Supercritical fluid chromatography matrix effects

Other separation techniques, such as capillary electrophoresis (CB) and supercritical fluid chromatography (SBC), have been shown to perform well for the separation of phenols. Several papers describing the use of CB for the separation of phenolic compoimds in water samples have been published lately [97-100]. The majority of these employ UV detection, but BSP-MS in the negative-ion mode [101] and indirect fluorescence detection [102] have also been used. In one study, a comparison between HPLC and CB was performed to assess their suitability for the determination of the 11 priority pollutants in water [16]. The authors claim that CB gave a shorter analysis time and smaller matrix effects. However, it was not possible to achieve the desired detection limits without a preconcentration on solid-phase material. 

If the analyte is soluble in a SF yet cannot be extracted from the matrix, the analyte matrix interactions may be too strong. The problem may be overcome by the addition of modifiers to the supercritical fluid, or by the direct addition of a modifier to the extraction vessel. Several papers dealing with the use of modifiers for supercritical-fluid chromatography (SFC) (89-91) and SFE have been published (92-94). Modifiers have two basic effects on the SFE of analytes from a matrix. They can interact with the surface of the matrix displacing the analyte into the SF. To distinguish between the two types of modifiers, they are commonly termed solvent modifiers and matrix modifiers, respectively. 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

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. 

---