Supercritical fluid chromatography coupled to mass spectrometry for lipidomics

Supercritical fluid chromatography (SFC) has experienced a particular revival in recent years thanks to the development of robust and efficient commercial systems. Because of its physico‐chemical properties, supercritical carbon dioxide (CO₂) mixed with cosolvents and additives is particularly suitable for SFC to allow the elution of compounds of different polarities and more particularly complex lipids. Hyphenation with mass spectrometry (MS) is increasingly described in the literature but still requires many further developments in order to be as user‐friendly as coupling with liquid chromatography. The basic concepts of SFC and MS hyphenation will be first considered. Then a representative example of method development in lipidomics will be introduced. In conclusion, the challenges and future needs in this field of research will be discussed.     

Supercritical fluid chromatography with redox chemiluminescence detection

Summary The new selective and sensitive redox chemiluminescence detector (RCD), originally developed for gas chromatography, has been successfully interfaced with a capillary column supercritical fluid chromatograph. Interfacing the SFC-RCD was accomplished simply through utilization of the redox reaction chamber presently used for both capillary and packed column GC. The feasibility of this new separation-detection scheme was demonstrated with SFC chromatograms of antioxidant isomers and some compounds of biological relevance. Mobile phase compatibility and signal quenching are also discussed.

---

Chromatographie mit überkritischen fluiden phasen unter Verwendung der Redox-Chemiluminescenz-Detektion

     

Supercritical fluid extraction coupled with supercritical fluid chromatography for the separation of sulfonylurea herbicides and their metabolites from complex matrices

Supercritical fluid chromatography and supercritical fluid extraction have been coupled to provide on-line evaluation of a dynamic extraction procedure. The simultaneous removal and analysis of representative sulfonylureas, their precursors and metabolites from complex matrices was accomplished with this configuration. The matrices examined include soil, plant materials and a cell culture medium. The simplicity and current applicability of this design to the agricultural products industry is demonstrated.     

Supercritical fluid chromatography of carbohydrates

Supercritical fluid chromatography (SFC) is a relatively new technique applied to polar solutes such as carbohydrates about 10 years ago. The developments in the SFC of carbohydrates are summarized and a comparison between capillary and packed column SFC is presented. High-efficiency capillary columns are suitable only for derivatized carbohydrates since various packed columns are well adapted for non derivatized mono-, di- and trisaccharides and provide complementary selectivities.     

Supercritical fluid chromatography with a slurry-packed capillary column

A versatile system with a slurry-packed capillary column was developed for supercritical fluid chromatography, which is capable of programming both inlet and outlet pressure independently, as well as using a restrictor to apply back pressure. This system revealed the relationships between pressure drop, flow rate, and linear velocity in pressure-programmed supercritical fluid chromatography. In the restrictor system, both the pressure drop and the flow rate increased almost linearly with inlet pressure, while under conditions of constant pressure drop characteristic behavior was observed which depended on the density-viscosity relationships of supercritical fluid. Resolution in the separation of polysiloxane oligomers was found to be increased by increasing the ratio of pressure drop to pressure-programming rate, although the sensitivity decreased due to the increase in peak volume. The system controlling both inlet and outlet pressure has distinct advantages over the restrictor system controlling both inlet and outlet pressure has distinct advantages over the restrictor system in practical in practical operations.     

Supercritical fluid chromatography of imidazole derivatives

Summary The aim of this work was to use SFC to separate simple, slightly basic, imidazole derivatives which are used for the synthesis of more complex molecules with therapeutic properties. Control of their purity utilizes separation techniques and this paper shows what SFC can do in comparison with LC which requires derivatization before detection and with GC where peak tailing can be a problem. Our results concern the use of sub-critical mixtures of CO₂ and polar modifiers because imidazole derivatives react with neat CO₂, thus failing to elute from packed columns, and are only partially resolved on capillary columns with neat N₂O. Therefore, separations with CO₂-alcohol-amine-water mixtures on aminopropyl-bonded silica with UV detection are discussed. The resolution and sensitivity limits allow real sample analysis within a very short time.     

Supercritical fluid extraction of nylon 6,6 oligomers and their characterization via liquid chromatography coupled with mass spectrometry.

This research extends previous studies regarding the application of supercritical fluid extraction (SFE) for the analysis of oligomers from nylon 6,6 fibers. The effects of CO2 pressure, extraction temperature, CO2-modifier percentage, static extraction time and dynamic extraction time on the SFE efficiency of nylon 6,6 oligomers were examined. Results from the SFE methods for oligomer extractions were compared to results from conventional solvent extraction. The extracted oligomers were identified by high-performance liquid chromatography (HPLC) with coupled on-line atmospheric pressure chemical ionization mass spectrometry and HPLC fractionation coupled with off-line liquid secondary ion mass spectrometry.     

Supercritical fluid chromatography: A technology update

Summary Significant advances continue to be made in supercritical fluid chromatography both in the instrumentation and application areas. Both packed and wider bore capillary columns are being extensively investigated. Detector designs and interfaces are being reviewed with a great deal of work also going into restrictor and injection technology. The paper focusses on these current developments and how they impact the role of supercritical fluid chromatography in the analytical laboratory.

---

Chromatographie mit überkritischen fluiden Phasen: Stand der Technik

     

Supercritical fluid chromatography of Fusarium mycotoxins.

Capillary- and packed-column supercritical fluid chromatography has been used for the separation of Fusarium mycotoxins of various structure types such as the trichothecenes including deoxynivalenol and its acetylated derivatives and T-2 toxin, as well as butenolide, culmorin, sambucinol and zearalenone. The effect of modifier concentration and column temperature and pressure was also studied. Retention indices based on alkylphenones were determined for these mycotoxins on two of the capillary columns employed.     

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.     

Supercritical fluid chromatography of industrial paraffins and waxes

Summary The application of capillary SFC for SIMDIST investigations of paraffins and waxes is reported. Pressure, density and temperature of the mobile phase are optimized to obtain high chromatographic resolution of complex mixtures of industrial products. Isothermal linear pressure programming and asymptotic pressure and density programming are used in the range from 15 to 35 MPa in the isothermic mode above 100°C. SFC chromatograms of natural and synthetic paraffins, microcrystalline wax and candle wax are compared. The retention behaviour of paraffin mixtures and waxes has been investigated on methyl/phenyl and biphenyl capillary columns. The polarity of the stationary phases influences the absolute retention time more than it influences the chromatographic resolution.     

SFC/FT-IR: Supercritical fluid chromatography with Fourier transform infrared detection

Fourier transform infrared spectroscopy has been utilized in the discrimination of chromatographic eluents with success. The technique of capillary supercritical fluid chromatography (SFC) offers the possibility of highly efficient and quantitative separations which avoid problems associated with gas chromatography. The technique of SCF/FT-IR utilizing a flow-through cell is described. Approaches to solutions of several of the problems inherent in flow-through cell SFC/FT-IR are described.     

Supercritical fluid chromatography hyphenated with twin comprehensive two-dimensional gas chromatography for ultimate analysis of middle distillates

This paper reports the conditions of online hyphenation of supercritical fluid chromatography (SFC) with twin comprehensive two-dimensional gas chromatography (twin-GC × GC) for detailed characterization of middle distillates; this is essential for a better understanding of reactions involved in refining processes. In this configuration, saturated and unsaturated compounds that have been fractionated by SFC are transferred on two different GC × GC columns sets (twin-GC × GC) placed in the same GC oven. Cryogenic focusing is used for transfer of fractions into the first dimension columns before simultaneous GC × GC analysis of both saturated and unsaturated fractions. The benefits of SFC–twin-GC × GC are demonstrated for the extended alkane, iso-alkane, alkene, naphthenes and aromatics analysis (so-called PIONA analysis) of diesel samples which can be achieved in one single injection. For that purpose, saturated and unsaturated compounds have been separated by SFC using a silver loaded silica column prior to GC × GC analysis. Alkenes and naphthenes are quantitatively recovered in the unsaturated and saturated fractions, respectively, allowing their identification in various diesel samples. Thus, resolution between each class of compounds is significantly improved compared to a single GC × GC run, and for the first time, an extended PIONA analysis of diesel samples is presented.     

Serially coupled capillary columns supercritical fluid chromatography with midpoint pressure control

Two capillary columns of different polarities were coupled in series by means of a coupling restrictor. The pressure of the first column and the midpoint pressure (between the coupling restrictor and the second column) were controlled independently of each other using two pumps. The selectivity of this separation system was highly dependent on the pressure difference and could be continuously changed between those of two columns. The pressure difference could be changed even in course of separation for fine tuning of the selectivity. Several examples were shown to demonstrate the utility of this method.     

Supercritical fluid injection of high-molecular-weight polycyclic aromatic compounds in capillary supercritical fluid chromatography

A sample introduction system for capillary supercritical fluid chromatography, which allows the dissolution of the sample in the supercritical mobile phase before being introduced into the column, was constructed and evaluated. Supercritical n-pentane was shown to solvate high-molecular-weight polycyclic aromatic compounds that could not be solvated using typical liquid solvents. In addition, split injection of a supercritical fluid solution was found to be more reproducible than split injections of a liquid solution. The potential of such an injection system was demonstrated, although further developments are needed in order to make the technique of practically utility.