Supercritical fluid chromatography/Fourier transform infrared microspectrometry

The feasibility of supercritical fluid chromatography/Fourier transform-infrared (SFC/FT-IR) microspectrometry is presented. In this approach to SFC/FT-IR, the chromatographic eluates are aspirated from the restrictor directly onto the surface of a moving window which then passes into the beam focus of an infrared microscope. Because the mobile phase is gaseous at ambient conditions, elimination of the mobile phase is easily accomplished. Detection limits in all interfaces between a chromatograph and an FT-IR spectrometer in which the mobile phase is eliminated are determined in large part by the area over which the sample is deposited. We have shown that SFC eluates can be condensed at ambient temperature into spots of 100 to 200μm in diameter. The microscope interface therefore serves to increase the sensitivity of the SFC/FT-IR measurements of these spots and detection limits in the low nanogram range are possible. Preliminary results obtained before any real attempts were made to optimize the deposition process indicate that identifiable spectra can be obtained in real time at the 50 ng level for chromatographic separations performed with a 100μm i.d. wall coated open tubular column. Reproducible reconstructed chromatograms are obtained as each deposited eluate travels through the beam focus of the microscope. The concentration profile of deposited peaks was determined by IR measurements performed at 50 μm increments over the width of the peak to ascertain the deposition size. The results described in this paper, while not yet optimized, indicate the great potential of SFC/FT-IR microspectrometry.     

Use of on-line mass spectrometric detection in capillary electrochromatography

Capillary electrochromatography (CEC) is a liquid phase analytical separation technique that is generally carried out with packed capillary columns by electroosmotically driven mobile phase at high electric field strength. The analytes are separated by virtue of the differences in their distribution between the mobile and stationary phases and, if charged in their electrophoretic mobilities as well. It is thus considered a hybrid of liquid chromatography and capillary electrophoresis and is expected to combine the high peak efficiency of capillary zone electrophoresis (CZE) with the versatility and loading capacity of HPLC. This review explores the potential use of on-line mass spectrometric detection for CEC. It discusses key design issues that focus on the physical and electrical arrangement of the CEC column with respect to the electrospray orifice inlet. The salient features of the sheathless, sheath flow and liquid junction interfaces that are frequently employed while coupling a CEC column to an electrospray ionization mass spectrometry system are also highlighted. Possible configurations of the CEC column outlet that would obviate the need for pressurizing the capillary column are also presented. While coupling CEC with MS both the nature of the interface and the configuration of the column outlet will determine the optimal arrangement. The review also discusses bandspreading that occurs when a connecting tube is employed to transfer mobile phase from the column outlet to the atmospheric region of the electrospray source with a concomitant loss in sensitivity. Selected examples that highlight the potential of this technique for a wide range of applications are also presented.     

Electrostatic and electrochemical nature of liquid-gated electric-double-layer transistors based on oxide semiconductors

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics because of the large interfacial capacitance coupling and its ability for high-density charge accumulation. Much effort has recently been devoted to the fundamental understanding and practical applications of such highly charged EDL interfaces. However, the intrinsic nature of the EDL charging, whether it is electrostatics or electrochemistry, and how to distinguish them are far from clear. Here, by combining electrical transport measurements with electrochemical impedance spectroscopy (EIS), we studied the charging mechanisms of highly charged EDL interfaces between an ionic liquid and oxide semiconductor, ZnO. The direct measure for mobile carriers from the Hall effect agreed well with that from the capacitance-voltage integration at 1 Hz, implying that the pseudocapacitance does not contribute to carrier transport at EDL interfaces. The temperature-frequency mapping of EIS was further demonstrated as a "phase diagram" to distinguish the electrostatic or electrochemical nature of such highly charged EDL interfaces with densities of up to 8 × 10(14) cm(-2), providing a guideline for electric-field-induced electronic phenomena and a simple method for distinguishing electrostatic and electrochemical charging in EDLTs not only based on a specific oxide semiconductor, ZnO, but also commonly applicable to all types of EDL interfaces with extremely high-density carrier accumulation.     

Retention mechanisms in super/subcritical fluid chromatography on packed columns

Whereas the retention rules of achiral compounds are well defined in high-performance liquid chromatography, on the basis of the nature of the stationary phase, some difficulties appear in super/subcritical fluid chromatography on packed columns. This is mainly due to the supposed effect of volatility on retention behaviours in supercritical fluid chromatography (SFC) and to the nature of carbon dioxide, which is not polar, thus SFC is classified as a normal-phase separation technique. Moreover, additional effects are not well known and described. They are mainly related to density changes of the mobile phase or to adsorption of fluid on the stationary phase causing a modification of its surface. It is admitted that pressure or temperature modifications induce variation in the eluotropic strength of the mobile phase, but effects of flow rate or column length on retention factor changes are more surprising. Nevertheless, the retention behaviour in SFC first depends on the stationary phase nature. Working with polar stationary phases induces normal-phase retention behaviour, whereas using non-polar bonded phases induces reversed-phase retention behaviour. These rules are verified for most carbon dioxide-based mobile phases in common use (CO(2)/MeOH, CO(2)/acetonitrile or CO(2)/EtOH). Moreover, the absence of water in the mobile phase favours the interactions between the compounds and the stationary phase, compared to what occurs in hydro-organic liquids. Other stationary phases such as aromatic phases and polymers display intermediate behaviours. In this paper, all these behaviours are discussed, mainly by using log k-log k plots, which allow a simple comparison of stationary phase properties. Some examples are presented to illustrate these retention properties.     

Reversed-phase high-performance liquid chromatography of several metal-8-quinolinethiol complexes.

The reversed-phase high-performance liquid chromatography of 8-quinolinethiol (Hqt) complexes of Fe, Co, Ni, Cu, Zn, Hg and Pb on an octadecyl-bonded silica gel stationary phase was examined. The Pb complex dissociated in the column. The retention and separation of other complexes depended on the composition of the mobile phase. EDTA as an additive displaced the Zn complex and eliminated its peak. All the other metal complexes and also Hqt and its disulphide were separated in 23 min by using methanol-water (82:18, v/v) as the mobile phase. The complexes formed by the reaction of Co(II) and Hqt gave three peaks, which were assigned as fac(S)-CoIII(qt)3, mer(S)-CoIII(qt)3 and CoII(qt)2, respectively. This method is applicable to the simultaneous determination of Fe, Co, Ni, Cu and Hg.     

乳状液对色谱过程的影响

运用特征线法和多流动相色谱的概念,从理论上研究了在非线性色谱过程中流动相为乳状液时其溶质浓度波的类型与特征。当流动相为乳状液时,溶质在乳状液的内相和外相之间发生分配并同时在固定相上进行吸附,此时其浓度波图像与流动相为均相时有很大不同。结合相应的算例分析并讨论了表面活性剂在乳状液内相和外相间的分配关系及其在固定相上的吸附等温线为Langmuir型时其浓度波的各种运动图像及形成机理,并与流动相为均相时的色谱过程作了比较。分析结果表明,多流动相的存在可能使简单波“陡峭化”而成为激波,或使激波溃散为简单波。     

Comparison of thermospray and ion spray mass spectrometry in an atmospheric pressure ion source

Ion spray is an approach to liquid chromatography, mass spectrometry which includes features common to the electrospray and ion evaporation interfaces. Thermospray is a liquid chromatographic/mass spectrometric technique which utilizes heat and electrolytes in the mobile phase to generate sample ions. In this paper the operation of these two techniques at atmospheric pressure are compared with respect to the effects of solvent composition and electrolyte ion concentration for the production of ions from compounds that are ionized in solution (safranin orange, acid black 1 and testosterone sulfate) and un-ionized in solution (methyl red, adenosine and diethylstilbestrol). The results indicate that at atmospheric pressure ion spray produces ions by the ion evaporation mechanism while thermospray produces ions by both gas-phase chemical ionization and ion evaporation processes.     

Open-tubular capillary electrochromatography using a capillary coated with octadecylamine-capped gold nanoparticles

Octadecylamine-capped gold nanoparticles (ODA-Au-NPs) were prepared and characterized by using UV-Vis adsorption spectrum, transmission electron chromatography (TEM), SEM, and FT-IR. A simple but robust hydrophobic coating was easily developed by flushing a capillary with a solution of ODA-Au-NPs, because the positive charges were carried by the nanoparticles which strongly adsorb to the negatively charged inner surface of a fused-silica capillary via electrostatic and hydrophobic interactions. The chromatographic characteristics of the coated capillary was investigated by varying the experimental parameters such as buffer pH, buffer concentration, and percentage of organic modifier in the mobile phase. The results show that (i) resolution between thiourea and naphthalene is almost the same when comparing the electrochromatograms obtained using pH 7 buffer as mobile phase after and before the capillary column was operated using pH 11 and 3 mobile phase; (ii) no significant changes in retention time and deterioration in peak efficiency were found after 60 runs of test aromatic mixtures; and (iii) column efficiency up to 189 000 theoretical plates/meter for testosterone was obtained. All of the results indicated that the coating could act as a stable stationary phase for open tubular CEC as well as for bioanalysis.     

Supercritical fluid chromatography-mass spectrometry for chemical analysis

Chromatography with a supercritical fluid as the mobile phase was suggested more than four decades ago (Klesper, E., Corwin, A. H., Turner, D. A., J. Org. Chem. 1962, 27, 700-701). Supercritical fluid chromatography (SFC) is basically a hybrid of GC and LC that eases the resolution of a mixture of compounds not conveniently resolved by either GC or LC. The mobile phases for SFC have low viscosities and high diffusion coefficients compared to those for HPLC and allow for high efficiency separations. SFC uses supercritical fluid as the mobile phase, polar organic solvents as the modifiers in conjunction with acidic/basic compounds as additives to run the chromatographic process like in HPLC. In many applications, SFC-based methods are advantageous over HPLC-based methods as a separation tool in terms of efficiency and economical impact perspectives. Today, the availability of commercial hardware and API interfaces with a mass spectrometer makes SFC even more widely applicable for chemical analysis in many research fields. This review summarizes a variety of novel SFC-MS methods for chemical analysis that have been reported in the peer-reviewed publications.     

Secondary isotope effects in liquid chromatography behaviour of 2H and 3H labelled solutes and solvents

The separation of solutes that differ only in the extent of isotopic substitution of their hydrogen atoms, using either mixtures of isotopically non-modified or perdeuterated solvents as mobile phases, is described. The occurrence of a secondary isotope effect is demonstrated in reversed-phase liquid chromatography, which is independent of the nature of the stationary phase (different octadecyl-bonded silicas, an embedded alkylamide-bonded silica, as well as one polymeric stationary phase were tested), and the water content and the nature of organic modifier of the mobile phase. The separation of 24 structurally different isotopologue pairs (apolar compounds and polar compounds with exchangeable or non-exchangeable hydrogen atoms) is examined using reversed-phase liquid chromatography. It is found that the greater the number of isotopically substituted hydrogen atoms in a given organic solute, the better is the separation of a particular isotopologue pair. The single secondary isotope effect is shown to be dependent on the number of isotopic substitutions. The greater the number of these substitutions, the smaller is the single isotope effect. The single secondary isotope effect is higher for aromatic hydrocarbons than for aliphatic hydrocarbons. A secondary isotope effect is also observed in chiral chromatography and normal-phase liquid chromatography, as well as on changing the nature of the substituting isotope, i.e.: tritium instead of deuterium. Thus, we have demonstrated that the total secondary isotopic effect for hydrogen/tritium is higher than for hydrogen/deuterium. This isotope effect involves only the consequences of changes in interactions due to nuclear motions. Overall this study confirms the predominance of hydrophobic effects in retention processes in reversed-phase liquid chromatography. In reversed-phase liquid chromatography, a secondary isotope effect related to mobile phase composition is also observed. The behaviour of deuterium oxide and water in mobile phases of the same composition (%, w/w) is compared. Independent of the nature of the organic modifier (methanol, acetonitrile or ethanol), the effect of replacing H2O with 2H2O in the mobile phase, is an increase in the retention factors and an improvement in the chromatographic resolution of isotopologue pairs. This increase in the resolution is not accompanied by a change in the chromatographic selectivity. The measurement of liquid-liquid extraction coefficients proves that the effect is mainly due to the modification of the phase ratio. In general the effect of 2H-labelled solvents (2H2O and C2H3CN) as mobile phase components, compared to their isotopically non-modified isomers, can be rationalized on the basis of their lower polarisabilities. Overall the use of perdeuterated rather than isotopically non-modified solvents as mobile phase components leads to the most efficient separation systems.     

Separation of Tetracycline Antibiotics by Hydrophilic Interaction Chromatography Using an Amino-Propyl Stationary Phase

In this work the potential of hydrophilic interaction chromatography (HILIC) is explored for the analysis of tetracycline antibiotics. The choice of the polar stationary phase is first discussed and it is demonstrated that aminopropyl stationary phases lead to higher efficiencies and peak symmetry than bare silica ones. The influence of the composition of the mobile phase is studied next : the concentration of the weaker solvent (acetonitrile), the nature and concentration of the more polar solvent (water or methanol), pH, the nature and ionic strength of the buffer. It is shown that high efficiencies are reached only with a citrate buffer that impairs the interactions with the residual silanol groups whatever the mobile phase pH is. We demonstrate that the citrate buffer strongly interacts with the cationic moiety of the aminopropyl stationary phase and thus reduces the accessibility of silanols. The separation of oxytetracycline, tetracycline and chlortetracycline is achieved in a few minutes at pH 3.5 or 5, with no peak tailing as usually observed in reversed phase liquid chromatography with an opposite elution order when compared with reversed phase liquid chromatography.     

On-line liquid chromatography/thermospray mass spectrometry in the analysis of oligosaccharides.

The analysis of intact neutral oligosaccharides by on-line liquid chromatography/thermospray mass spectrometry is described. Molecular-weight information on oligomers up to a degree of polymerization of 10 is obtained using an aqueous mobile phase containing 10(-4) mol/L sodium acetate, which was found to be compatible with thermospray interfacing and ionization. Ions due to sodiated and disodiated oligosaccharides are observed under these conditions without fragmentation. The aqueous 10(-4) mol/L sodium acetate mobile phase is demonstrated to be applicable in the separation of mixtures of oligosaccharides on a reversed-phase octadecyl-modified silica column.     

Advantages and drawbacks of popular supercritical fluid chromatography/mass interfacing approaches--a user's perspective

Mobile phases in supercritical fluid chromatography (SFC) have low viscosities and high diffusion coefficients with respect to those of traditional high performance liquid chromatography (HPLC). These properties allow higher mobile phase flow rates and/or longer columns in SFC, resulting in rapid analyses and high efficiency separations. In addition, chiral SFC is becoming especially popular. Mass spectrometry (MS) is arguably the most popular "informative" detector for chromatographic separations. Most SFC/MS is performed with atmospheric pressure ionization (API) sources. Unlike LC/MS, the interface between the SFC column and the API source must allow control of the downstream (post-column) pressure while also providing good chromatographic fidelity. Here we compare and contrast the popular interfacing approaches. Some are simple, such as direct effluent introduction with no active back-pressure-regulator (BPR) in high speed bioanalytical applications. The pressure-regulating-fluid interface is more versatile and provides excellent chromatographic fidelity, but is less user friendly. The pre-BPR- split interface and an interface which provides total-flow-introduction with a mechanical BPR are good compromises between user friendliness and performance, and have become the most popular among practitioners. Applications of SFC/MS using these various interfaces are also discussed.     

色氨酸在TOYOPEARL HW-40S 凝胶填料上非理想排阻行为的研究

在发现１８种氨基酸特别是色氨酸在ＴＯＹＯＰＥＡＲＬＨＷ－４０Ｓ树脂所填色谱柱上表现出非理想排阻行为的基础上,以色氨酸在ＨＷ柱上的保留时间为指标,探讨不同的流动相改良剂对ＨＷ凝胶与色氨酸之间相互作用的影响,从而阐明这种反常行为是疏水、离子交换以及氢键三者综合作用的结果,而疏水作用是最主要的影响因素。实验结果说明流动相组成对ＨＷ凝胶色谱行为有明显的影响作用,这不但有助于确立ＨＷ凝胶理想的排阻色谱条件,而且为利用其所具有的吸附性能分离分析肽类和氨基酸等小分子物质奠定了基础。     

Feasibility of enhancing high-performance liquid chromatography using microwave radiation

High-performance liquid chromatography (HPLC) is the most versatile of the chromatographic techniques because it is applicable to a wide variety of analytes. Unfortunately, liquid mobile phases have relatively low diffusivities. A novel approach is presented for increasing the diffusivity of a liquid mobile phase. The method advocated in this study is the first process that has been described by which it is possible to increase the diffusivity of a liquid while having essentially no effect on other physical properties of the liquid, such as the temperature or solvating power. It is demonstrated that it is possible to sharpen peaks, in an HPLC separation, by the application of microwave radiation.     

Chromatographic shape selectivity with carbon dioxide-acetonitrile mobile phases. Effect of mobile phase composition and density

Trends in chromatographic shape selectivity with mobile phases consisting of mixtures of carbon dioxide and acetonitrile are investigated. Selectivity is evaluated as a function of mobile phase composition, temperature, and column bonding chemistry. SRM (standard reference material) 869a is used as a probe of shape selectivity, while the selectivity between triphenylene and o-terphenyl is used to investigate planarity selectivity. Four molecular mass 228 polyaromatic hydrocarbon isomers are used to investigate shape selectivity based on differences in length-to-breadth ratio. Shape selectivity trends as a function of temperature and column type are found to be similar to what is seen in reversed-phase liquid chromatography, while the trend seen as the amount of acetonitrile in the mobile phase increases is found to be different than in reversed-phase liquid chromatography. In addition, the effect of mobile phase density, i.e., solvent strength, on shape selectivity is investigated by examining shape selectivity as a function of density with neat carbon dioxide as the mobile phase.     

On the influence of strain on ion transport: microstructure and ionic conductivity of nanoscale YSZ|Sc2O3 multilayers

Multilayer samples of the type (YSZ|Sc2O3) × n with layer thicknesses between 8 nm (n=100) and 250 nm (n=5) were prepared on (0001) sapphire substrates by pulsed laser deposition (PLD). The samples were characterised using X-ray diffraction (XRD), scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM/HRTEM, SAED (selected-area electron diffraction) and quantitative EELS (electron energy-loss spectroscopy)). The polycrystalline layers show a columnar microstructure, which is typical for the used preparation technique. The layers are highly textured and only one axial orientation relation is found between yttria-stabilised zirconia (YSZ), scandium oxide and the substrate: (0001) Al2O3‖(111) Sc2O3‖(111) YSZ. A preferred orientation relationship also exists for the azimuthal rotation of the crystallites, which was demonstrated by SAED, XRD pole figure measurements and fast Fourier transformation (FFT) of HRTEM micrographs. The interfaces between YSZ, Sc2O3 and the substrate are sharp and do not contain diffuse transition regions. Dislocations appear not to be arranged in regular arrays. With increasing interface density (thinner individual layers in the multilayer), the conductivity of the multilayers decreases. We relate this to the negative nominal misfit present at the YSZ|Sc2O3 interfaces (compressive stress in YSZ at the phase boundaries). This observation agrees well with the previously investigated case of YSZ|Y2O3 (A. Peters et al., Phys. Chem. Chem. Phys., 2008, 10, 4623), where tensile misfit strain was present in YSZ at the phase boundaries, leading to a conductivity increase.     

Thin-layer chromatography with high-boiling mobile phases.1. Investigation of the flow of the high-boiling mobile phases

This paper considers some aspects of a new TLC technique using a molten mobile phase which is solid under ambient conditions. The flow of high-boiling mobile phase at elevated temperature in thin-layer chromatography has been investigated and it is shown that the equation Z_f² = kt is not applicable to migration of the high boiling mobile phase front. The flow stability of the high-boiling mobile phase is noted. It is suggested on the basis of studies of concentration profiles of the solidified mobile phases by scanning photometers that the shape of the mobile phase concentration profile be taken into account in calculation of R_f Values.     

Separation of different oligomers by supercritical fluid chromatography using gradient elution

Summary In supercritical fluid chromatography (SFC), the solubilizing nature of the mobile phase enables the use of composition gradients (gradient elution) like in liquid chromatography. With this gradient technique, oligomer separations have been achieved over wide molecular weight ranges. For separating oligomers which absorb only at low wavelengths, a carbon dioxide/acetonitrile mobile phase was used which allows UV detection down to 200 nm. As an ideally suited detection assembly for the chromatography of oligomers, a combination of mass-sensitive and evaporative light-scattering detection is suggested.

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Trennung von verschiedenen Oligomeren durch Chromatographie mit überkritischen fluiden Phasen unter Verwendung der Gradientenelution