potential use of immobilized chiral adsorptive media operated with EtOAc:hexanes mixtures for preparative separations of DHPMs have been evidenced.     

Recent development of monolithic stationary phases with emphasis on microscale chromatographic separation

The column technologies play a crucial role in the development of new methods and technologies for the separation of biological samples containing hundreds to thousands compounds. This review focuses on the development of monolithic technology in micro-column formats for biological analysis, especially in capillary liquid chromatography, capillary electrochromatography and microfluidic devices in the past 5 years (2002-2007) since our last review in 2002 on monoliths for HPLC and CEC. The fabrication and functionalization of monoliths were summarized and discussed, with the aim of presenting how monolithic technology has been playing as an attractive tool for improving the power of existing chromatographic separation processes. This review consists of two parts: (i) the recent development in fabrication of monolithic stationary phases from direct synthesis to post-functionalization of the polymer- and silica-based monoliths tailoring the physical/chemical properties of porous monoliths; (ii) the application of monolithic stationary phases for one- and multi-dimensional capillary liquid chromatography, fast separation in capillary electro-driven chromatography, and microfluidic devices.