Photoionization detector
Encyclopedia
A photoionization detector or PID is a type of gas detector
.
Typical photoionization detectors measure volatile organic compound
s and other gases in concentrations from sub parts per billion
to 10 000 parts per million
(ppm). The photoionizaton detector is an efficient and inexpensive detector for many gas and vapor analytes. A PID may produce instantaneous readings and operate continuously. These hand-held, battery-operated detectors are widely used in military, industrial, and confined working facilities for safety.
PIDs are used as monitoring solutions for:
. In a photoionization detector high-energy photon
s, typically in the ultraviolet
(UV) range, break molecule
s into positively charged
ion
s. As compounds elute from the GC's column they are bombarded by high-energy photons and are ionized
when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current
, which is the signal output of the detector
. The greater the concentration of the component, the more ions are produced, and the greater the current.
The current is amplified
and displayed on an ammeter
. It is widely held that the ions recombine after passing the detector to reform their original molecules, however only a small portion of the airborne analytes are ionized to begin with so the practical impact of this (if it occurs) is probably negligible.
similar to or lower than the energy of the photons produced by the PID lamp used in the detector. This selectivity can be useful when analyzing mixtures in which only some of the components are of interest.
The PID is usually calibrated using isobutylene
, and other analytes may produce a relatively greater or lesser response on a concentration basis. Although many PID manufacturers provide the ability to program an instrument with a correction factor for quantitative detection of a specific chemical, the broad selectivity of the PID means that the user must know the identity of the gas or vapor species to be measured with high certainty. If a correction factor for benzene is entered into the instrument, but hexane vapor is measured instead, the lower relative detector response (higher correction factor) for hexane would lead to underestimation of the actual airborne concentration, and the user would not know that hexane had been measured instead of benzene.
PIDs are non-destructive detectors. They do not appreciably destroy/consume the components they detect. Therefore they can be used before other detectors in multiple-detector configurations. The signal produced by a PID may be quenched when measuring in high humidity environments, or when a compound such as methane is present in high concentration This attenuation is due to the ability of water, methane, and other compounds with high ionization potential (IP) values to absorb the photons emitted by the uv lamp without leading to the production of ion current. This reduces the number of energetic photons available to ionize target analytes.
Gas detector
A gas detector is a device which detects the presence of various gases within an area, usually as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down...
.
Typical photoionization detectors measure volatile organic compound
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...
s and other gases in concentrations from sub parts per billion
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
to 10 000 parts per million
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
(ppm). The photoionizaton detector is an efficient and inexpensive detector for many gas and vapor analytes. A PID may produce instantaneous readings and operate continuously. These hand-held, battery-operated detectors are widely used in military, industrial, and confined working facilities for safety.
PIDs are used as monitoring solutions for:
- Lower explosive limit measurements
- AmmoniaAmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
detection - Hazardous materials handling
- ArsonArsonArson is the crime of intentionally or maliciously setting fire to structures or wildland areas. It may be distinguished from other causes such as spontaneous combustion and natural wildfires...
investigation - Industrial hygieneHygieneHygiene refers to the set of practices perceived by a community to be associated with the preservation of health and healthy living. While in modern medical sciences there is a set of standards of hygiene recommended for different situations, what is considered hygienic or not can vary between...
and safetySafetySafety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be... - Indoor air quality
- Environmental contaminationContaminationContamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
and remediation - CleanroomCleanroomA cleanroom is an environment, typically used in manufacturing or scientific research, that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. More accurately, a cleanroom has a controlled level of contamination that is specified by...
facility maintenance
Principle
The first application of photoionization detection was as a gas chromatography (GC) ion detectorGC ion detector
Many GC detectors are ion detectors with varying methods of ionizing the components eluting from the GC's column.-Principle of Operation:An ion detector is analogous to a capacitor or vacuum tube. It can be envisioned as two metal grids separated by air with inverse charges placed on them. An...
. In a photoionization detector high-energy photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s, typically in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) range, break molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s into positively charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s. As compounds elute from the GC's column they are bombarded by high-energy photons and are ionized
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
, which is the signal output of the detector
Counter
In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...
. The greater the concentration of the component, the more ions are produced, and the greater the current.
The current is amplified
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
and displayed on an ammeter
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
. It is widely held that the ions recombine after passing the detector to reform their original molecules, however only a small portion of the airborne analytes are ionized to begin with so the practical impact of this (if it occurs) is probably negligible.
Application
As a stand alone detector PIDs are broad band detectors and are not selective, as these may ionize everything with an ionization energy less than or equal to the lamp output. A PID is highly selective when coupled with a chromatographic technique or a pre-treatment tube such as a Benzene specific tube. The PID will only detect components which have ionization energiesIonization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
similar to or lower than the energy of the photons produced by the PID lamp used in the detector. This selectivity can be useful when analyzing mixtures in which only some of the components are of interest.
The PID is usually calibrated using isobutylene
Isobutylene
Isobutylene is a hydrocarbon of significant industrial importance. It is a four-carbon branched alkene , one of the four isomers of butylene. At standard temperature and pressure it is a colorless flammable gas.-Uses:...
, and other analytes may produce a relatively greater or lesser response on a concentration basis. Although many PID manufacturers provide the ability to program an instrument with a correction factor for quantitative detection of a specific chemical, the broad selectivity of the PID means that the user must know the identity of the gas or vapor species to be measured with high certainty. If a correction factor for benzene is entered into the instrument, but hexane vapor is measured instead, the lower relative detector response (higher correction factor) for hexane would lead to underestimation of the actual airborne concentration, and the user would not know that hexane had been measured instead of benzene.
PIDs are non-destructive detectors. They do not appreciably destroy/consume the components they detect. Therefore they can be used before other detectors in multiple-detector configurations. The signal produced by a PID may be quenched when measuring in high humidity environments, or when a compound such as methane is present in high concentration This attenuation is due to the ability of water, methane, and other compounds with high ionization potential (IP) values to absorb the photons emitted by the uv lamp without leading to the production of ion current. This reduces the number of energetic photons available to ionize target analytes.