When measuring samples, the extraction amount depends not only on the polarity and thickness of the fiber coating but also on the extraction method, the volume of the water sample and the headspace phase, extraction time, stirring conditions, extraction temperature, pH, concentration and analysis of inorganic salts conditions and many other factors are related, so when using this method to analyze samples, the measurement conditions must be strictly controlled to make them exactly the same as when determining the calibration curve. Some of these parameters have been optimized to effectively improve the extraction efficiency.
The application of fiber technology to extract target compounds can use three extraction methods: direct immersion extraction; headspace extraction; membrane protection extraction, and three extraction methods.
Direct immersion extraction is to insert the fiber into the sample, and the analyte migrates directly from the sample matrix to the extracted fiber. It is suitable for the analysis of organic compounds in gas samples and clean water samples, but not suitable for the analysis of organic compounds in the complex sample matrix.
At present, the direct method is used for the extraction of most organic compounds, especially for the poorly volatile compounds in the water sample. The extraction head is in direct contact with the analyte during direct extraction, which is better than the headspace method.
For gas samples, the natural flow of air can make volatile compounds quickly reach equilibrium, but for liquid samples, a stable thin liquid layer will form around the fiber coating, which hinders the diffusion and migration of target compounds to the coating, so some form The stirring is very necessary. The disadvantage of applying the direct extraction method to the determination of liquid samples is that the contact between the fiber and the sample matrix will greatly shorten the service life of the fiber.
Headspace extraction is the process of exposing the fiber to the gas phase above the sealed sample and extracting the analyte that volatilizes into the headspace of the solid or liquid sample. It is suitable for analyzing wastewater, grease, humic acid, and other complex matrix samples and solid samples. Semi-volatile organic compounds. Compared with the direct extraction method, the headspace method has more advantages for volatile substances with larger diffusion coefficients.
Because part of the analyte has entered the headspace phase before extraction; here, the diffusion coefficient of the analyte is 4 orders of magnitude higher than that in the liquid phase. The interface between the gas and liquid phases is continuously updated by mechanical methods to make the analyte more Effectively adsorb to the fiber, so the time required for equilibrium is greatly shortened.
For example, for BTEX in water, the sampling time can be shortened from 5 min for the direct method to 1 min for the headspace method, and the detection limit reaches 10-9. In addition, in the headspace extraction method, the fiber does not directly contact the sample, avoiding the interference of the matrix, and the method’s reproducibility is better than that of the direct method.
As the two most commonly used extraction methods, choosing the headspace method or the direct method has no effect on the sensitivity of the method, but should be determined according to the nature of the target compound. Because for a system composed of liquid and its headspace phase, the number of analytes extracted by the fiber is closely related to the concentration of the compound in the sample, regardless of whether the fiber is in a liquid or a gas, it has nothing to do with the location of the fiber.
Only when the direct method is used to extract extremely volatile compounds, and the system does not have a headspace phase, its sensitivity is different from that of headspace extraction.
Membrane protection extraction is suitable for heavily polluted water samples. The fiber is separated from the sample by a selective membrane. The analyte can be adsorbed to the fiber through the selective membrane, while the high molecular weight compounds in the sample cannot pass through, thereby eliminating matrix interference.
Unlike headspace extraction, the membrane protection method can be used for the analysis of non-volatile compounds. However, since the analyte must diffuse through the membrane before it can be adsorbed on the coating, the extraction time is longer. To shorten the extraction time, a thinner membrane can be used and the sample temperature can be increased.