Thermal conductivity detector for gas chromatograph detector
Gas chromatography is a commonly used testing instrument in modern analytical laboratories. The detector is an important part of the chromatograph. This article introduces the thermal conductivity detector of the gas chromatograph detector.
First, several detectors commonly used in gas chromatography :
1. Thermal conductivity detector (TCD)
2. Hydrogen flame ionization detector (FID)
3. Electron capture detector (ECD)
4, flame photometric detector (FPD)
5, nitrogen and phosphorus detector (NPD) also known as thermionic detector (TID)
6, atomic emission detector (AED)
7, sulfur fluorescence detector (SCD)
Thermal conductivity detectors are based on the principle that different materials have different thermal conductivity. The thermal conductivity detector is the most widely used and most mature detector because of its simple structure, stable performance, wide linear range, low price and good versatility. The main disadvantage is the low sensitivity.
Gas chromatography is a commonly used testing instrument in modern analytical laboratories. The detector is an important part of the chromatograph. This article introduces the thermal conductivity detector of the gas chromatograph detector.
First, several detectors commonly used in gas chromatography :
1. Thermal conductivity detector (TCD)
2. Hydrogen flame ionization detector (FID)
3. Electron capture detector (ECD)
4, flame photometric detector (FPD)
5, nitrogen and phosphorus detector (NPD) also known as thermionic detector (TID)
6, atomic emission detector (AED)
7, sulfur fluorescence detector (SCD)
Thermal conductivity detectors are based on the principle that different materials have different thermal conductivity. The thermal conductivity detector is the most widely used and most mature detector because of its simple structure, stable performance, wide linear range, low price and good versatility. The main disadvantage is the low sensitivity.
Second, the structure and working principle of the thermal conductivity pool
The thermal conductivity cell is composed of a cell body and a heat-sensitive element, and can be divided into two types of arms and a four-arm thermal conductivity cell. Since the resistance of the hot wire of the four-arm heat guide pool is doubled that of the double-arm heat guide cell, the sensitivity is also doubled. At present, four arm thermal conduction layers composed of four wires are used in the instrument. The two arms are reference arms, the other two arms are measuring arms, the reference arm and the measuring arm are connected to the Wheatstone bridge, and the constant current heating constitutes the thermal conductivity measuring circuit.
Shenzhen Yixin Instrument Equipment Co., Ltd. is a high-tech enterprise mainly engaged in R&D, production and sales of quality control instruments and laboratory test equipment. Its main products cover high performance liquid chromatography, gas chromatography and gas chromatography. Instrument, atomic absorption spectrophotometer, ultraviolet spectrophotometer, precision analytical balance, spectrometer, ROHS detector, water quality analyzer, measuring instrument, chemical analysis instrument, metallographic microscope, physical measuring instrument, nondestructive flaw detector, blue light three-dimensional scanner Large-scale professional precision instrumentation of various products such as high-precision three-coordinates, two-dimensional elements, testing machines and instruments commonly used in laboratories
Third, factors affecting the sensitivity of the thermal conductivity detector
(l) Bridge current The bridge current increases, the tungsten wire temperature increases, the temperature difference between the tungsten wire and the thermal conductivity cell body increases, and the gas easily transfers heat, and the sensitivity is improved. The response value is proportional to the cube of the operating current. Therefore, increasing the current is beneficial to increase the sensitivity, but too much current will affect the life of the tungsten wire. Generally, the bridge current is controlled to be about 100 to 200 mA (100 to 150 mAH for N2 as a carrier gas and 150 to 200 mA for a carrier gas).
(2) Pool body temperature The temperature of the cell body is lowered, which can increase the temperature difference between the cell body and the tungsten wire, which is beneficial to improve the sensitivity. However, the temperature of the cell body is too low and the sample to be tested will condense in the detector. The temperature of the cell body should generally not be lower than the column temperature.
(3) Type of carrier gas The greater the difference between the thermal conductivity of the carrier gas and the sample, the higher the sensitivity. Therefore, selecting hydrogen or helium gas with a large thermal conductivity as a carrier gas is advantageous for sensitivity improvement. When nitrogen is used as the carrier gas, some samples (such as methane) have a higher thermal conductivity than it will have a peak.
(4) Resistance of the thermal element The thermal element with high resistance and large temperature coefficient has high sensitivity. Tungsten wire is a widely used thermal element. Its resistance increases with temperature. Its temperature coefficient of resistance is 5.5×10-3cm·Ω-1·°C-1, and its resistivity is 5.5×1O- 6 Ω·cm. To prevent vaporization of the tungsten wire, gold or nickel may be applied to the surface.
Shenzhen Yixin Instrument Equipment Co., Ltd. is a high-tech enterprise mainly engaged in R&D, production and sales of quality control instruments and laboratory test equipment. Its main products cover high performance liquid chromatography, gas chromatography and gas chromatography. Instrument, atomic absorption spectrophotometer, ultraviolet spectrophotometer, precision analytical balance, spectrometer, ROHS detector, water quality analyzer, measuring instrument, chemical analysis instrument, metallographic microscope, physical measuring instrument, nondestructive flaw detector, blue light three-dimensional scanner Large-scale professional precision instrumentation of various products such as high-precision three-coordinates, two-dimensional elements, testing machines and instruments commonly used in laboratories
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