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  • Factors affecting analysis quality of direct reading spectrometer
Factors affecting analysis quality of direct reading spectrometer
Factors affecting analysis quality of direct reading spectrometer




1. Argon



The main function of argon blowing is to drive away the air in the spark chamber when the sample is excited, and reduce the absorption of the air to the spectral line of the ultraviolet light region. The main reason is that oxygen and water vapor in the air have strong absorption band in far ultraviolet region, which has a great influence on the analysis results, and it is not conducive to the excitation stability, forming or strengthening diffusion discharge, and white spots will be generated during the excitation. In addition, the alloy elements in the sample may react with the air components to form molecular compounds at high temperature, which will cause interference to the atomic spectrum we need. Therefore, it is necessary to require that the purity of argon be over 99.999%. In addition, the pressure and flow of argon also have a certain influence on the analysis quality. It determines the impact capacity of argon on the discharge surface. This excitation ability must be appropriate and too low to flush out the oxygen produced during the sample excitation and the oxide formed by it. These oxides agglutinate on the electrode surface, thus inhibiting the continuous excitation of the sample; the argon flow rate is too large, one is to make It's unnecessary waste. Second, it also has some damage to the spectrometer. Therefore, the pressure and flow rate of argon must be appropriate. According to the practice, the pressure and flow of argon should be adjusted according to different materials. The analysis of low alloy steel shows that the argon pressure input into spectrometer should reach 0.5-1.5mpa, the flow rate of dynamic argon is 12-20 readings, and the static argon flow rate is 3-5 readings.



2. Slit



The spectrometer adopts a complex and sensitive optical system. The changes of the ambient temperature, humidity, mechanical vibration and atmospheric pressure of the spectrometer will cause slight changes in the spectral line and cause the deviation of the spectrum line. The change of air pressure and humidity will change the refractive index of the medium, which will offset the spectral line. The increase of humidity will not only increase the refractive index of the air, but also corrode the optical parts. The light transmittance of the instrument is lowered. The humidity should be controlled below 55% to 60%. The influence of temperature on grating mainly changes the grating constant, which causes the change of dispersion rate and the shift of spectral line. These changes will make the spectral line not fully aligned with the corresponding ejection slit, thus affecting the analysis results. Therefore, the optical system shall be adjusted at least once a day. If the indoor temperature control is constant, the slit shall be adjusted twice a week even if the weather is not changed.



3. Lens of incident window



The lens leading to each chamber, especially the lens to the air chamber, is blown with argon when the sample is excited, which makes the dust generated during the exposure of the sample blown to the lens, which prevents the light from passing through, and affects the accuracy of the measurement results. Therefore, it is necessary to clean frequently, usually twice a week to keep it clean, and ensure that all light enters the light chamber through the lens for measurement. It is especially reminded that several waste samples should be excited after cleaning the lens, and then standardized operation should be carried out after the strength is stable, otherwise, it will affect the analysis quality.



4. Excitation station



Cleaning the inner surface of the excitation table is mainly to avoid the dust on the residual inner wall from releasing the film sound analysis results. Usually clean every 100-200 times of excitation. The distance between the electrode and the excitation surface must be adjusted according to the requirements of the polar distance. If the distance from the excitation surface is too large, the sample is not easy to be excited. If the distance between the electrode and the excitation surface is too small, the discharge current is too large during exposure, so that it does not match with the parameters of the instrument, so that there is a difference between the measurement result and the actual result, which affects the accuracy of the measurement. Therefore, the distance between the electrode and the excitation surface must be adjusted accurately. After cleaning the excitation table and electrode, it is necessary to pay attention to this problem.



5. Correction of working curve



Although the photoelectric direct reading spectrometer is not limited by the sensitive plate, the working curve will change after drawing it. For example, the pollution of lens, the contamination of electrode, the change of temperature and humidity, the influence of argon, the fluctuation of power supply, etc. can make the curve change. After a period of time, the curve may drift to the position of B in the original curve. In order to analyze the curve, it is necessary to recover curve B to the position of curve A. therefore, the working curve must be standardized. The following points must be noted in the standardization of curves:



(1) After cleaning the sample excitation table, it is necessary to stimulate more than 10 times or argon for an hour before daily standardization work can be done.



(2) Standardized samples should be uniform, sample preparation should be careful, the surface of the sample is flat, and the pattern is clear. The analysis clearance is accurate and the sample holder is kept clean.



(3) The standardization frequency is determined according to the number of samples analyzed, and it must be standardized twice a day.



6. Control sample



In practice, due to the difference between metallurgical process and some physical states of sample and standard sample, the working curve is often changed. Usually, the standard samples are mostly forged and rolled, while the daily analysis is casting state. In order to avoid the influence of metallurgical state on the analysis results, a control sample which is the same as the metallurgical and physical state of the sample should be used to control the analysis results. The element content of the control sample should be within the content range of the working curve, and the closer the content of the sample to the analysis sample, the better. Meanwhile, the element content of the control sample should be accurate and reliable, the composition distribution is uniform, and the appearance is free of physical defects such as porosity, sand hole, crack and so on.



7. Sample



The results of spectral analysis depend on the sample to a large extent, and the preparation and processing technology of the samples should be paid attention to. The failure of flat surface or sample placement due to porosity segregation and error caused by operation error will greatly affect the quality of analysis. Therefore, the sample processing must meet the following requirements:



(1) The entire surface of the specimen shall be uniform (its shape and size shall be suitable for the excitation table so that the gas flushing chamber can be sealed).



(2) There are no sandholes.



(3) Clean the rust and oil on the back of the sample to ensure the sample and the excitation table contact well.



(4) The surface of the sample shall not be polluted, and the grinding sample shall have a pattern.



(5) The excitation point of the sample is usually located at 1/2 of the sample radius, where the chemical composition is relatively uniform, the results are representative and the accuracy of the measurement is high. In conclusion, through many years of practice, several factors affecting direct reading spectrum determination are summarized, which is of great value in improving the quality of elemental analysis.
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