Optical Emission Spectrometer

Content
1. What is optical emission spectrometer?
    1.1 Working principle of optical emission spectrometer
    1.2 Development of optical emission spectrometer
2. Composition of optical emission spectrometer
3. Feature of optical emission spectrometer
    3.1 Advantage of optical emission spectrometer
    3.2 Disadvantages of optical emission spectrometer
4. How to buy optical emission spectrometer?

Optical emission spectrometer, also known as a spark emission spectrometer, is an emission spectroscopy instrument that applies the photoelectric conversion reception method for simultaneous analysis of multiple elements. Due to the widespread use of inductively coupled high-frequency plasma light sources, the OES spectrometer occupies a major position in the spectrometer. It is widely used in material analysis in many industries such as steel, non-ferrous metal, metallurgy, machinery, chemical equipment, and quality inspection system, as well as the pre-furnace analysis and factory identification analysis of the metal smelting industry.

The characteristic wavelength of each element is emitted by direct vaporization of each element in the sample from the solid state by the high temperature of electric arc (or spark) and excitation. After spectroscopy with a grating, the spectrum is arranged by wavelength. The characteristic spectral lines of these elements pass through the exit slit and are injected into the respective photomultiplier tube (PMT) or CCD image sensor, where the optical signal becomes an electrical signal. The electrical signal is taken and converted to modulus by the instrument's control measurement system, and then processed by a computer and printed out as a percentage of each element.

Spectroscopy originated in the 17th century when Newton, a physicist, conducted the first experiment on the dispersion of light in 1666. He introduced a beam of sunlight in a dark room, let it pass through a prism, and saw the seven colors of red, orange, yellow, green, blue, indigo, and violet scattered in different positions on a white screen behind the prism. This phenomenon was called spectroscopy.

In 1802, the English chemist Wollaston discovered that the solar spectrum was not a perfect rainbow, but was cut by a number of black lines.

In 1814, Fraunhofer, a German optical instrument expert, studied the relative positions of black spots in the solar spectrum and used a slit device to improve the imaging quality of the spectrum to plot those major black lines on a spectral map.

In 1825, Talbot, studying the spectra of sodium and potassium salts on an alcohol lamp, pointed out that the red spectrum of potassium salts and the yellow spectrum of sodium salts were both properties of this element.

In 1859, Kirchoff and Bunsen designed and built a perfect spectroscopic device in order to study the spectra of metals. This device was the world's first practical spectroscopic instrument to study the spectral lines of various metals in flames and electric sparks, thus establishing the initial foundation of spectral analysis. The shift from the determination of the absolute intensity of spectral lines to the measurement of the relative intensity of spectral lines laid the foundation for the development of spectral analysis methods from qualitative to quantitative analysis, thus allowing spectral analysis methods to gradually move out of the laboratory and be applied in the industrial sector.

After 1928, as spectral analysis became an industrial analysis method, spectral instruments were developed rapidly, and progress was made in improving the stability of the excitation source and the performance of the spectral instruments themselves.

The earliest excitation light source was flame and later developed into the application of simple electric arc and electric spark as excitation light sources. In the 1930s and 1940s, the stability of spectroscopic analysis was improved by using improved controlled arc and spark as excitation sources. The development of industrial production and the progress of spectroscopy prompted further improvement of optical instruments, and the latter in turn reacted to the former, promoting the development of spectroscopy and the development of industrial production.

In the 1960s, with the development of computer and electronic technology, the optical emission spectrometer began to develop rapidly. In the 1970s, almost 100% of the spectroscopic instruments were computer-controlled, which not only improved the analysis accuracy and speed but also realized the data processing of the analysis results and the automatic control of the analysis process.

The optical emission spectrometer is composed of a light source part, light gathering part, light dividing part and light measuring part. The light source part is to excite the specimen to emit light; the light gathering part is to gather the emitted light into the spectral part; the spectral part is to disperse the light into the spectral lines of each element; the photometric part is to measure the intensity of the spectral lines of each element by photoelectric method, and indicate and record it, or convert the photometric reading into the mass fraction of the element to express it.

The light source generators used for optical spectroscopy are spark generators, arc generators, low-voltage capacitive discharge generators, etc.

The electrode holder of the light source is used to load block specimens, rod specimens, and counter electrodes. The block electrode holder can generally be used to load flat specimens with a diameter of 20mm or more, and some of them can be used to load rod specimens, small specimens, and thin plate specimens using various sample clamps. In the vacuum photoelectric spectrometer, the light source electrode holder has the structure of using an argon atmosphere, and the flow of argon can be adjusted and controlled by a flow meter and automatic valve.

Light gathering device is composed of a concentrating mirror system, whose role is to gather the light from the light source and make it shoot into the spectroscopic system. This system is generally required to make full use of the light radiation from the light source to get a large light intensity. At the same time, it should give full play to the function of the instrument to achieve the proper resolution. Usually, the use of single-lens imaging method, three-lens intermediate imaging method, and circular cylindrical lens imaging method to make the light emitted from the light source imaging in the collimator.

The beam splitter is composed of an incident slit, a beam splitting element, and an exit slit system. The light entering the incident system is divided by the beam splitting element, and the spectral lines of each element are selected by the exit slit system. Since there are many spectral lines of iron, it is better to use a spectroscopic element with a large dispersion. The spectrometer can be divided into two categories: vacuum type and non-vacuum type according to whether it is used under a vacuum or non-vacuum inside.

The photometric device consists of a photomultiplier, an integrating unit, a recorder or an indicator, etc. The photomultiplier tube of the inner standard line and the analysis line turn the light received from the outgoing slit into current and then charges the integrating capacitor respectively.

Since the sensitive lines of elements such as sulfur, phosphorus, carbon, and nitrogen are located in the wavelength range below 200 nm, and the radiation in these wavelengths is absorbed by air, the optical system of the photoelectric spectrometer must be placed in a vacuum to perform the analysis of these elements. For this reason, a vacuum optical emission spectrometer must be used for the determination of elements such as sulfur, phosphorus, and carbon. In addition to the general photoelectric spectrometer device, the vacuum system and the controlled atmosphere are added to the vacuum photoelectric spectrometer.

a. The wavelength range of spectral lines that can be used for analysis with a photoelectron direct reading spectrometer is wide. This range is determined by the performance of the photomultiplier tube. For example, with a PMT with a quartz window aperture, coupled with the optical system of the spectrometer placed in a vacuum, the available wavelength can be as short as 150 nm. this makes it possible to use spectral lines located in the band for analysis.

b. Wide range of calibration curves. Since the PMT has a large amplification capacity for the signal, different amplifications are available for PMTs used for spectral lines of different strengths and weaknesses. The difference can be up to 10000 times. Therefore, the photoelectric method can be used for the analysis of many elements in the sample under the same analytical conditions. Although the content range varies widely, many elements from high to low content can be analyzed simultaneously.

c. The photographic plate and photometric aspect of the spectroscopy method introduces an error of more than 1% in general. The photometric error of the optical emission spectrometer can be reduced to less than 0.2%. It has a high degree of accuracy. Favorable to the analysis of high content elements in the sample, and accurate.

d. The analysis speed of the optical direct reading spectrometer is fast, generally within 2~3min after receiving the sample, more than 20 alloy elements in steel can be measured at the same time. It can control the smelting process and accelerate the steel-making process, which is an effective means to save energy and reduce emissions.

a. Since the outgoing slit is used, spectral lines of similar wavelength cannot be utilized.

b. Due to the use of the exit slit, the PMT receives the spectral lines while also receiving the background (using the BKG 175.7 nm background channel, the effect of the background can be deducted).

c. The position of the exit slit is fixed, the elements analyzed are limited, and changes to the analysis task require changing the channel and selecting another exit slit.

d. The presence of background makes the analysis of trace elements somewhat difficult.

e. It is not a stand-alone method and needs to rely on chemical analysis. The chemical analysis is required to provide the accurate content of the standard sample of the spectrum and to calibrate the analytical results of the spectrum.

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