Sample introduction supercritical fluid

Chapter 5 Sample Introduction Supercritical Fluid Chromatography... 

Smith and Udseth [154] first described SFE-MS in 1983. Direct fluid injection (DFT) mass spectrometry (DFT-MS, DFI-MS/MS) utilises supercritical fluids for solvation and transfer of materials to a mass-spectrometer chemical ionisation (Cl) source. Extraction with scC02 is compatible with a variety of Cl reagents, which allow a sensitive and selective means for ionising the solute classes of interest. If the interfering effects of the sample matrix cannot be overcome by selective ionisation, techniques based on tandem mass spectrometry can be used [7]. In these cases, a cheaper and more attractive alternative is often to perform some form of chromatography between extraction and detection. In SFE-MS, on-line fractionation using pressure can be used to control SCF solubility to a limited extent. The main features of on-line SFE-MS are summarised in Table 7.20. It appears that the direct introduction into a mass spectrometer of analytes dissolved in supercritical fluids without on-line chromatography has not actively been pursued. 

The introduction of commercial instrumentation in this automated area has been too slow and too disappointing to meet the need for routine analysis of numerous samples. The options have been the extraction of one sample at a time or individual samples in parallel. Either of these options make the repetitive analysis of the same sample or the sequential analysis of different samples exceptionally time consuming. Parallel analysis, proposed by one manufacturer, is susceptible to cross-contamination and across the board sample loss with clogging of one extraction vessel. In order to move supercritical fluid extraction into the realm of routine operations for residue analysis, rapid analysis of multiple samples needed to be addressed. 

The soil samples which were extracted in our experiments were saturated with methanol prior to the introduction of the mobile phase. Other samples, especially those which have been field aged, are saturated with water and water/methanol mixtures before the supercritical fluid extraction process begins. Our experience indicates that this saturation step is crucial to obtain high recoveries. 

Another powerful means of sample introduction is direct, on-line supercritical fluid extraction. A small amount of solid (or liquid) sample can be placed in an extraction vessel, followed by the introduction of the supercritical fluid. The material can be... 

Most FTMS instrument and method development research has been focussed on demonstration experiments. Examples include coupling FTMS with various sample introduction schemes (e.g., GC, LC, supercritical fluid chromatography), sample ionization (e.g., LD, pulsed SIMS, Cf-252 PDMS, etc.), and demonstrating application to various interesting classes of chemical compounds. These demonstrations are useful because they are indications of the potential of the technique. However, few reports of the routine use of FTMS for trace analysis, for accurate mass, and for structure determination of unknowns have yet appeared. One reason is that FT mass spectrometers are not widely spread in the hands of users. Another is that FTMS is not yet routine. Most of the demonstration experiments have been done in expert laboratories by committed and highly focussed graduate students and postdoctoral researchers. 

Kirschner, C.H. and L.T. Taylor. 1993. Recent advances in sample introduction for supercritical fluid chromatography. J. High Resolut. Chromatogr. 16 73-84. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Clifford, A.A., Introduction to supercritical fluid extraction in analytical science. In Supercritical Fluid Extraction and Its Use in Chromatographic Sample Preparation, S.A. Westwood (Ed.), Blackie Academic and Professional, Glasgow, 1-38,1993. 

In addition to gases, supercritical fluids can be used to carry samples into an IMS after chromatography or directly after extraction. Supercritical fluids have two primary advantages for sample introduction. First, unlike gases, supercritical fluids... 

Greibrokk, T. and Berg, B. E. (1993) Trace analysis in capillary supercritical fluid chromatography sample introduction. Trends Anal Chem., 12, 303-8. 

An automatic method for the separation and determination of RF vitamin in food samples (chicken liver, tablet, and powder milk) is proposed by Zougagh and Rios [2], The method is based on the online coupling of supercritical fluid extraction (SFE) with a continuous flow-CE system with guided optical fiber fluorometric detection (CE-CE-ED). The whole SFE-CF-CE-FD arrangement allowed the automatic treatment of food samples (cleanup of the sample followed by the extraction of the analytes), and the direct introduction of a small volume of the extracted material to the CE-ED system for the determination of RF vitamins. Fluorescence detection introduced an acceptable sensitivity and contributed to avoidance of interferences by nonfluorescent polar compounds coming from the matrix samples in the extracted material. Electrophoretic responses were linear within the 0.05-1 pg/g range, whereas the detection limits of RE vitamins were in the 0.036-0.042 pg/g range. 

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