SummarySolid Phase Micro Extraction (SPME) involves exposing a fused silica fiber coated with stationary phase to a contaminated water sample. The organic analytes become partitioned between the stationary phase and the water and when equilibrium is reached the fiber is removed from the solution and the analytes are thermally desorbed in the injector of a gas chromatograph.The fiber is contained in a syringe to facilitate handling.Factors which affect linear range, limit of detection, and total analysis time are discussed with regard to the development of a method for analysis of volatile compounds in environmental water samples.The sensitivity of the method was determined by the thickness of the film of stationary phase; the equilibration time, however, increased with the film thickness, although it can be minimized by use of a cross-shaped stirrer bar.Increasing the thickness of stationary phase in the analytical column enables the cryofocusing temperature to be increased from -40 to -15°C. With an ion trap mass spectrometer, detection limits required by the US Environmental Protection Agency are met for all compounds except chloromethane and chloroethane. The method has been applied to environmental water samples.
A method for sampling volatile chlorinated hydrocarbons in either gaseous or aqueous samples using solid-phase microextraction (SPME) is presented. In the liquid phase, method limits of detection of 1-130 ng I-' can be achieved with an electron-capture detector. The method is linear over at least 2-4 orders of magnitude and has a relative standard deviation of 1-5%. In the gaseous phase, the method has limits of detection in the parts per trillion ( v h ) range when used with an electron-capture detector. The linear range is at least two orders of magnitude and the relative standard deviation is 1-7%. Sample preparation times range from 10 min for gaseous samples t o 20 min for liquid samples. The method is comparable to US Environmental Protection Agency (EPA) methods 502.2 and TO-14. The precision of the technique is relatively independent of the number of injections made per container. Samples can be stored for up to 20 min on the fibre if they are capped and refrigerated. The amount absorbed by the fibre decreases with increasing temperature and increasing humidity.
Two mathematical descriptions of the solvent-free extraction process occurring In a hollow fiber membrane system are described. A model solving the diffusion equation and considering the concentration distribution across the diameter and along the axis of the fiber has an analytical solution. Mass transfer through the aqueous phase, membrane, and stripping phase has a semlemplrlcal solution. These two solutions were compared to each other and to experimental data for typical extraction conditions when gas flow rates are much higher than sample flow. Excellent agreement was observed, confirming the assumption that at these conditions diffusion through the aqueous phase Is the rate-limiting step. Extraction recoveries were Investigated by varying several parametersgeometry of the fiber, Its length, diameter, and thickness, linear velocity If the aqueous phase, gas/sample flow ratios, analyte Henry constant, and stripping-phase diffusion coefficients. The velocity of the stripping phase must be at least 20 times faster than that of the aqueous phase otherwise It will adversely affect the removal of volatile organic compounds (VOCs). The porous membrane thicknesses used In this study have no effect on the removal of VOCs from water; however, when the membrane wall Is 1 order of magnitude thicker than the Inner diameter of the fiber It becomes a factor In the rate of overall mass transfer. Spiralling the hollow fiber Increases the removal efficiency of the system at higher flow rates. This Is caused by Increased mixing of the components In the aqueous phase. The porous membrane extraction method Is effective for organic compounds which have Henry constants above 0.1. The Investigations were extended to a nonporous silicone hollow fiber membrane. It was found, again, that for the typical experimental conditions, extraction rate Is controlled by the diffusion of the analyte In the aqueous phase.
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