Traditional environmental monitoring only tests the total amount of toxic metal elements, but modern scientific studies have shown that the toxicity of many elements is related to their chemical form, and the different forms of the same elements have different effects on the environment and humans. In the field of biology, the way in which metals act on biological systems is determined by the chemical form of the metal elements rather than its total amount. Therefore, in modern environmental science research, not only the total amount of elements should be measured, but also the morphological analysis is needed to understand the occurrence pattern, migration and transformation law and toxicological effects of compounds in the environment.

Method principle

Chemical morphological analysis mainly uses chromatography. For some volatile compounds, gas chromatography (GC) is the main analytical tool. However, due to the insensitivity and special effects of most chromatographic detectors on most metal elements, GC has many limitations in the morphological analysis of organometallic compounds. Atomic absorption spectroscopy (AAS), as an elemental effect detector combined with GC, has become the main method of metal speciation analysis. This combination technology has the advantages of strong GC separation ability, low matrix interference, and high sensitivity and selectivity of AAS. The good features make up for the inadequacy of the GC detector for metals and the indistinguishability of AAS for different forms of the same element.

The GC-AAS combined device is shown in Figure 1. The principle is that the sample to be tested is injected from the inlet and is carried by the carrier gas into the column after gasification in the gasification chamber. Since the components to be separated entering the column have different physical or chemical properties, they are in the stationary phase. And the mobile phase has different partition coefficients. When these components move with the mobile phase (carrier gas), they are repeatedly distributed between the two phases, so that the components are completely separated, and the separated components are The combined action of the carrier gas and the auxiliary makeup gas enters the heated stainless steel tube, and after decomposition, the elemental state of the analyte is obtained and passed through the absorption tube of the T-tube into the quartz atomizer and measured by an atomic absorption spectrometer.

Method application

The development of GC-AAS technology has led to the sensitive determination of many trace organometallic compounds that cannot be detected by a single chromatographic method. It also extends the application of AAS from the determination of the total amount of a single element to the field of morphological analysis.

The key to the GC-AAS combination is the design of the interface part. The interface of the GC-AAS joint technology developed by this laboratory has the following advantages:

1. The T-type quartz tube is longer, which can increase the retention time of the atom in the optical path, and the sample is atomized before entering the absorption tube. Before reaching the absorption tube, there is a distance to cool it, which is better than the sensitivity obtained by directly heating the T-tube. High, which is beneficial to improve the sensitivity of the measurement;

2. The use of 0.5-1.0mm inner diameter stainless steel tube transmission overcomes the band broadening caused by the diffusion of the sample during the transfer process, and obtains a satisfactory separation effect;

3. The stainless steel nozzle enters the intersection of the T-shaped absorption tube so as to be as close as possible to the beam of the hollow cathode lamp without blocking the beam, and at the same time avoids the diffusion of the component to be tested which is decomposed therein before the absorption tube, and obtains good Peak type.

At present, the GC-AAS combination method has been widely used in environmental and biological sample analysis, feed detection, fine chemical analysis and drug detection, etc., mostly for the separation and determination of trace toxic organometallic compounds, typically organic mercury, organic Morphological analysis of selenium, organic germanium, and organotin. The research group has completed the morphological analysis and biological samples of dimethylmercury (DMM), diethylmercury (DEM), methylmercury (MMC), ethylmercury (EMC) and phenylmercury (PMC) in the air. Analysis of MMC, separation of MMC, EMC, PMC in soil and sediment (Fig. 2) and morphological analysis of dimethyl selenium, diethyl selenium and dimethyl diselenium. The relevant research won the second prize of the Natural Science of Chinese Academy of Sciences in 1998.

Prospects

Since the combination of GC-AAS avoids the complicated operation of AAS for separating the measured morphology from the matrix when used for morphological analysis, it overcomes the shortcomings of traditional GC detectors that are insensitive to trace organometallic compounds in the environment. Therefore, it is widely applicable to organometallic compounds such as alkyl mercury, selenium, tin, antimony and lead which are not closely related in polarity in the environment and biological samples, are volatile at a certain heating temperature, but are thermally stable and have a low atomization temperature. Morphological analysis of et al. If combined with emerging sample pretreatment techniques, such as microwave extraction, solid phase extraction, solid phase microextraction, etc., the analysis time of GC-AAS can be greatly shortened, making it more suitable for the fast and accurate requirements of modern environmental analysis. However, for the separation of Hg2+ and saturated alkyl mercury, Se4+ and saturated alkyl selenium, and the separation of non-volatile, thermally unstable compounds such as arsenide, the use of GC-AAS is also subject to certain limit.

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