Investigation of parameters affecting the supercritical fluid extraction of polymer additives from polyethylene

Polymer additives were extracted from polyethylene with supercritical carbon dioxide. The two-film theory, which considers mass transfer across a phase boundary, is applied to qualitatively describe the kinetics of mass transfer from the core of the polymer particles into the supercritical fluid extractant. The effects of pressure, temperature, addition of benzene as a modifier, properties and concentrations of the solutes, static time, and supercritical fluid extractant flow rate on the extraction process are investigated systematically. At constant temperature the extraction rates first increase with increasing pressure. When pressure reaches a certain level, a further increase of the pressure does not further increase the extraction rates. At constant pressure, the extraction rates were found to increase first and then decrease with increasing temperature. In addition to pressure and temperature, the SFE extraction kinetics is also influenced by the solute concentration, and the rate-limiting parameter in the extraction can be changed from solubility to diffusion during the course of the extraction. The magnitude of the effects of the experimental parameters depends on the properties and molecular weights of the solutes. The role of benzene as a modifier in the extraction of polymer additives from polyethylene is swelling the polymer particles and improvement of the solvent strength of the supercritical fluid extractant. Modifier effects were found to be more pronounced at lower temperatures

Compressibility effects in packed and open tubular gas and supercritical fluid chromatography

The influence of the pressure drop on the efficiency and speed of anal. in packed and open tubular supercrit. fluid chromatog. (SFC) is described: methods previously developed to describe the effects of mobile phase compressibility on the performance of open tubular columns in SFC have been extended to packed columns. The Horvath and Lin equation has been used to elucidate the influence of variations in velocity, diffusivity, and capacity factor along the column on the overall efficiency of packed column SFC. In packed columns, in contrast with the situation in open tubular columns, because the diffusion coeffs., the increase in both linear velocity and capacity factor which result from a significant pressure drop cause the plate height to increase along the column. The effect of liq. decompression along the length of the column on the speed of anal. in SFC has been studied and numerical expressions derived which enable calcn. of compressibility correction factors for the plate height. Both the f1 and f2 correction factors remain very close to unity for acceptable pressure drops, which means that the pressure drop has virtually no effect on the no. of plates generated per unit time for an unretained component. For retained species, the decompression of the mobile phase across the column causes the capacity factor to increase and hence leads to increased anal. time

Supercritical fluid extraction-capillary gas chromatography: on-line coupling with a programmed temperature vaporizer

A simple and versatile system is described for the on-line coupling of SFE to capillary GC. The interfacing consists of a programmed temperature vaporizer (PTV) injector. With this injector it is possible to combine solute trapping, elimination of a high flow of extraction fluid, and quantitative transfer of solutes to the seperation column. The problems caused by impurities in the extraction fluid in on-line SFE-GC are discussed. Simple methods are described for the purification of commercially available carbon dioxide. The trapping efficiency of the PTV injector is studied. Applications of the SFE-PTV-GC system are given for the analysis of polymer anti-degradants, polar compounds, and samples with environmental relevance

Electron capture detection in high-speed narrow-bore capillary gas chromatography: fast and sensitive analysis of PCBs and pesticides

In this work the combination of high-speed narrow-bore capillary GC with electron capture detection is evaluated. The make-up gas flow rate is a key parameter in the successful coupling of narrowbore columns and ECD detection. The make-up flow has to be as high as possible to eliminate peak tailing caused by the large detection cell volume. The sensitivities at these elevated make-up flow rates (400 to 1000 ml/min), measured for some pesticides like HCB and dieldrin, were very good. Detection limits for these compounds of 0.1 pg were obtained, resulting in minimum detectable concentrations of approximately 0.2 ppb. The performance of the system is illustrated by several high-speed analyses of environmentally relevant samples of PCBs and pesticides

Use of an open-tubular trapping column as phase-switching interface in on-line coupled reversed-phase liquid chromatography-capillary gas chromatography

The applicability of open-tubular traps for phase switching in coupled RPLC—GC was studied. The phase-switching process involves sorption of the analytes of interest from a methanol—water mobile phase into the stationary phase of an open-tubular column, removal of the aqueous phase by purging the trap with nitrogen and desorption of the analytes with hexane. Water elimination carried out in this manner appears to be highly efficient. In the sorption step the sampling flow-rate and the capacity factors of the analytes in the trap are critical parameters. Using a 2 m × 0.32 mm I.D. trap with a swollen 5-μm stationary phase at flow-rates not exceeding 100 μl/min, polycyclic aromatic hydrocarbons are trapped quantitatively from 300 μl of aqueous phases containing up to 65% (v/v) of methanol. For desorption 70–125 μl of hexane are needed. These volumes are easy to handle in solvent elimination carried out using a PTV injector prior to transfer of the analytes to a GC column

High-speed narrow-bore capillary gas chromatography in combination with a fast and double-focusing mass spectrometer

In this work the application of high-speed narrow-bore capillary GC in combination with a fast scanning double focusing magnetic sector mass spectrometer is evaluated. Special emphasis is placed upon detection limits and scan speed in the full scan mode and in the selected ion monitoring mode (SIM). In the full scan mode, up to 20 scans per second could be obtained. The detection limits are in the low picogram range in the full scan mode and improve even to 5 to 50 fg in the SIM mode, depending on the sample complexity and mass resolving power. It will be illustrated that by increasing the resolution in the SIM mode, interferences from ions of the same nominal mass-to-charge ratio as the ions of interest are significantly reduced. Chemical background noise can therefore be largely eliminated, thus enhancing the signal-to-noise ratio