The principle and basis of sample handling method determined by ray energy dispersion spectrometer

Sample treatment is an important link in the whole process of analysis and testing, its purpose is to use various chemical methods to measure the element from the solid (liquid) state of the sample quantitatively in the form of ions into the test solution. Choosing a reasonable sample decomposition method can greatly simplify the analytical procedures, and greatly improve the adaptability and accuracy of the analytical method.

The principles for designing the best sample treatment are:

Ensure that all the tested elements in the sample are transferred quantitatively into the test solution, that is, the sample decomposition is complete;

Avoid the introduction of interfering elements in the process of sample processing, and at the same time be conducive to removing interfering elements;

The decomposition method should be as simple as possible, easy to operate, economic, rapid and safe, and minimize the pollution to the environment;

The principle and basis of sample handling method determined by ray energy dispersion spectrometer

Convenient for batch processing samples.

To design a sample handling method that meets these conditions, it is necessary to have an in-depth understanding of:

a) the physicochemical properties of the element under test and its compounds;

b) the content range and occurrence form of the measured element in the sample;

c) composition and properties of the sample matrix;

d) the final test methods and techniques used.

The sample treatment methods used for the determination of As, Se and Hg at home and abroad are taken as an example

For the determination of As, Se and Hg in various samples at home and abroad, the sample treatment methods used can be roughly divided into three categories:

1) Wet acid/alkali decomposition;

2) closed system combustion method (oxygen bomb combustion method);

3) sintering (semi-melting) method.

In the process of qualitative and quantitative detection of trace elements in coal, the sample pretreatment is often more important than the detection technology itself. Especially for various atomic spectroscopy techniques, it is generally necessary to make liquid samples or liquid samples to achieve a higher accuracy of 1 degree. In order to ensure the accuracy of detection, the coal sample with complex composition should be fully dissolved at the same time, and the loss of the volatile single substance or compound form of the element to be detected should be avoided. At present, there are the following methods at home and abroad:

(1) Direct digestion of sealed melting crucible.

The method uses a closed container, with mixed acid to decompose the sample directly, while using microwave oven heating digestion. The method has the advantages of less weighing sample, high dissolution efficiency, simple operation, safety, easy control and avoiding volatilization. But the direct decomposition method inevitably has the disadvantage of incomplete decomposition. This method is often used to detect trace elements in burned coal ash or coal extract samples.

(2) Low temperature ashing method.

Plasma technology is used to ash the sample at about 150℃, and then decompose it with mixed acid in a polytetrafluoroethylene container. This method is recognized as a better pretreatment technology at present, but it also has the disadvantages of high equipment operating cost and long time.

The principle and basis of sample handling method determined by ray energy dispersion spectrometer

Wet acid decomposition

In the determination of geological samples, HNO3-HF-HCIO4 system is often used for acid dissolution. When the sample is decomposed by this method, if the heating intensity is slightly high and the evaporation is too dry, part of the Se in the sample will volatilize and lose. The reason may be that Se(CIO4)2 generated by acid decomposition of the sample can be divided into HC and SeO2, and these two compounds can react to form SeC2 which can be sublimed at a lower temperature. According to the information, when the sample is processed, the Se in the sample can be lost by about 40% after evaporation to dry. Therefore, using this method to decompose the sample, must be careful to master the heating 1 temperature and time, steam to 1~2mL transparent solution, that is, should stop heating. There is also the use of HNO3-H2SO4 system acid dissolution and HNO3-HCIO4 system decomposition. The operation procedure of wet decomposition is more complicated, and accompanied by acid mist volatilization into the atmosphere, which is not beneficial to environmental protection. In addition, for coal, due to the large amount of organic matter contained in coal, it brings inconvenience to acid decomposition.

Oxygen bomb combustion method

This method is to place the coal sample in the stainless steel cartridge filled with hyperbaric oxygen, and power the coal sample, the organic matter in the coal is fully burned, and the inorganic minerals also occur oxidation, decomposition and other reactions; The Se element in the coal is converted into oxide, and then dissolved in gaseous form into the absorption liquid (water or dilute alkali) in the elastic. Some laboratories abroad use this method to measure se in coal. Its advantage is that the sample processing in addition to oxygen, do not introduce other reagents, reduce the opportunity to introduce interfering elements. The disadvantage is that the decomposition of high ash coal may not be complete (can not be completely burned), which can lead to low analysis results; The operation is more troublesome, and the efficiency of processing samples is low; It is not conducive to batch decomposition of samples.

Sintering (semi-melting) method using Aldrin and sample mixed evenly, heating and burning, so that As, Se in coal is oxidized to oxide, and then react with Na2CO3, MgO to form arsenate and selenate. Then use HC to dissolve the burning matter, As, Se in the form of ions into the solution. Taking coal 1 as an example, the reaction equation is:

The advantage of this method is that it is easy to operate and decompose samples in batches. As long as the burning conditions are controlled correctly, the measured elements can be quantitatively transformed. Considering the analysis standards of other elements in coal at present, there are a variety of methods using Aldrin semi-melting to decompose coal samples, which are easy to be accepted by analysts and have high working efficiency; National standards and American standards also use Aldrin semi-melting method to treat samples. Through the comparison of various treatment methods, this test adopts the semi-melting method to treat the sample.In order to transfer the tested element from the burned sample to the solution, the burned matter must be dissolved with a suitable acid. To this end, experiments are also carried out with regard to the type of acid and the method of adding acid.

The principle and basis of sample handling method determined by ray energy dispersion spectrometer

Types of acids

The use of different acids to treat the sample has an effect on the results, so the test examined the dissolution of HCI, H2SO4 and HNO3 in the treated cautery and the effect on the determination.H2SO4: It was found in the test that dissolving the burned matter with H2SO4 would lead to CaSO4 precipitation. In addition, H2SO4 contains As and Se, which leads to too high blank space. If it is removed in advance, the experimental workload will be increased. Therefore, H2SO4 should not be selected as the dissolved acid.HNO3: The test found that when HNO3 dissolved the sample, the atomic absorption signal was lower than that of HC medium. The reason may be that HNO3 reacted with the reducing substance in the sample as an oxidizing agent to form nitrous acid, and NO2- inhibited the formation of hydride of Se. Some foreign laboratories use HNO3 as the medium to determine the hydride generation atomic absorption of Se, in order to reduce the sensitivity of the cost, in exchange for better stability.HCI: The use of HC to dissolve the calcined matter is a common method at home and abroad. Most chlorides are soluble salts, HC has reducing properties, and it is a reducing agent that reduces Se6+ to Se4+. The content of As.Se in HCI is small, and the reagent blank is low, which is advantageous for determination. lHCI is used in this method to dissolve the burned matter.