Most direct-reading spectrometers are used in the pre-furnace analysis of the smelting or casting process. In order to obtain an accurate analysis result, in addition to the good performance of the spectrometer itself, the correct use, operation, maintenance and management of the instrument can fully play its role To get accurate analysis results.
It is inevitable that errors occur during the analysis. There are many sources of error. As far as photoelectric spectroscopic analysis is concerned, in addition to the uneven composition of the standard sample and the analysis sample, and the inconsistent tissue conditions, the unstable spectral performance, improper surface treatment of the sample, and insufficient purity of argon can cause errors. Therefore, it is very important for each analyst to understand the causes of errors and further study the methods of eliminating them.
The excitation light source is an extremely important part of the photoelectric spectrometer, and its role is to provide the analysis sample with the energy of evaporation, atomization or excitation. There is no clear boundary between evaporation, atomization, or excitation of a sample during spectroscopic analysis, and these process equations occur almost simultaneously. These series of processes directly affect the analysis results. The evaporation, dissociation, excitation, ionization, emission of spectral lines, and the intensity of spectral lines of the group analysis elements in the sample are in addition to the melting point, boiling point, atomic weight, chemical reaction, dissociation energy of the compound, ionization energy, and excitation energy of the sample components. In addition to the physical and chemical properties of the energy levels of atoms and ions, it is also closely related to the characteristics of the light source used. Different excitation light sources have different evaporation behaviors and excitation energies for various samples and various elements. Therefore, according to different analysis objects, an excitation light source with corresponding characteristics should be selected.
At present, there are two types of light sources commonly used. One is the classic light source, including arc and spark light sources. Among them, high-voltage wave-controlled light sources, low-pressure spark high-speed light sources, and high-energy pre-spark light sources are widely used in metallurgical analysis. Most are widely adopted in different fields.
There are several discharge methods for common light sources for spectral analysis:
1. The maximum current of the high-energy pre-spark discharge can reach 150 amps and the burning time is 150 microseconds. This makes the tissue structure in the burning spots in the sample more uniform, thereby eliminating the effects of interference between elements and bonding between elements.
2. Spark discharge is reproducible for most elements.
3. The reproducibility of arc discharge is 2-3 times worse than that of spark discharge, but the detection limit for trace elements is much lower.
Therefore, you should try to meet the following requirements when choosing a light source:
1. High sensitivity, with slight changes in the element concentration in the sample, the detected signal has a large change;
2. Low detection limit, can detect trace and trace components;
3. Good stability, the sample can be stably evaporated, atomized and excited, so that the result has higher precision:
4.The ratio of spectral line intensity to background intensity is large (large signal-to-noise ratio);
5. Fast analysis speed and short pre-burn time;
6. Simple structure, easy operation and safety;
7. Small self-absorption effect and wide linear range of calibration curve.
The selection of the excitation conditions of the light source should be determined according to the test of the analysis object. For different samples, the pre-ignition time is different under different light sources, which mainly depends on the evaporation process of the sample during spark discharge. It is not only closely related to the energy of the light source and the discharge atmosphere, but also to the sample. Composition, structural state, type of inclusions, size and so on are closely related.
Spark discharges in an argon atmosphere can generally be divided into two extreme states: concentrated discharges and diffusion discharges. When the discharge is performed on the metal phase, it is called a concentrated discharge, and when the discharge is performed on the non-metal phase, it is called a diffusion discharge. When discharging in argon, there are three main reasons for diffusive discharge: 1. Purity of argon; 2. Leakage of argon inlet and outlet pipes or discharge cells is the second source of oxygen introduced during discharge. : 3. The sample itself is the third source of oxygen introduced during the discharge (such as inclusions, bubbles and cracks); when discharging in argon, the argon pressure, argon flow and flushing time also affect the photoelectric spectrum Results of the analysis.
Matrix effect, also known as coexisting element, third element, or concomitant effect, mainly refers to the effect of all other components in the sample except the analyte, which is the main reason for the change in the intensity of the spectral line in the spectral analysis and the error in the analysis result. This effect should also be called interference effect, which is the most complicated problem in spectral analysis.
In actual work, due to the difference in the smelting process and physical state of the analysis samples and standards, the calibration curve often changes. Usually the standards are mostly forged and rolled, and the analysis samples are mostly cast. The impact of the change in the metallurgical state of the sample on the analysis. A control sample that is consistent with the metallurgical process and physical state of the sample is often used to control the analysis result of the sample.
In spectral analysis, the sampling method and the processing of the sample are very important, it directly affects the accuracy and accuracy of the analysis. During the analysis in front of the furnace, a rapid red cut of the as-cast steel sample in the furnace was found, and it was found that the sample had cracks, inclusions and pores, and it was necessary to re-sample. In the case of low-carbon steel, the red material is put into running water for rapid cooling, which promotes the formation of martensite and austenite in the sample structure, ensuring the accuracy of carbon analysis results. In the case of high carbon samples, warm and cold should be used to avoid cracks. For the analysis of cast iron and nodular cast iron, the analysis sample must be fully whitened, and standardization of sampling is required, such as sampling temperature, demolding time, and cooling rate. It is also important to analyze different materials and choose different grinding tools. Generally, alumina abrasive wheels are used, and the particles are medium. The surface of the sample should be removed from 0.5 to 1.5 mm, because the oxide layer on the surface of the sample often causes erroneous analysis results, especially for carbon.
In short, the detection and analysis error always exists. It is important to treat it correctly and eliminate it. For optoelectronic spectroscopic analysis, errors are mainly caused by changes in the following five factors:
People: Operator's quality awareness, technical level, proficiency and physical fitness.
Equipment: Equipment maintenance is crucial, the performance and reproducibility of the light source, the stability of the argon system, and the sample processing equipment and its maintenance status.
Sample: The uniformity, representativeness, heat treatment state and structure state of the components of the sample to be tested. The uniformity of the composition of the standard sample and the control sample, the reliability of the standard value of the component content, and the identity of the organizational structure. The method of grinding the sample and its effect are both key.
Analytical method: The production of the calibration curve and the degree of fitting, the standardization process and its effect, the selection and setting of the control sample must be strictly operated.
Environment: Control the temperature, humidity, electromagnetic interference and cleaning conditions of the analysis room. With a stable operating environment, the stability of the instrument will be good.
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