The operation of the photoionization detector (PID) is based on measuring the current induced by the ionization of gases and vapors by photons emitted by an ultraviolet source. The energy of the photons emitted by the source is about 10 eV and depends on the type of lamp. The radiation from the ultraviolet source (UVC lamp) enters the ionization chamber, where two electrodes are installed, one of which is connected to a power supply and the other to an electrometer. A sample is introduced into the ionization chamber. Under the influence of the radiation, components with ionization energy lower than the photon energy emitted by the lamp are ionized. A current flows in the ionization chamber, the magnitude of which is proportional to the concentration of impurities. At the same time, the components of clean air, namely oxygen, nitrogen, argon, methane, carbon monoxide, and carbon dioxide, are not ionized. The advantages of FID include high sensitivity, fast response, resistance to concentration overloads, stability, long service life, and unlimited shelf life. FID is used in portable and stationary gas analyzers to measure a very wide range of compounds: alkanes, alkenes, alcohols, aromatic hydrocarbons, aldehydes, ketones, carbon disulfide, and others in concentration ranges from mg/m3 to fractions of % vol.
For many components, photoionization detection (PID) is the only method for rapid measurement of their content in the air. PID is used at temperatures ranging from minus 30 to 50°C.
photoionization detector VOC Pro
The drawbacks of PID include non-selectivity: if the analyzed air contains a mixture of components, the detector measures their total concentration. The detector detects all substances with an ionization potential lower than the energy emitted by the UV lamp. Typically, a lamp with an emission energy of 10.6 eV is common. Lamps with higher emission energy have a significantly shorter lifespan. Sensitivity greatly depends on the ionization potential of the gas. Increased humidity in the ionization zone also leads to a shift in readings by several ppm. The presence of methane in large quantities can significantly reduce the output signal. The measurement is also affected by contamination of the lamp surface and the presence of dust and other fine particles in the measurement zone.
The idea of using the current generated in a gas mixture under the influence of vacuum ultraviolet (VUV) radiation to measure the concentration of its components emerged in the early 1950s. In the first photoionization detectors, VUV radiation was excited in a flow of inert gas, where the ionization of the analyzed components also occurred, which was inconvenient for practical application. In the mid-1960s, it was proposed to separate the volumes where excitation and ionization occur, and to use a separate radiation source for the ionization of the analyzed gas - a detached lamp with a window for radiation output. The first such design was described in a USSR author's certificate obtained by A.S. Mironov in 1964. More than 10 years later, in the USA, J. Driscoll patented a similar design, and the first serial photoionization detector (PID) was created. Nowadays, thousands of PIDs are used in gas chromatographs and gas analyzers.
What types of gases can photoionization detectors measure
how does photoionization detector work
UV radiation passes through the lamp window into the ionization chamber, where two electrodes are installed, one of which is connected to a power source, and the other to an electrometer. A sample is introduced into the ionization chamber. Under the influence of the radiation, components with ionization energies lower than the energy of the photons emitted by the UV lamp become ionized, generating a current signal whose magnitude is proportional to the concentration of impurities. At the same time, components of clean air, namely oxygen, nitrogen, and argon, which have higher ionization potentials, do not get ionized. The photoionization detection method allows for the measurement of concentrations of a large number of organic substances, including components of oil and petroleum products, in the range from units to thousands mg/m3.
A photoionization detector (PID) is a type of gas detector that measures the concentration of volatile organic compounds (VOCs) and other hazardous gases in the air. PIDs are highly sensitive and are used across a wide range of industries for safety and environmental monitoring.
How a PID Works
PIDs operate on the principle of photoionization, which is a process where a molecule loses an electron when it's exposed to high-energy light.
UV Lamp: A PID contains an ultraviolet (UV) lamp that emits high-energy photons. The energy of these photons is key, as it determines which compounds the detector can "see."
Ionization: As a gas sample enters the detector, it's exposed to the UV light. If a molecule in the gas has an ionization potential (the energy required to remove an electron) that's less than the energy of the UV light, the molecule will absorb a photon. This absorption ejects an electron, creating a positively charged ion and a free electron.
Current Measurement: The detector has two electrodes. The newly formed ions and electrons are attracted to the oppositely charged electrodes, creating a small electrical current.
Signal Output: The strength of this current is directly proportional to the concentration of the gas in the sample. The detector's circuitry measures this current, amplifies it, and displays the concentration on a screen, often in parts per million (ppm) or parts per billion (ppb).
Key Features and Applications
PIDs are widely used for a number of reasons:
High Sensitivity: PIDs can detect gases at very low concentrations, down to the parts-per-billion range. This makes them ideal for monitoring exposure to toxic substances.
Fast Response: They provide nearly instantaneous readings, which is critical in situations where a quick response to a gas leak is needed.
Broad Detection Range: A single PID can detect a wide variety of VOCs and some inorganic gases, making it a versatile tool for many applications.
Non-Destructive: The ionization process doesn't destroy the gas sample. This allows the sample to be used for further analysis by other sensors or instruments.
Portability: Many PIDs are small, lightweight, and battery-operated, making them perfect for field use.
Common applications include:
Industrial Hygiene: Monitoring worker exposure to solvents, fuels, and other chemicals in manufacturing plants and confined spaces.
Environmental Monitoring: Detecting air pollutants and monitoring air quality at industrial sites or during environmental cleanup efforts.
Hazardous Materials Response: Used by emergency responders to quickly assess the risks of a chemical spill or leak.
Leak Detection: Finding gas leaks in industrial settings, chemical plants, and oil refineries.
Explosives and Narcotics Detection: Used in trace detection systems for security applications.
PID vs. Other Gas Detectors
PIDs offer several advantages over other types of gas detectors, such as catalytic bead sensors or flame ionization detectors (FIDs).
PIDs vs. FIDs:
PIDs are generally smaller, lighter, and easier to use. They don't require an open flame or hydrogen gas, making them safer and more portable.
FIDs can detect a broader range of organic compounds, including methane, which PIDs cannot. However, FIDs are destructive and require a constant supply of hydrogen, which can be a safety and logistical issue.
PIDs vs. LEL Sensors:
PIDs are designed for toxic gas detection at very low concentrations (ppm/ppb) to protect human health.
LEL (Lower Explosive Limit) sensors are used to detect combustible gases at much higher concentrations (percent levels) to prevent explosions. They're not suitable for monitoring long-term toxic exposure.
In summary, PIDs are a crucial tool for professionals who need to quickly and accurately measure low levels of VOCs and other hazardous gases, especially in environments where portability and high sensitivity are a priority.
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