The photoionization detector or PID is a type of gas detector.
The common photoionization detector measures volatile organic compounds and other gases in concentrations from sub sections per billion to 10 000 parts per million (ppm). The photoionization detector is an efficient and inexpensive detector for many gas and steam analyzers. PID produces an instantaneous, continuous, and generally used as a detector for gas chromatography or as a handheld portable instrument. Hand-held, battery-operated versions are widely used in military, industrial, and work facilities limited to health and safety. Its main use is to monitor the possible exposure of workers to volatile organic compounds (VOCs) such as solvents, fuels, lubricants, plastics & amp; their precursors, heat transfer fluids, lubricants, etc. during the manufacturing and waste handling process.
Portable PID is used as a monitoring solution for:
- Industrial hygiene and safety
- Environmental contamination and restoration
- Handling of hazardous materials
- Detection of ammonia
- Lower explosion limit measurements
- Arson's investigation
- Indoor air quality
- Maintenance of clean space facility
Video Photoionization detector
Principles
In photonization photons of high energy photons, usually in the ultraviolet vacuum range (VUV), break the molecule into positively charged ions. As the compound enters the detector, they are bombarded with high-energy U disks and ionized when they absorb UV light, producing electron ejection and the formation of positively charged ions. Ion generates an electric current, which is the output signal from the detector. The greater the component concentration, the more ions are generated, and the greater the current. The current is amplified and displayed on the ammeter or digital concentration display. The ions can undergo various reactions including reaction with oxygen or moisture, rearrangement, and fragmentation. Some of them can recapture the electrons inside the detector to reform the original molecule; Yet only a small part of the ionized air analyte is to start so that the practical impact of this (if it happens) is usually negligible. Thus, PID is not destructive and can be used before other sensors in multi-detector configurations.
PID will only respond to components that have ionisation energy equal to or lower than the photon energy generated by the PID lamp. As a stand-alone detector, PID is broad band and not selective, since it can ionize everything with ionisation energy that is less than or equal to the energy of the light photon. The more common commercial lights have upper limit photon energy of about 8.4 eV, 10.0 eV, 10.6 eV, and 11.7 eV. The main and minor components of clean air all have ionization energies above 12.0 eV and thus do not interfere significantly in VOC measurements, which normally have ionisation energies below 12.0 eV.
Maps Photoionization detector
The type of light and detectable compound
PID lamp photon emissions depend on the type of fill gas (which determines the energy of light produced) and the lamp window, which affects the energy of photons that can come out of the lamp:
The ev 10.6 lamp is the most common because it has a strong output, has the longest life and responds to many compounds. In the approximate order of the most sensitive to least sensitive, these compounds include:
- Aromatic
- Olefins
- Bromides & amp; Iodide
- Sulfide & amp; merkaptan
- Organic amines
- Keton
- Ether
- Esther & amp; acrylates
- Aldehyde
- Alcohol
- Alkana
- Some inorganic, including NH 3 , H 2 S, and PH 3
Apps
The first commercial application of photoionization detection in 1973 as a handheld instrument for the purpose of detecting VOC leakage, particularly vinyl chloride monomer (VCM), in chemical manufacturing facilities. The photoionization detector was applied to gas chromatography (GC) three years later, in 1976. PID was highly selective when combined with chromatographic techniques or pre-treatment tubes such as benzene-specific tubes. Wider selectivity cuts for ionized compounds can be obtained using lower energy UV lamps. This selectivity can be useful when analyzing a mixture where only a few components are of interest.
PIDs are usually calibrated using isobutylene, and other analytes can produce relatively larger or smaller responses on the concentration basis. Although many PID manufacturers provide the ability to program instruments with correction factors for quantitative detection of certain chemicals, the wide selectivity of PID means that the user must know the identity of the gas or steam species to be measured with high certainty. If the correction factor for benzene is incorporated into the instrument, but the hexane vapor is measured instead, a lower relative detector response (a higher correction factor) for hexane will cause to underestimate the actual hexane air concentration.
Matrix gas effect
With gas chromatography, filter tubes, or other separation techniques in the upper PID, the matrix effect is generally avoided because the analyte enters the detector isolated from the disturbing compound.
The response to stand-alone PIDs is generally linear from the ppb range to at least several thousand ppm. Within this range, the response to the component mix is ​​also a linear additive. At higher concentrations, the response gradually diverges from linearity because recombination of opposite charged ions is formed at close range and/or 2) absorption of UV light without ionization. Signals generated by PID can be quenched when measuring in high humidity environments, or when compounds such as methane are present in high concentrations> = 1% of volume. This attenuation is caused by the ability of water, methane, and other compounds. with high ionisation energy to absorb the photons emitted by UV lamps without leading to the production of ion currents. This reduces the amount of energetic photons available to ionize the target analytes.
References
Source of the article : Wikipedia
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