Retention In Supercritical Fluid Chromatography Research Articles

Development progress of stationary phase for supercritical fluid chromatography and related application in natural products

Supercritical fluid chromatography (SFC) is an environment-friendly and efficient column chromatography technology that was developed to expand the application range of high performance liquid chromatography (HPLC) using a supercritical fluid as the mobile phase. A supercritical fluid has a temperature and pressure that are above the critical values as well as relatively dynamic characteristics that are between those of a gas and liquid. Supercritical fluids combine the advantages of high solubility and diffusion, as their diffusion and viscosity coefficients are equivalent to those of a gas, while maintaining a density that is comparable with that of a liquid. Owing to the remarkable compressibility of supercritical fluids, analyte retention in SFC is significantly influenced by the density of the mobile phase. Thus, the column temperature and back pressure are crucial variables that regulate analyte retention in SFC. Increasing the back pressure can increase the density and solubility of the mobile phase, leading to reductions in retention time. The column temperature can affect selectivity and retention, and the degree to which different analytes are affected by this property varies. On the one hand, increasing the temperature reduces the density of the mobile phase, thereby extending the retention time of the analytes; on the other hand, it can also increase the energy of molecules, leading to a shorter retention time of the analyte on the stationary phase. CO2, the most widely employed supercritical fluid to date, presents moderate critical conditions and, more importantly, is miscible with a variety of polar organic solvents, including small quantities of water. In comparison with the mobile phases used in normal-phase liquid chromatography (NPLC) and reversed-phase liquid chromatography (RPLC), the mobile phase for SFC has a polarity that can be extended over a wide range on account of its extensive miscibility. The compatibility of the mobile phase determines the diversity of the stationary phase. Nearly all stationary phases for HPLC, including the nonpolar stationary phases commonly used for RPLC and the polar stationary phases commonly used for NPLC, can be applied to SFC. Because all stationary phases can use the same mobile-phase composition, chromatographic columns with completely different polarities can be employed in SFC. The selectivity of SFC has been effectively expanded, and the technique can be used for the separation of diverse analytes ranging from lipid compounds to polar compounds such as flavonoids, saponins, and peptides. The choice of stationary phase has a great impact on the separation effect of analytes in SFC. As new stationary phases for HPLC are constantly investigated, specialized stationary phases for SFC have also been continuously developed. Researchers have discovered that polar stationary phases containing nitrogen heterocycles such as 2-EP and PIC are highly suitable for SFC because they can effectively manage the peak shape of alkaline compounds and provide good selectivity in separating acidic and neutral compounds.The development of various stationary phases has promoted the applications of SFC in numerous fields such as pharmaceuticals, food production, environmental protection, and natural products. In particular, natural products have specific active skeletons, multiple active groups, and excellent biological activity; hence, these materials can provide many new opportunities for the discovery of novel drugs. According to reports, compounds related to natural products account for 80% of all commercial drugs. However, natural products are among the most challenging compounds to separate because of their complex composition and low concentration of active ingredients. Thus, superior chromatographic methods are required to enable the qualitative and quantitative analysis of natural products. Thanks to technological improvements and a good theoretical framework, the benefits of SFC are gradually becoming more apparent, and its use in separating natural products is expanding. Indeed, in the past 50 years, SFC has developed into a widely used and efficient separation technology. This article provides a brief overview of the characteristics, advantages, and development process of SFC; reviews the available SFC stationary phases and their applications in natural products over the last decade; and discusses prospects on the future development of SFC.

An attempt to apply a subtraction model for characterization of non-polar stationary phase in supercritical fluid chromatography

This study verified the feasibility of using a subtraction model to characterize the non-polar stationary phases (including C4, C8, and phenyl-type) in supercritical fluid chromatography (SFC). The model with 6 terms was expressed as log α = η'H + θ'P + β'A + α'B + κ'C + σ'S, where a term θ'P indicating dipole or induced dipole interaction was intentionally supplemented. Ethylbenzene and SunFire C8 were respectively defined as the reference solute and column. A 7-step modeling procedure was proposed: in the first 6 steps, except σ'S, by the use of a bidirectional fitting method, other parameters were calculated based on the equation: log α = log (ki/kref) ≈ η'H + θ'P + β'A + α'B + κ'C; and in the 7th step, residual analysis was employed to describe the σ'S term according to the equation: σ'S = log αexp. - log αpre. Furthermore, six columns that were not involved in modeling process and 12 compounds with unknown retention were used for methodology validation. It showed good predictions of log k, as demonstrated by adjusted determination coefficient (R2adj) from 0.9927 to 0.9998 (column) and from 0.9940 to 0.9999 (compound), respectively. The subtraction model emphasized the contribution of dipole or induced dipole interaction to the retention in SFC, and it obtained the σ'S term through residual analysis. Moreover, it made reasonable physical-chemical sense as the linear solvation energy relationship (LSER) model did, with the distinct advantages of better fitting and more accurate prediction. This study provided some new insights into the characterization of non-polar stationary phases in SFC.

Retention mechanisms of imidazoline and piperazine-related compounds in non-aqueous hydrophilic interaction and supercritical fluid chromatography based on chemometric design and analysis

The experimental design methodology based on central composite design of experiments was applied to compare the retention mechanisms in supercritical fluid chromatography (SFC) and non-aqueous hydrophilic interaction liquid chromatography (NA-HILIC). The selected set consists of 26 compounds that belong to imidazoline and serotonin receptor ligands. The different chemometric tools (multiple linear regression, principal component analysis, parallel factor analysis) were used to examine the retention, as well as to identify the most significant retention mechanisms. The retention mechanism was investigated on two different stationary phases (diol, and mixed-mode diol). In NA-HILIC, the mobile phase contains acetonitrile as a main component, and methanolic solution of ammonium formate (+ 0.1% of formic acid) as a modifier. The same mobile phase modifier was used in SFC, with a difference in the main component of the mobile phase which was CO2.The retention behaviour differs significantly between HILIC and SFC conditions. The retention pattern in HILIC mode was more partition-like, while in SFC the solute-sorbent interactions allowed retention. The retention mechanism between mixed-mode diol and the diol phases varies depending on the applied chromatographic mode, e.g., in HILIC the type of stationary phase significantly affects the elution order, while in SFC this was not the case. The HILIC retention behaviour was influenced by the number of tertiary amines-aliphatic, and N atom-centred fragments in tested compounds. On the other hand, the number of pyrrole and pyridine rings in the structure of the compound showed correlation with their SFC retention, simultaneously increasing the molecular weight and rapid elution of larger compounds. It was found that temperature surprisingly plays a major role in SFC mode. The increase in temperature reduces the relative contribution of enthalpy factors to total retention, so the separation in SFC was more entropy-controlled. For further pharmaceutical research and optimization, the SFC would be considered more beneficial compared to HILIC since it gives good selectivity in separation of chosen impurities.

Improving retention-time prediction in supercritical-fluid chromatography by multivariate modelling

The prediction of chromatographic retention under supercritical-fluid chromatography (SFC) conditions was studied, using established and novel theoretical models over ranges of modifier content, pressure and temperature. Whereas retention models used for liquid chromatography often only consider the modifier fraction, retention in SFC depends much more strongly on pressure and temperature. The viability of combining several retention models into surfaces that describe the effects of both modifier fraction and pressure was investigated. The ability of commonly used retention models to describe retention as a function of modifier fraction, expressed either as mass or volume fraction, pressure and density was assessed. Using the multivariate surfaces, retention-time prediction for isocratic separations at constant temperature improved significantly compared to univariate modelling when both pressure and modifier fractions were changed. The “mixed-mode” model with an additional exponential pressure or density parameter was able to predict retention times within 5%, with the majority of the predictions within 2%. The use of mass fraction and density further improves retention modelling compared to volume fraction and pressure. These variables however, do require extra computations.

Some aspects of additives effects on retention in supercritical fluid chromatography studied by linear free energy relationships method

This work is dedicated to the investigation of additives effects on retention mechanisms in supercritical fluid chromatography. Additives are compounds which are added to the mobile phase in small quantities and greatly affect retention factors, peak shape, separation selectivity and other chromatographic parameters. Linear free energy relationship (LFER) method with an expanded set of descriptors including the ones taking ionic interactions into account was used to probe the effect of four types of additive: trifluoroacetic acid, diethylamine, ammonium acetate and water – on retention on four polar stationary phase bearing different functional groups: bare silica, cyano, 2-ethylpyridine and zwitter-ionic sulfobetaine. Effects of all additives were shown to be complex, involving different intermolecular interactions and not uniform. The direction and magnitude of retention change depends not only on additive concentration, but on all other component of a chromatography system: type of a stationary phase, mobile phase composition and the nature of the solute. Various subtle effects were registered, the most peculiar among them being the fact that diethylamine and ammonium acetate provide identical changes of LFER constants. We hypothesize that this is caused by the convergence of diethylamine into a methylcarbamate via reaction with carbon dioxide and methanol, which then behaves as a salt additive.

Evaluation of temperature and pressure effects on retention in supercritical fluid chromatography on polar stationary phases

Four polar stationary phases (ethylene-bridged hybrid silica, cyanopropyl, 2-ethylpyridine, and zwitterionic sulfobetaine) have been characterized in supercritical fluid chromatography (SFC) by linear free energy relationships (LFER) method with an extended set of Abraham's descriptors. Temperature (25–55 °C) and pressure (110–180 bar) effects on analyte retention, separation selectivity and LFER-coefficients of chromatographic systems have been studied using the 89 test compounds of various chemical classes and carbon dioxide – methanol (9:1 v/v) binary solvent as a mobile phase.It was found that for the selected stationary phases temperature and pressure had only moderate effects on selectivity. The retention times of all analytes decrease, as can be expected, if the pressure rises at the isothermal conditions due to the increase of the fluid density and its eluting power. The effect of temperature on retention is complicated and depends both on the chemical class of analyzed compounds and the stationary phase type.Temperature and pressure variations lead to small changes in the LFER-coefficients, and general trends observed do not depend much on the stationary phase type. It may be difficult to interpret the LFER-analysis results because of the evident, more significant chromatographic phenomena.

Exploration and optimization of conditions for quantitative analysis of lignans in Schisandra chinensis by an online supercritical fluid extraction with supercritical fluid chromatography system

An online supercritical fluid extraction with supercritical fluid chromatography system could provide sequential extraction and quantitative analysis of lignans in Schisandra chinensis. Supercritical fluid extraction conditions were optimized at 15MPa, 50°C, and 4min with supercritical CO2 adding 1% methanol; the elution volume and flow rate were set at 6mL and 2mL/min to blow extract out of the tank completely. The split-flow rate was confirmed at 2.5%, which determines injection volume and accuracy of quantitative detection. The factors having negative influences on supercritical fluid chromatography retention in the online system, including sample loading forms and backpressure settings, are discussed in the paper. At last, an extraction-quantitative method for lignans in Schisandra chinensis was developed, which could be finished within 19.5min. The total content percentage of four lignans (Schisandrin, Schisandrin A, Schisandrin B and Schisandrol B) in four batches was respectively measured to be 1.42, 1.54, 1.62, and 1.90%.

The comparison of retention behaviour of imidazoline and serotonin receptor ligands in non-aqueous hydrophilic interaction chromatography and supercritical fluid chromatography

This work presents an investigation of retention characteristics of imidazoline and serotonin receptor ligands in non-aqueous hydrophilic interaction liquid chromatography (NA-HILIC) and supercritical fluid chromatography (SFC). The separation has been carried out by using methanol as a mobile phase modifier with addition of two types of additives (NH4HCOO; NH4HCOO/HCOOH) and two different stationary phases (diol; mixed-mode diol). The selectivity characteristics were observed based on S-factors, logk-logk plots and radar plots. NA-HILIC vs. SFC retention of tested compounds was also described by considering the molecular properties of the analytes within the LSER analysis. The differences between SFC vs. NA-HILIC retention of imidazoline and serotonin receptor ligands grow with the acid addition to a mobile phase, noticeably on mixed-mode diol stationary phase (S ≥ 87). In addition, the good selectivity performances of the certain NA-HILIC and SFC conditions were confirmed by good separation of structurally related compounds (α ≥ 2). The molecular basis of NA-HILIC and SFC retention were explained by using Abraham’s equation. The dominant analyte descriptors influencing retention were hydrogen bonding and dipolar interactions. The current study will present the theory, and discuss the applicability within the SFC vs. NA-HILIC regimes. In this way, it was provided the placing of two relatively new methods (SFC, NA-HILIC) in the map of modern analytical chromatography in terms of the pharmaceutical analysis.

Relationship between Passive Permeability and Molecular Polarity Using Block Relevance Analysis.

EPSA is an experimental descriptor of molecular polarity obtained from chromatographic retention in supercritical fluid chromatography (SFC) systems, previously shown by Goetz et al. to correlate with passive permeability of cyclic peptides. The present study focuses on EPSA in relation to passive permeability of small molecules. We applied block relevance (BR) analysis to interpret the relative significance of mechanistic forces prevailing in EPSA. The BR analysis is a computational tool that allows the interpretation of the balance of intermolecular interactions governing systems such as the aforementioned chromatographic retention in EPSA. EPSA and passive permeability determined by Ralph Russ canine kidney cells (RRCK) or low efflux Madin Darby canine kidney cells (MDCK-LE) and human epithelial colorectal adenocarcinoma cells (Caco-2), studied on a data set of commercial drugs, indicated that EPSA is relevant in describing permeability of hydrophilic drugs (CLogP < 1). We then verified, on a data set of 1699 Rule of 5 compliant Pfizer compounds, that when CLogP < 1, a value of EPSA < 100 significantly increases the likelihood of high permeability.

Effect of reference conditions on flow rate, modifier fraction and retention in supercritical fluid chromatography

When using compressible mobile phases such as fluidic CO2, the density, the volumetric flow rates and volumetric fractions are pressure dependent. The pressure and temperature definition of these volumetric parameters (referred to as the reference conditions) may alter between systems, manufacturers and operating conditions. A supercritical fluid chromatography system was modified to operate in two modes with different definition of the eluent delivery parameters, referred to as fixed and variable mode. For the variable mode, the volumetric parameters are defined with reference to the pump operating pressure and actual pump head temperature. These conditions may vary when, e.g. changing the column length, permeability, flow rate, etc. and are thus variable reference conditions. For the fixed mode, the reference conditions were set at 150bar and 30°C, resulting in a mass flow rate and mass fraction of modifier definition which is independent of the operation conditions. For the variable mode, the mass flow rate of carbon dioxide increases with system pump operating pressure, decreasing the fraction of modifier. Comparing the void times and retention factor shows that the deviation between the two modes is almost independent of modifier percentage, but depends on the operating pressure. Recalculating the set volumetric fraction of modifier to the mass fraction results in the same retention behaviour for both modes. This shows that retention in SFC can be best modelled using the mass fraction of modifier. The fixed mode also simplifies method scaling as it only requires matching average column pressure.

Enantiomeric Separation of Original Heterocyclic Organophosphorus Compounds in Supercritical Fluid Chromatography

New and original heterocyclic α-enamido phosphine chiral solutes were prepared: four structurally similar racemates with the chirality center placed on the phosphorus atom, and four other related pairs of enantiomers with chirality borne by the carbon atoms of the phospholane ring. The structural variations were placed on an aliphatic heterocycle (six- or seven-member rings) and on the carbamate function (methyl or t-butyl). Their separation was achieved on a commercial cellulose tris-(3,5-dimethylphenylcarbamate) stationary phase (Lux Cellulose-1, Phenomenex) in supercritical fluid chromatography (SFC). The effects of molecular structure on SFC retention and enantioresolution were studied. Among these eight pairs of enantiomers, some reversal of elution order between similar compounds was observed. The effect of changing the organic solvent (methanol and ethanol) and its proportion (between 5 and 40%) in the mobile phase was investigated. Retention data were collected over the temperature range 0-50 °C, and the results interpreted from thermodynamic aspects.

Effects of mobile phase composition and temperature on the supercritical fluid chromatography enantioseparation of chiral fluoro-oxoindole-type compounds with chlorinated polysaccharide stationary phases

The indole ring is present in many pharmaceutically active compounds, as in MaxiPost™ (or BMS-204352), an active pharmaceutical ingredient for post-stroke neuroprotection. Two different fluoro-oxindole-type chiral solutes derived from MaxiPost were prepared. The structural modification was placed on the NH indole function, which is probably taking part in the bioactivity, but also probably in the chromatographic enantiomer separation. Baseline resolution of the enantiomers of MaxiPost and the two derivatives was achieved on two commercial chlorinated polysaccharide stationary phases: cellulose tris-(3-chloro-4-methylphenylcarbamate) and amylose tris-(5-chloro-2-methylphenylcarbamate) (Lux Cellulose-2 and Lux-Amylose-2 from Phenomenex) in supercritical fluid chromatography (SFC). The effects of molecular structure on SFC retention and enantioresolution are studied. The effect of temperature, modifier nature (methanol or ethanol) and proportion were investigated in order to select the best conditions for preparative purposes. A temperature study led to Van’t Hoff plots that were strongly dependent on the mobile phase composition. From linear portions of the plots for separation, thermodynamic parameters of the separation could be calculated. Incidentally, particular adsorption–desorption effects were observed. Semi-preparative resolution was then achieved, allowing retrieving about 10mg purified enantiomers for bioactivity testing.

Prediction of supercritical fluid chromatographic retention factors at different percents of organic modifiers in mobile phase

In this work quantitative structure-retention relationship models were developed to predict the solute retention factors in supercritical fluid chromatography (SFC) in various organic modifiers. Data set contains the retention data of 35 various organic compounds in 0, 2, 4 and 6% of methanol in mobile phase. The obtained 140 data points were divided into training, internal and external test sets which have 93, 23 and 24 retention data. The diversity validation test was performed on data the set to ensure that the structure of the training and/or test sets can represent those of the whole ones. Descriptors which were selected by stepwise multiple linear regression (MLR) methods are: the percent of organic modifier in mobile phase, salvation connectivity index chi-2, salvation connectivity index chi-5, H attached to heteroatom, the 2(nd) structural information content and polarity parameter. These descriptors were used as features in generation of linear and non-linear models using MLR and artificial neural network (ANN) methods, respectively. The root mean square error of MLR model are 0.116, 0.138 and 0.260 for training, internal and external test sets, respectively, while these values are 0.036, 0.097 and 0.244 for ANN model, respectively. Comparison between these values and other statistical parameters obtained from these two models reveals the credibility of ANN in prediction of solute retention factors in SFC.

Investigation of retention behavior of drug molecules in supercritical fluid chromatography using linear solvation energy relationships

In this study, the retention behaviors of pharmaceutical and drug-like compounds in supercritical fluid chromatography (SFC) with polar stationary phases were evaluated using linear solvation energy relationships (LSERs). We used more than 200 compounds, including mono-, di-substituted benzenes, and other marketed drugs with a wide range of solvation descriptors. The dominant contribution to positive retention was the hydrogen bond donor acidity of the solutes, particularly for pyridine and amino columns. That factor is determined by the difference in basicity between the mobile and stationary phases. Another significant contributor to retention behavior was the hydrogen bond acceptor basicity of the solutes made particularly for diol and amino columns. The hydrogen bond donating ability is small for cyano and pyridine stationary phases (or should be zero) since those two phases lack hydrogen bond donor groups. The LSER results showed that the SFC retention behavior of compounds using carbon dioxide–methanol with polar stationary phases was close to that reported for normal phase LC using hexane–methanol. As expected, mobile phases containing high percentages of modifier resulted in decreased retention times for most compounds. A greater knowledge of the nature of solute/stationary phase and solute/mobile phase allows a more rapid and efficient choice in high-throughput screening of compound libraries.

Relative Retention in Supercritical Fluid Chromatography as a Source of Partial Molar Properties

In addition to its significance as an analytical tool, supercritical fluid chromatography also provides a useful technique for measurement of thermodynamic properties. The purpose of this contribution is to point out some advantages of using the relative retention of a pair of solutes to obtain the solute partial molar properties in supercritical solvents. First, the particular benefits of using the relative retention are shown at the general thermodynamic level of the problem. Next, the Flory−Huggins equation of state (EOS) is employed to derive the correction terms for the composition derivatives of the solute chemical potentials that can readily be used in reducing the experimental retention data to the partial molar properties. When the Flory−Huggins EOS is combined with the regular solution theory, the corrections require pure-component parameters only. Further, as thermodynamic data on isomeric solutes are often of interest in applications of supercritical solvents, the additional simplifications in the correction terms for isomeric solutes are explicitly included.

Partial molar properties from solute retention in supercritical fluid chromatography: thermodynamic framework, advantages, and limitations

The changes in chromatographic retention of a solute with the operating parameters such as pressure, temperature and composition of the mobile-phase fluid may be employed to measure the infinite-dilution partial molar volume of the solute in the mobile-phase fluid, the infinite-dilution partial molar enthalpy of the solute in the mobile-phase fluid, and the change of the infinite-dilution fugacity coefficient of the solute in a binary mobile-phase fluid with composition of the fluid. The theoretical background, benefits and limitations of this technique are discussed with emphasis on supercritical fluid chromatography.

Study of Interactions in Supercritical Fluids and Supercritical Fluid Chromatography by Solvatochromic Linear Solvation Energy Relationships

Linear solvation energy relationships were used to study the retention process in supercritical fluid chromatography (SFC) and to gain a better understanding of intermolecular interactions in supercritical fluids. Correlation of SFC retention data with a set of solute solvatochromic parameters, which are also applicable to gas and liquid chromatography, yields information regarding the relative contributions of dispersion, cavity formation, dipolar, and hydrogen-bonding processes to retention. Dispersion interactions and cavity formation processes dominate retention on an open tubular poly(dimethylsiloxane) stationary phase with pure carbon dioxide as the mobile phase. Dipolar interactions and hydrogen-bonding interactions are of decidedly less importance but do contribute significantly to retention. Based on prior solvatochromic studies of poly(dimethylsiloxane) and carbon dioxide, the changes in the regression coefficients with temperature and pressure are interpreted chemically. The relative importance of these contributions changes with temperature and pressure. As pressure increases, the carbon dioxide becomes more dense, and dispersion interactions between the solute and the mobile phase increase. A temperature increase at constant pressure decreases dispersion interactions with the stationary phase, as in gas chromatography, but also decreases dispersion interactions with the mobile phase, due to a decrease in carbon dioxide density. On the basis of the solvatochromic coefficients, carbon dioxide acts as both a Lewis base and a Lewis acid. The quality of fit for these correlations is very high and compares favorably with similar studies in gas chromatography and liquid chromatography, permitting the prediction of retention behavior from a solute's solvatochromic parameters.

Modifier effects in open tubular capillary column supercritical fluid chromatography: Retention mechanisms from linear solvation energy relationships

Recently, the solvatochromic comparison method was successfully applied to determine the effects of adding a modifier to the mobile phase in packed capillary columns, and we have extended this method to understand the modifier effect in open tubular capillary column supercritical fluid chromatography (SFC). SFC retention data for a wide variety of compounds at different modifier concentrations were obtained using a 10 m×50 μm I.D. SB-methyl-100 polydimethylsiloxane capillary column. Supercritical carbon dioxide at a pressure of 260 atm was used as mobile phase and methanol as modifier. The LSER regression equation used was log k′=SP 0+ l log L 16+ s π 2 H+ a α 2 H+ b β 2 H+ r R 2, where k′ represents the capacity factor and L 16 represents the partition coefficient of the solute between the gas phase and n-hexadecane at 298 K, and l, s, a, b and r are the regression coefficients. For open tubular column SFC, variation of the l and s coefficients showed a similar pattern to the packed capillary column SFC. However, the b coefficient showed a different pattern from packed capillary SFC, i.e., a gradual decrease with increasing modifier concentration. Based on these observations, the effects of a small amount of a modifier in the supercritical mobile phase and its effects on solute retention mechanisms have been discussed. For open tubular column SFC, the swelling of the chromatographic stationary phase by a modifier and carbon dioxide and the increase of dipolarity of the mobile phase are the most important contributing factors in substantiating the modifier effect on solute retention.

Capillary supercritical fluid chromatography of cosmetic ingredients and formulations

Capillary supercritical fluid chromatography (SFC) is an extremely versatile technique for the characterization of natural products that are used in the cosmetics industry. In this study, the utility of capillary SFC for separating components contained in natural lanolin, cosmetic grade lanolin, and lanolin derivatives and for profiling the components in cosmetic formulations, such as lipsticks and lip balms, is demonstrated. Capillary SFC is also shown to be facile for separating reaction products from starting materials for substrates frequently used in cosmetic formulating. Optimal stationary phases for the separation of nonpolar and lipophilic components found in cosmetic products have been determined. SFC profiles generated on SB-octyl columns using linear density programs have been particularly useful for cosmetic deformulation. Retention data on complex natural mixtures correlate with the overall polarity of the solutes and traditional SFC retention patterns based on the molecular weight of the solutes. B

Temperature and pressure effects on solubility in supercritical carbon dioxide and retention in supercritical fluid chromatography

Solubilities of some polycyclic aromatic hydrocarbons (PAHs) in supercritical carbon dioxide were measured with a procedure based on a direct on-line combination of a saturation cell to a flame ionization detector. Acenaphthene, anthrance and chrysene were selected as the test solutes. A method was developed and evaluated which enables the measurement of the contribution of solute vapor pressure to the overall solubility. The effects of temperature and pressure on solubility in supercritical carbon dioxide were investigated and discussed in detail. The trends of solubility changes in supercritical carbon dioxide and the variations in observed retention in supercritical fluid chromatography (SFC) were correlated. Equations were derived to estimate the effects of temperature on the solute's affinity for the stationary phase in SFC.