On-line multidimensional supercritical fluid chromatography/capillary gas chromatography

A new analytical technique combining on-line supercritical fluid chromatography with capillary gas chromatography has been developed. The supercritical fluid sample effluent is decompressed through a restrictor directly into a conventional capillary gas chromatographic injection port. This technique allows for not only direct (100%) sample transfer from the supercritical fluid chromatograph to the gas chromatograph but also for selective or multi-step heartcutting of various sample peaks as they elute from the supercritical fluid chromatograph. Heartcut times are determined by monitoring the responses from the flame ionization or ultraviolet absorbance detectors on the supercritical fluid chromatograph. This report describes the operational setup and provides the results of heartcut reproducibility experiments using normal hydrocarbon and aromatic test mixtures. Results from studies where operational parameters were varied, such as GC injector temperature, will also be provided. The potential usefulness of this new technique for selective heartcutting will also be demonstrated using complex hydrocarbon streams.     

Effect of mobile phase density gradients on efficiency in packed column supercritical fluid chromatography

Summary The efficiency of packed columns was measured as a function of flow rate, temperature, outlet density, and the density differential across the column, unsing pure carbon dioxide as the mobile phase. Although density differentials are often blamed for a serious loss in efficiency in packed column supercritical fluid chromatography, the results show that efficiency was not a function of the density differential. Peak shapes suggest that apparent loss in efficiency is actually due to inadequate solubility of the solute in carbon dioxide.     

Packed column supercritical fluid chromatography with light-scattering detection. I. Optimization of parameters with a carbon dioxide/methanol mobile phase

Summary Evaporative light scattering detectors have, in recent years, gained acceptance in chromatography with dense mobile phases i.e. liquid and supercritical fluid chromatography. In the present work an instrument of this type has been used in packed column supercritical fluid chromatography with carbon dioxide/methanol mixtures. Detector response and signal-to-noise ratios have been determined using squalane as test compound. Nebulizer gas flow, evaporator temperature, photomultiplier sensitivity, and mobile phase composition were found to have an influence on instrument performance. With this type of detector the field of packed column SFC applications can be extended to include non-UV-absorbing substances even when mixed mobile phases or composition gradients are necessary for the separation.     

Computer-assisted retention prediction of polycyclic aromatic hydrocarbons in supercritical fluid chromatography using carbon dioxide as the mobile phase

Summary The dependence of the capacity factor of polycyclic aromatic hydrocarbons on column temperature and on the density of the mobile phase in supercritical-fluid chromatography was investigated using carbon dioxide as the mobile phase. Logarithmic capacity factors of polycyclic aromatic hydrocarbons were obtained as a linear function of the reciprocal column temperature at a constant molar volume of carbon dioxide.The application of the Retention Prediction System to supercritical-fluid chromatography is demonstrated: one can predict the retention of polycyclic aromatic hydrocarbons using equations including column temperature, density and the physico-chemical properties of the solutes as the parameters.     

The effects of modifiers in supercritical fluid chromatography

Investigations have been initiated to examine the basic elements of resolution and how they vary individually and collectively in terms of modifiers used in supercritical fluid chromatography (SFC). Capacity factors (k′) have been determined for a mixture of polynuclear aromatic hydrocarbons from SFC experiments as function of modifier identity and concentration using different stationary phases. Using carbon dioxide as the primary mobile phase, the modifiers investigated included methanol, 2-methoxy ethanol, 1-propanol, tetrahydrofuran, dimethyl sulfoxide, acetonitrile, sulfur hexafluoride, and freon 11.     

Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase

Summary New polyacrylate liquid crystalline compounds were coated onto glass or fused-silica capillary columns as stationary phases and applied to supercritical fluid chromatography. These stationary phases, were very stable: no bleeding was observed at 200°C and up to 200kg/cm² pressures of carbon dioxide mobile phase. The wide working range of the capillary column was extended below the g-n transition temperature. Isomeric compounds such as - and -methoxynaphthalene, anthracene and phenanthrene and several phenolic compounds were separated.     

Direct on-line coupling of small subcritical and supercritical fluid extractors with packed column supercritical fluid chromatography

Summary A mini extractor of 85 L void volume and a micro extractor of 3–4 L void volume have been coupled directly with a packed column SFC and used under sub- and supercritical conditions. The mini extractor is suitable for holding adsorbates which can be on-line extracted and the extract chromatographed (direct SFE-SFC). The micro extractor can be used for direct sample introduction of liquid and solid materials under SF conditions. Thus any solvent interference with the sample and the chromatographic conditions is excluded. Standard samples of wood tar residue, engine oil, and metal organic compounds have been tested.     

The dependence of reduced plate height on reduced velocity in carbon dioxide supercritical fluid chromatography with packed columns

Summary The relation is proposed for the dependence of reduced plate height, h, on reduced velocity in carbon dioxide, supercritical fluid chromatography with packed columns. The classical Knox model does not take into account the density drop in the column which produces a peak-broadening. A supplementary coefficient with a parabolic dependence on velocity must be added and then a good fit between experimental results and theory is observed. Finally, the influence of other parameters such as pressure, capacity factor and solute molecular area is discussed.     

Small-scale preparative supercritical fluid chromatography of cyproterone acetate

Summary A method for the micropreparative fractionation of steroid hormones from process solutions was developed. Using a commercial SFC equipment and raw cyproterone acetate (CPA) as a model sample, we showed the value of SFC as a lab scale purification method and also its potential as an environmentally friendly approach to preparative scale chromatography.     

Low flow rate modifier addition in packed capillary column supercritical fluid chromatography

A new method to accurately deliver small amounts (0.5 to 20 mol%) of modifier into CO₂ was used to study the effects of three different modifiers (methanol, water, and formic acid) in packed capillary column SFC. The method allows the use of different modifiers, with minimal instrument modification. The effects of the different modifiers at different concentrations on retention and peak shape are shown by analyzing a polarity test mixture and a sample of free fatty acids.     

Synthetic ceramic clay material as stationary phase in packed column supercritical fluid chromatography

Summary A novel inorganic synthetic clay material (SC) has been evaluated as the stationary phase in packed-column, supercritical fluid chromatography (SFC). The molecular recognition capability of the SC stationary phase in SFC for polycyclic aromatic hydrocarbons has been evaluated using carbon dioxide and carbon dioxide modified with methanol as the mobile phase. This recognition derives from the layer structure of the SC material which acts as a slit to distinguish non-planar solutes from the molecular-molecular interaction between solute and stationary phase and leads to smaller retention for non-planar solutes. The recognition capability is also dependent on the SFC conditions such as column pressure and column temperature.     

Abnormal composition of commerical waxes revealed by supercritical fluid chromatography

Summary The analysis of natural waxes is a complicated process because they occur as complex mixtures. A study using supercritical fluid chromatography with packed columns made it possible to observe the behavior of each family of hydrocarbons, fatty esters, alcohols, acids and triglycerides. The latter were then separated according to their function type and alkyl chain lengths using octadecyl silica as stationary phase. With a polar modifier added to the supercritical fluid, it was possible to analyse certain waxes. Composition anomalies were observed which related to an overabundance of certain compounds which, in excess, are harmful to the quality of waxes intended for use in cosmetics.     

Fast supercritical fluid chromatography using capillaries packed with nonporous particles

Summary Packed columns containing microparticles provide high column efficiency per unit time and strong retention characteristics compared with open tubular columns, and they are favored for fast separations. Nonporous particles eliminate the contribution of solute mass transfer resistance in the intraparticle void volume characteristic of porous particles, and they should be more suitable for fast separations. In this paper, the evaluation of nonporous silica particles of sizes ranging from 5 to 25 μm in packed capillary columns for fast supercritical fluid chromatography (SFC) using neat CO₂ is reported. These particles were first deactivated using polymethyl-hydrosiloxanes and then encapsulated with a methylphenylpolysiloxane stationary phase. The retention factors, column efficiencies, column efficiencies per unit time, separation resolution, and separation resolution per unit time for fast SFC were determined for various length capillaries packed with various sizes of polymerencapsulated nonporous particles. It was found that 15 μm nonporous particles provided the highest column efficiency per unit time and resolution per unit time for fast packed capillary SFC. Under certain conditions, separations were completed in less than 1 min. Several thermally labile silylation reagent samples were separated in times less than 5 min. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996     

Aspects of retention behaviour of steroids in packed column supercritical fluid chromatography

Summary A selection of steroids of different polarities and different polar functional groups was investigated in terms of retention and elution profile. It was found that the apparent polarity of steroids is determined not only by the amount and the nature of polar functional groups but also by their intramolecular interactions, shielding effects and molecular shape due to the conformation of the molecule.     

Liquid crystal stationary phases for gas chromatography and supercritical fluid chromatography

Novel monomeric and polymeric liquid crystalline compounds were synthesized as stationary phases for gas chromatography (GC) and supercritical fluid chromatography (SFC). Monomeric liquid crystalline compounds were used in packed column gas chromatography for the separation of isomeric aromatic compounds and insect sex pheromones. Liquid crystalline polymers possess long nematic ranges and a uniform coating was easily achieved in glass and fused silica capillaries, which could stand temperatures up to 250°C in GC and pressures of 200 MPa at 160°C in SFC. The columns provide excellent selectivity and resolution for fused ring aromatic compounds such as the isomers anthracene and phenanthrene or triphenylene and chrysene.     

Fast supercritical fluid chromatography of polymers using packed capillary columns

Summary Fast separations of perfluorinated polyethers and polymethylsiloxanes that are composed of 50–80 oligomers were demonstrated in packed capillary column supercritical fluid chromatography (SFC) using a carbon dioxide mobile phase. Separations were accomplished within 10 min using a 13 cm×250 μm i.d. column packed with 2 μm porous octadecyl bonded silica (ODS) particles. Effects of particle diameter of the packing material and pressure programming on separation were investigated, and packed column SFC was compared with open tubular column SFC. Results show that as the particle diameter was decreased from 5 to 3 to 2 μm and the column length was reduced from 85 to 43 to 13 cm, the separation time could be reduced from 70 to 20 to 10 min while still maintaining similar separation (resolution). Short columns packed with small porous particles are very suitable for fast SFC separations of polymers.     

Current technological challenges in capillary supercritical fluid chromatography

In cases where high efficiency is required to resolve complex mixtures of either thermally labile or nonvolatile organic compounds, capillary supercritical fluid chromatography may be the most desirable analytical method. While great strides in this new technology have been made over the last few years, several problem areas are requiring increased attention. These include sample introduction systems, pressure reduction at the end of the column, column stability in various supercritical mobile phases, and migration of polar solute molecules. This paper describes the state-of-the-art in capillary SFC with emphasis on the progress made and future needs in the solutions to these specific problems.     

Sampling considerations in supercritical fluid chromatography

Summary Packed column supercritical fluid chromatography, like HPLC, utilizes a sample loop to introduce materials onto the column for analysis. Unlike HPLC the mobile phase in SFC cannot be used to dissolve the sample. In practice, this causes a solvent peak, which can create a problem in the chromatographic interpretation. This paper describes one approach to solving this problem. A valving scheme is used to extract materials with the supercritical CO₂ mobile phase and introduce them onto the column with no external handling. The viability of this method is demonstrated and separations of the CO₂ extracts for several materials are shown on various columns. Comparisons are made for coal and coffee extracts using this on-line method and conventional off-line CH₂Cl₂ extracts. Advantages of the on-line procedure as they apply to chromatography and high information detectors are also discussed.