Factors Affecting Analysis Quality of Optical Emission Spectrometer

1, argon krypton the main role of argon blowing is to drive away the air in the spark chamber when the sample is excited, and reduce the absorption of air to the spectral lines in the ultraviolet region.  The main reason is that the oxygen and water vapor in the air have strong absorption bands in the far ultraviolet region, which greatly affects the analysis results, and is not conducive to the stabilization of excitation, the formation or enhancement of diffusion discharge, and the generation of white spots during excitation.  In addition, the alloy elements in the sample may chemically react with the constituents in the air to generate molecular compounds at high temperatures, so that the analysis spectrum will interfere with the atomic spectrum we need.  Therefore, the purity of argon must be above 99.999%.  In addition, the pressure and flow rate of argon gas also have a certain effect on the quality of analysis. It determines the impact ability of argon gas on the discharge surface. This excitation ability must be appropriate and too low to be sufficient for the oxygen and the formed oxides are washed away, and these oxides condense on the electrode surface, thereby suppressing the continued excitation of the sample; the excessive flow of argon gas causes unnecessary waste.  The second is the damage to the Optical Emission Spectrometer.  Therefore, argon pressure and flow must be appropriate.  According to practice, the pressure and flow rate of argon gas should be adjusted according to different materials. For the analysis of medium and low alloy steel, the pressure of argon gas input to the Optical Emission Spectrometer should reach 0.5-1.5 MPa, and the flow rate of dynamic argon is 12-20 readings the static argon flow is 3 to 5 readings.  

2. Slit Optical Emission Spectrometer uses a complex and sensitive optical system.  The changes in the ambient temperature, humidity, mechanical vibration, and atmospheric pressure of the Optical Emission Spectrometer will cause slight changes in the spectral lines and cause the spectral lines to shift.  Changes in air pressure and humidity will change the refractive index of the medium, which will cause the spectral lines to shift. Increasing the humidity will not only increase the refractive index of the air, but also have a corrosive effect on optical parts.  Generally, should be controlled below 55% -60%.  The effect of temperature on the grating mainly changes the grating constant, changes the divergence of the character, and causes spectral line drift.  These changes can cause the spectral lines to not be perfectly aligned with the corresponding exit slits, thereby affecting the analysis results.  Therefore, the optical system should be adjusted at least once a day if the indoor temperature control is constant.  Even if the weather does not change much, adjust the slit twice a week.

3. The lens of the entrance window leads to the lens of each chamber, especially the lens leading to the air chamber. Because the sample is blown with argon when the sample is excited, the dust generated during the exposure of the sample is blown onto the lens and prevents the transmission of light, Affects the accuracy of the measurement results.  Therefore, it should be cleaned frequently, usually twice a week to keep it clean, and ensure that all light passes through the lens and enters the light chamber for measurement.  It is especially reminded that several waste samples should be excited after cleaning the lens, and the standardization operation should be performed after the intensity is stabilized, otherwise the analysis quality will be affected.  

4. Excitation table cleaning the inner surface of the excitation table is mainly to avoid the dust discharge on the inner wall from affecting the analysis results.  Normally, it should be cleaned every 100-200 times.  The distance between the electrode and the excitation surface must be adjusted according to the requirements of the pole distance. If the distance from the excitation surface is too large, the sample is not easily excited. If the distance between the electrode and the excitation surface is too small, the discharge current is too large during exposure, so it does not match the parameters of the instrument, which makes the measurement result different from the actual result and affects the accuracy of the measurement.  Therefore, the distance between the electrode and the excitation surface must be adjusted accurately. This problem must be paid attention to after cleaning the excitation table and the electrode.

 5. Calibration of working curve. Although the method of direct reading photoelectric spectrometer is not limited by the photosensitive plate, the working curve will also change after a period of time after it is drawn.  For example: lens contamination, contamination of the electrodes, changes in temperature and humidity, effects of argon, fluctuations in power, etc., can make the curve change.  The position of A in the original graph may drift to the position of B after a period of time.  In order to use the curve for analysis, we must try to restore curve B to the position of curve A.  To this end, the working curve must be standardized.  The following points must be paid attention to when standardizing the curve: (1) After cleaning the sample excitation table, it must be excited more than 10 times or argon for one hour before doing the normalization.  2 (2) The standardized sample should be uniform, the sample preparation should be careful, the surface of the sample should be flat, and the texture should be clear.  The analysis gap is accurate and the sample holder is kept clean.  3 (3) The normalization frequency is determined by the number of samples analyzed. Generally, the frequency must be standardized twice a day.

6. Control sample in actual work, due to the difference between metallurgical process and some physical conditions of the sample and the standard sample, the working curve often changes. Usually the standard sample is mostly forged and rolled, and the daily analysis is cast status.  In order to prevent the sample from affecting the analysis results due to changes in the metallurgical state, a control sample that is the same as the metallurgical and physical state of the analysis sample should always be used to control the analysis result. The element content of the control sample should be within the content of the working curve, And the closer to the content of the analysis sample, the better.  At the same time, the element content of the control sample should be accurate and reliable, the composition should be evenly distributed, and the appearance should be free of physical defects such as pores, trachoma and cracks.  

7. Sample The quality of the spectral analysis result depends on the sample to a great extent. Pay attention to the preparation and processing techniques of the sample.  Failure to obtain a flat surface or sample placement due to stomatal segregation, as well as errors caused by operating errors, will have a significant impact on the quality of the analysis.  Therefore, the sample processing must meet the following requirements: (1) The entire sample surface should be uniform (its shape and size are suitable for the excitation table so that the gas flushing chamber can be sealed).  (2) No trachoma.  (3) Clean the rust and oil on the back of the sample to ensure that the sample is in good contact with the excitation table.  (4) The surface of the sample should not be contaminated, and the sample should be grained.  (5) When the sample is excited, the excitation point is generally located at 1/2 of the radius of the sample, where the chemical composition is relatively uniform, the results are representative, and the measurement accuracy is high.  In summary, through many years of practice, several factors affecting Optical Emission Spectrometer are summarized, which have important application value for improving the quality of elemental analysis.