On-line packed column supercritical fluid chromatography--microwave-induced plasma atomic emission

Interfacing and evaluation of packed column supercritical fluid chromatography (SFC)-microwave-induced plasma atomic emission detection (AED) is described. Via a flow splitter and an integral restrictor, efficient transfer of solutes from column to detector without band broadening is obtained. Variation of CO2 flow-rate during pressure gradients has little influence on both AED signal and baseline drift while it provides similar sensitivity as in capillary SFC. Continuous introduction of CO2 in the plasma reduces the available range of emission domains; nevertheless the region of detection which is free of CO2 interferences allows selective detection of C1 and Br as reported in this paper. reserved.     

Separation of divalent transition metal beta-diketonates and their adducts by supercritical fluid chromatography

A method for separation and detection of divalent transition metal beta-diketonates by adduct formation/supercritical fluid chromatography (SFC) with an open-tubular capillary column and a FID detector is described. The crystal structures of copper (Cu)-hexafluoro-acetylacetone (HFA) and Cu bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (THD) complexes have been determined by X-ray crystallography. The SFC behavior of Cu beta-diketonates shows a strong correlation with the structure of the complexes. The hydrated cu beta-diketonate complexes usually exhibit strong intermolecular interactions or decomposition in SFC. Formation of adducts with a neutral donor, such as tributyl phosphine oxide (TBPO), can greatly improve the SFC behavior and detection sensitivity of Cu(II) and Mn(II) beta-diketonates. The stoichiometry and thermal stability of the adducts Cu(II) and Mn(II) beta-diketonates with TBPO in supercritical CO(2) have also been investigated. The implications of utilizing adduct formation for supercritical fluid extraction (SFE) of divalent transition metals and for on-line coupled SFE/SFC analysis of divalent transition metals are discussed.     

Numerical modeling of the elution peak profiles of retained solutes in supercritical fluid chromatography

In supercritical fluid chromatography (SFC), the significant expansion of the mobile phase along the column causes the formation of axial and radial gradients of temperature. Due to these gradients, the mobile phase density, its viscosity, its velocity, its diffusion coefficients, etc. are not constant throughout the column. This results in a nonuniform flow velocity distribution, itself causing a loss of column efficiency in certain cases, even at low flow rates, as they do in HPLC. At high flow rates, an important deformation of the elution profiles of the sample components may occur. The model previously used to account satisfactorily for the retention of an unsorbed solute in SFC is applied to the modeling of the elution peak profiles of retained compounds. The numerical solution of the combined heat and mass balance equations provides the temperature and the pressure profiles inside the column and values of the retention time and the band profiles of retained compounds that are in excellent agreement with independent experimental data for large value of mobile phase reduced density. At low reduced densities, the band profiles can strongly depend on the column axial distribution of porosity.     

超临界二氧化碳二元体系相平衡性质的研究

采用固定体积可视观察法测定了CO₂+甲苯、CO₂+环己烷、CO₂+正丁醛、CO₂+异丁醛、CO₂+甲醇及CO₂+乙醇二元体系的临界点性质,为超临界萃取和化学反应提供基础数据.在对二元体系相行为与单组分超临界相行为进行比较的基础上,对不同化学物质及不同配比的二元体系临界点与二氧化碳临界点之间的关系进行了讨论.     

An attempt to extend ordinary capillary gas chromatography to supercritical fluid chromatography (SFC)

Since capillary columns with well immobilized stationary phases are expected to withstand contact with supercritical fluids, we wished to study their amenability to SFC. Simultaneously, we wished to learn how far SFC can be accomplished with the ordianary tools of capillary GC. The study demonstrates that truly supercritical, not just relatively high, pressure is required to ensure the typical effects of SFC. Results obtained with sub-and supercritical pressure are compared and discussed. A comprehenshive study of the parameters permitting SFC with capillary GC equipment showed a clear preference for CO₂ as a carrier, FID detection, and oncolumn sampling. While no additional equipment is required, a critical feature is the flow restrictor to be mounted on the end of the column. The production and properties of this restrictor are discussed in detail. It is reasonable to hope that SFC with 0.1 mm id capillary columns can be realized in the pressure range of 100–150 bar, where substances which cannot be eluted from a capillary colum under GC donditions are expected to be analyzed.     

Numerical modeling of elution peak profiles in supercritical fluid chromatography. Part I—Elution of an unretained tracer

When chromatography is carried out with high-density carbon dioxide as the main component of the mobile phase (a method generally known as “supercritical fluid chromatography” or SFC), the required pressure gradient along the column is moderate. However, this mobile phase is highly compressible and, under certain experimental conditions, its density may decrease significantly along the column. Such an expansion absorbs heat, cooling the column, which absorbs heat from the outside. The resulting heat transfer causes the formation of axial and radial gradients of temperature that may become large under certain conditions. Due to these gradients, the mobile phase velocity and most physico-chemical parameters of the system (viscosity, diffusion coefficients, etc.) are no longer constant throughout the column, resulting in a loss of column efficiency, even at low flow rates. At high flow rates and in serious cases, systematic variations of the retention factors and the separation factors with increasing flow rates and important deformations of the elution profiles of all sample components may occur. The model previously used to account satisfactorily for the effects of the viscous friction heating of the mobile phase in HPLC is adapted here to account for the expansion cooling of the mobile phase in SFC and is applied to the modeling of the elution peak profiles of an unretained compound in SFC. The numerical solution of the combined heat and mass balance equations provides temperature and pressure profiles inside the column, and values of the retention time and efficiency for elution of this unretained compound that are in excellent agreement with independent experimental data.     

Kinetic behaviour in supercritical fluid chromatography with modified mobile phase for 5 μm particle size and varied flow rates

After much development of stationary phase chemistry, in recent years the focus of many studies in HPLC has shifted to increase the efficiency and analysis speed. Ultra high pressure liquid chromatography (UHPLC) using sub-2 μm particles, and high temperature liquid chromatography (HTLC), using temperatures above 100°C have received much attention. These new approaches allow the use of flow rates higher than those classically used in HPLC, reducing the analysis duration. Due to the low viscosity of supercritical fluids, high velocities, i.e. high flow rates, can be achieved with classical pumping systems typically used in supercritical fluid chromatography (SFC). The effects of the flow rate increase with CO(2)/methanol mobile phase was studied on the inlet pressure, t(0), the retention factor of the compounds, and on the efficiency. Simple comparisons of efficiencies obtained at varied temperature between SFC and HPLC, with a packed column containing 5 μm particles, show the greater kinetic performances achieved with the CO(2)/methanol fluid, and underline specific behaviours of SFC, occurring for high flow rates and sub-ambient temperature. Some values (N/t(0)) are also compared to UHPLC data, showing that good performance can be achieved in SFC without applying drastic analytical conditions. Finally, simple kinetic plots (t(0) vs N) at constant column length are used to select combinations of temperature and flow rate necessary to achieve a required theoretical plate number.     

Construction of a new interface for comprehensive supercritical fluid chromatography x reversed phase liquid chromatography (SFC x RPLC)

The design of a new interface to hyphen high efficiency supercritical fluid chromatography (SFC) and fast RPLC in a comprehensive configuration is described. SFC x RPLC is a viable alternative to normal phase (NP) LC x RPLC and is characterized by high orthogonality. Compared to NPLC x RPLC an additional advantage is the expansion of supercritical carbon dioxide (CO(2)) when exposed to atmospheric pressure leading to fractions consisting of solvents that are miscible with the second dimension RPLC mobile phase. The interface consists of a two-position/ten-port switching valve equipped with two packed octadecyl silica (C(18)) loops for effective trapping and focusing of the analytes after elution from the SFC dimension. The addition of a water make-up flow to the SFC effluent prior to entering the loops is of fundamental importance to efficiently focus the solutes on the C(18) material and to reduce interferences of expanded CO(2) gas on the second dimension separation. The features of the system are illustrated with the analysis of a lemon oil sample.     

Feasibility of ultra high performance supercritical neat carbon dioxide chromatography at conventional pressures

The implementation of columns packed with sub-2 μm particles in supercritical fluid chromatography (SFC) is described using neat carbon dioxide as the mobile phase. A conventional supercritical fluid chromatograph was slightly modified to reduce extra column band broadening. Performances of a column packed with 1.8 μm C18-bonded silica particles in SFC using neat carbon dioxide as the mobile phase were compared with results obtained in ultra high performance liquid chromatography (UHPLC) using a dedicated chromatograph. As expected and usual in SFC, higher linear velocities than in UHPLC must be applied in order to reach optimal efficiency owing to higher diffusion coefficient of solutes in the mobile phase; similar numbers of theoretical plates were obtained with both techniques. Very fast separations of hydrocarbons are presented using two different alkyl-bonded silica columns.     

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.     

Evaluation of an ion mobility detector for supercritical fluid chromatography with solvent-modified carbon dioxide mobile phases

Summary An ion mobility detector (IMD) was evaluated for open tubular column supercritical fluid chromatography (SFC) when organic solvent-modified supercritical CO₂ was used as mobile phase. It was found that the SFC/IMD interface design in which the SFC capillaray restrictor was directly inserted into the ionization region of the IMD was not acceptable because of low sensitivity that resulted from the effect of the modifier on detector temperature and mechanism of detection. A new interface utilizing a heated nebulizer gas to provide heat to the restrictor and to minimize the formation of ion clusters, and a bent nozzle for enhancing the ionization efficiency of the solute in the IMD ion source are described. Using 5% acetonitrile in CO₂, the minimum detectable quantity (S/N=3) for pyrene was improved from 25.2 ng to 2.1 ng with the new detector design. This compares to a minimum detectable quantity of 0.1 ng when using neat CO₂ as mobile phase. The use of molecular connectivity calculations to predict the drift times of selected analytes is also successfully demonstrated.     

Enhancement of Structural fluctuation in the region connecting two kinds of critical points in temperature-pressure-composition three-dimensional phase diagram: Raman studies of benzene/CO2 binary systems up to supercritical region

Pressure dependence of Raman spectra of benzene/CO2 two-component systems was systematically studied at different temperatures and compositions. We estimated the magnitude of inhomogeneous component in Raman bandwidth to get information on the structural fluctuation in the system. It was found that the inhomogeneous bandwidth attains a maximum on an isothermal plane in the temperature-pressure-composition three-dimensional phase diagram when the state point crosses the line connecting the region where the density fluctuation is large (the vicinity of the critical point of neat CO2) and the region where the concentration fluctuation in a binary system is enhanced (the vicinity of the critical solution point). By accumulating such data, we found that the points of large structural fluctuation comprise a sheet that includes the extension line of the gas-liquid equilibrium line in the phase diagram of neat CO2 and the line connecting critical solution points of the two-component system at different temperatures. Interaction between benzene and CO2 molecules in the supercritical region is briefly discussed.     

Retention behaviour of carboxylic acid methyl esters in supercritical fluid chromatography

The study on retention behavior in supercritical fluid chromatography (SFC) is necessary to understand the mechanism of the various interactions in SFC. The retention of SFC in carboxylic acid methyl ester/polymethylsiloxane/CO2 system was studied systematically and the retention behavior of this kind of compounds under various typical operation conditions was described using the method of an alternative unified theory of chromatographic retention. The results illustrated that expression: Ink.= a + b/T + cp + dp/T + ep2/T can be used to describe quantitatively the retention behavior of carboxylic acid methyl ester/polymethylsiloxane/CO2 system in the ranges of reduced density from 0.549 to 1.411. It was also found that the entropy of solute in stationary phase is dependent on the density of supercritical fluid (SF) under typical operating conditions of SFC.     

Two-dimensional supercritical fluid chromatography/mass spectrometry for the enantiomeric analysis and purification of pharmaceutical samples

A new analytical two-dimensional supercritical fluid chromatography/mass spectrometry system (2D SFC/SFC/MS) has been designed and implemented to enhance the efficiency and quality of analytical support in drug discovery. The system consists of a Berger analytical SFC pump and a modifier pump, a Waters ZQ 2000 mass spectrometer, a set of switching valves, and a custom software program. The system integrates achiral and chiral separations into a single run to perform enantiomeric analysis and separation of a racemic compound from a complex mixture without prior clean up. The achiral chromatography in the first dimension separates the racemate from all other impurities, such as un-reacted starting materials and by-products. Mass-triggered fractionation is used to selectively fractionate the targeted racemic compound based on its molecular weight. The purified racemate from the achiral chromatography in the first dimension is then transferred to the chiral column in the second dimension to conduct the enantiomeric separation and analysis. A control software program, we coined SFC2D, was developed and integrated with MassLynx to retrieve acquisition status, current sample information, and real time mass spectrometric data as they are acquired. The SFC2D program also monitors the target ion signal to carry out mass-triggered fractionation by switching the valve to fractionate the desired peak. The 2D SFC/SFC/MS system uses one CO(2) pump and one modifier pump for both first and second dimension chromatographic separations using either gradient or isocratic elution. Similarly, a preparative 2D SFC/SFC/MS system has been constructed by modifying an existing Waters preparative LC/MS system. All components except the back pressure regulator are from the original LC/MS system. Applications of the 2D SFC/SFC/MS methods to the separation and the analysis of racemic pharmaceutical samples in complex mixtures demonstrated that an achiral separation (in first dimension) and a chiral separation (in second dimension) can be successfully combined into a single, streamlined process both in analytical and preparative scale.     

Capillary and packed column SFC/MS

A frit restrictor interface for capillary column supercritical fluid chromatography/mass spectrometry (SFC/MS) has been constructed and used for the analysis of high boiling point alkanes. Packed column SFC/MS is described using both a moving belt liquid chromatographic/mass spectrometric interface and a thermospray source in the filament-on mode.     

Compressibility effects in packed and open tubular gas and supercritical fluid chromatography

The influence of the pressure drop on the efficiency and speed of analysis in packed and open tubular supercritical fluid chromatography (SFC) is described: methods previously developed to describe the effects of mobile phase compressibility on the performance of open tubular columns in SFC have been extended to packed columns. The Horvath and Lin equation has been used to elucidate the influence of variations in velocity, diffusivity, and capacity factor along the column on the overall efficiency of packed column SFC. In packed columns, in contrast with the situation in open tubular columns, because the increase in velocity is no longer compensated by an increase in diffusion coefficients, the increase in both linear velocity and capacity factor which result from a significant pressure drop cause the plate height to increase along the column. The effect of fluid decompression along the length of the column on the speed of analysis in SFC has been studied and numerical expressions derived which enable calculation of compressibility correction factors for the plate height. Both the f₁ and f₂ correction factors remain very close to unity for acceptable pressure drops, which means that the pressure drop has virtually no effect on the number of plates generated per unit time for an unretained component. For retained species, the decompression of the mobile phase across the column causes the capacity factor to increase and hence leads to increased analysis times.     

Use of isopycnic plots in designing operations of supercritical fluid chromatography: II. The isopycnic plots and the selection of the operating pressure-temperature zone in supercritical fluid chromatography

In SFC, the key chromatographic parameters, the retention factors and the column efficiency, strongly depend on the density of the mobile phase. This indicates that the isodensity or isopycnic plots, drawn on the pressure-temperature plane, can provide an effective tool to help analyzing how the chromatograms obtained in SFC evolve, when the experimental conditions, the inlet and outlet pressures and the column temperature, are changed. In a companion paper, we analyzed the role of density in controlling the physical properties of the mobile phase, which in turn controls solute retentions and column efficiencies. In this report, we analyze the operating conditions in SFC with reference to the isopycnic plots of carbon dioxide. This analysis clarifies the differences and similarities between the operating conditions selected in the subcritical zone and those located in the supercritical zone. It also sets out an operational map illustrating how retention factors vary with respect to the operating temperatures and pressures. This study is focused on the use of pure carbon dioxide as the mobile phase, but the same method of investigation is also applicable when the mobile phase contains a modifier.     

Separation of enantiomers of ibuprofen on chiral stationary phases by packed column supercritical fluid chromatography.

A packed column supercritical fluid chromatography (SFC) method for the separation of ibuprofen enantiomers on a chiral stationary phase and CO2 with modifier as mobile phase has been developed at an analytical scale. Among 11 different stationary phases the Kromasil CHI-TBB phase showed by far the best separation properties. The influence of different modifiers, injection solvents, temperature, and pressure, and density of the fluid, respectively, on the separation behavior has been studied. It was found that the separation behavior strongly depends on the type of modifier and the modifier content. Temperature and pressure are of less influence.     

Relationship between the density of supercritical CO_2 + ethanol binary system and its critical properties

The solvent strength and selectivity of supercritical fluids (SCF) can be greatly enhanced by addition of one or two entrainers into the system. The amount of entrainer added is usually less than 5% (mole fraction). However, even with such slight amount, solubility of organic solutes has been observed to increase by several orders magnitude[1]. Therefore, critical pressure and tem-perature data of these supercritical fluid + cosolvent systems are imperative for the reasonable design of effici…     

饲料中D-泛酸钙的超临界流体色谱测定

采用超临界流体色谱(SFC)测定了饲料中D-泛酸钙的含量.在二氧化碳流动相中添加14.85%甲醇改性剂及0.15%三氟乙酸添加剂,流速2 mL/min,在Kromasil ODS柱上分离,检测波长为210 nm.结果表明D-泛酸钙能在3 min内完成分离,在1.0 ～25.0 mg/L范围内,样品的质量浓度与色谱峰面积呈良好的线性关系(r=0.999 5).方法的精密度良好,相对标准偏差在2.3%以内,回收率为89% ～98%.方法操作简单,结果准确,适合普及应用.