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The gas chromatography detection system is mainly composed of components such as a detector, an amplifier and a recorder. Performance requirements of gas chromatograph detector: strong versatility or speciality; wide response range, can be used for constant and trace analysis; good stability, low noise; small dead volume, fast response; wide linear range, easy to quantify; Simple and durable. Gas chromatograph detectors can be classified into concentration detectors and mass detectors according to their classification characteristics. The commonly used detectors for gas chromatography are described below:
1. thermal conductivity detector (TCD)
Structure: The thermal element is placed in the detection pool body to form a thermal conductivity cell, and then the thermal conductivity cell and the resistor form a Wheatstone bridge.
Principle: The heat generated by the thermistor consumes heat and the heat of the carrier gas and forced convection loses thermal equilibrium. When the components in the carrier gas enter the thermal conductivity cell, the thermal conductivity of the component is different from the carrier gas. The heat balance is destroyed, the temperature of the thermistor changes, and the resistance value changes accordingly. The output voltage of the Wheatstone bridge is unbalanced, and the signal is recorded to obtain a peak.
Application: The thermal conductivity detector is a general-purpose non-destructive concentration detector that can theoretically be applied to the detection of any component, but is generally used for constant analysis because of its low sensitivity.
2. Hydrogen ionization detector (FID)
Structure: The metal cylinder is used as the outer casing, and the inside is equipped with a combustion nozzle. The carrier gas and components flow out of the column and then escape from the nozzle together with the hydrogen (and if necessary, the makeup gas)Gas Chromatography vials and burn with the air around the nozzle. An emitter and a collector are arranged near the nozzle, and an electric field is formed between the two poles.
Principle: FID is the energy generated by the combustion of hydrogen in air. The components generate ions when burned, and form an ion current under the action of an electric field. The mechanism by which components form ions in a flame is not well understood.
Working conditions: The temperature should generally be above 150 °C to prevent water accumulation; hydrogen: nitrogen: air = 1:1:10.
Performance and Application: FID is a versatile destructive mass detector. High sensitivity and wide linear range, widely used in constant and trace detection of organic materials.
3. Nitrogen-phosphorus detector (NPD)
Structure: Similar to the hydrogen flame ionization detector, but between the flame nozzle and the collector, bead beads (barium silicate, Rb2O·SiO2).
Principle: Some researchers have proposed some different mechanisms, but they cannot explain the experimental phenomena satisfactorily.
Working conditions: two modes of operation, NP mode and P mode, the working conditions are also different.
Performance and application: NPD is a selective detector. In the NP mode of operation, it can be used to determine organic compounds containing nitrogen and phosphorus; in the P mode of operation, it can be used to determine phosphorus-containing organic compounds. As a selective detector, the sensitivity of the detected compound is very high and is not as good as other detectors.
4. Electron capture detector (ECD)
Structure: There are positive and negative electrodes and β-ray source in the detection chamber. The current source of Zui is Ni63. There is no γ radiation in the decay, and the generated β-ray energy is low, and the half-life is long. It can be used up to 400 °C.
Principle: The radioactive source in the detection chamber emits β-ray particles (primary electrons), and collides Gas Chromatography vials with the carrier gas passing through the detection chamber to generate secondary electrons and positive ions. Under the action of the electric field, respectively, the electrodes are moved to the opposite polarity to form a detection. The chamber background current, when the negatively charged component (ie, the component capable of trapping electrons) enters the detection chamber, captures electrons in the detection chamber and becomes negatively charged ions, which are detected due to electrons being trapped by the components. The chamber background current is reduced, producing a reversed peak signal.
Working conditions: The carrier gas is generally selected from high-purity nitrogen. Traces of oxygen and traces of water in the gas will contaminate the test chamber and must be removed with a purge tube.
Performance and application: ECD is a concentration-selective detector Gas Chromatography vials that gives a very significant response to negatively charged components.
For the analysis of halogen compounds, polynuclear aromatic hydrocarbons, some metal chelates and steroids.
5. Flame-photometric detector (FPD)
Structure: generally divided into two parts: combustion and photoelectric; the former is a flame combustion chamber, similar to FID, the latter consists of a filter and a photomultiplier tube.
Principle: When components are burned in a hydrogen-rich (H2:O2>3) flame, the components become fragments or atoms to varying degrees, and the outer electrons are excited by collision with each other, when the electrons return to the low energy state from the excited state or In the ground state, a spectrum of characteristic wavelengths is emitted, and the spectrum of this characteristic is measured by a selected interference filter (a compound containing phosphorus, sulfur, boron, nitrogen, halogen, etc. can produce such a spectrum). For example, sulfur produces a spectrum of 350-430 nm in a flame, and phosphorus produces a spectrum of 480-600 nm.Gas Chromatography vials
Working conditions: The amount of hydrogen supplied must be greater than the amount of hydrogen required for normal combustion, that is, it is burned in the presence of hydrogen to obtain a flame.
Performance and application: FPD is a mass-selective detector, mainly used to determine sulfur-containing and phosphorus-containing compounds, and its signal is almost 10,000 times higher than that of hydrocarbons. It is widely used in the analysis of trace sulfur compounds in petroleum products and organophosphorus compounds in pesticides.
6. Other detectors: mass spectrometer, Fourier transform infrared spectrometer, AED, SCD, ELCD, PID, HID, etc.
This is the end of the introduction of Introduction to Gas Chromatography Detection System. I hope it can help you.