Elemental analysis is an important research field in modern analytical science...
Elemental analysis is an important research field in modern analytical science. The physiological and toxicological effects, bioaccessibility, environmental behavior and mobility of an element depend to a large extent on its form. One form of an element may be toxic, while another form of the same element may be non-toxic, even necessary for biological functions. The physicochemical properties and biological effects of compounds of the same family may be quite different. Therefore, only relying on the determination of the total amount of elements cannot fully explain their toxicity and biological effects, and the chemical forms of the elements must also be analyzed.
According to the characteristics of speciation analysis and the complexity of the sample, element speciation analysis requires the use of a combination of chemical separation and instrument detection, that is, combined technology. Separate the trace or trace elements of different valence states and different forms with separation equipment first. Then, the content of these trace or trace elements in different valence states and different forms are respectively measured with detection equipment.
Atomic absorption spectroscopy (AAS) is an instrumental analysis method with high selectivity, high sensitivity, simple instrumentation, and fast analysis speed. According to the different atomization methods used, atomic absorption spectroscopy can be divided into flame atomic absorption spectroscopy (FAAS), graphite furnace atomic absorption spectroscopy (GFAAS), and quartz furnace atomic absorption spectroscopy (QFAAS). Atomic absorption spectrometry is used to determine various elements in samples, and has important applications in the detection of metal ion limits and the determination of rare elements. However, a single atomic absorption spectrometry (AAS) can only determine the total amount of elements, and cannot directly provide information on the chemical forms of the elements.
Chromatography-atomic absorption spectroscopy hyphenated technology combines the high separation efficiency of chromatogram and the high selectivity and sensitivity of atomic absorption spectrometry, and it is one of the most effective methods for analyzing element forms.
Gas chromatography (GC) has the advantages of simple principle, convenient operation, high separation efficiency, fast analysis speed, and low sample consumption. It is widely used in the separation and analysis of volatile substances. GC-AAS technology can perform morphological analysis on complex mixtures containing metals and certain non-metal elements. GC-AAS technology can directly introduce the gaseous components separated by gas chromatography and the carrier gas into the atomic spectrum for direct analysis and determination.
GC-FAAS is a flame atomizer that directly introduces the separated components of GC into FAAS through a heated transfer line.
GC-FAAS has the advantages of continuous operation and simple instrumentation, and real-time chromatograms can be obtained. However, the residence time of the atomic vapor in the flame atomizer is short, and the analyte is diluted by the fuel gas and the supporting gas. Therefore, the sensitivity of GC-FAAS is low, and it cannot meet the current requirements of morphological analysis, and it has fewer applications.
GC-QFAAS has high sensitivity and is the mainstream technology of GC-atom absorption spectroscopy. Quartz furnace can work continuously, with good reproducibility, simple production, and slightly lower sensitivity than graphite furnace.
GC-GFAAS has good separation performance, high selectivity and sensitivity. However, because the graphite furnace lasts for a long time at high temperature (above 1000 degrees Celsius), the life of the graphite furnace is shortened, the repeatability is poor, and the operating cost is high.
The development of GC-AAS technology has enabled the sensitive determination of the morphology of many trace organometallic compounds that cannot be detected by a single chromatographic method. At the same time, the application of AAS has been extended from the total amount of a single element to the field of morphological analysis. GC-AAS is widely applicable to the morphological analysis of organometallic compounds (such as organic mercury, organic selenium, organic tin, organic germanium and organic lead, etc.) in environmental and biological samples with little difference in polarity, thermal stability, volatility, and low atomization temperature.
High-performance liquid chromatography (HPLC) can directly analyze high-boiling, hard-to-volatile substances. And HPLC has more operating parameters to choose from. HPLC-AAS technology has the characteristics of high sensitivity, low detection limit, and simple operation. In recent years, it has gradually become one of the widely used techniques in speciation analysis. In HPLC-AAS, the sample is converted into a solution and then enters the nebulizer. The atomized sample enters the atomizer again. In order to improve the sensitivity of certain elements, the sample in HPLC-AAS can also enter the hydride generator after being converted into a solution. The produced hydride goes directly into the atomizer.
The joint interface of HPLC and FAAS is very simple. However, due to the low spray efficiency and the absorption of the flame itself, the sensitivity of HPLC-FAAS is low, and it is rarely used for the speciation analysis of trace elements.
The graphite furnace atomic absorption process is discontinuous, and there is currently no ideal way to combine the continuous process of liquid chromatography with the discontinuous process of graphite furnace atomic absorption.