Why use FID gas detection?

July 1, 2005
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There are two primary groupings of gases that are generally monitored in industry: 1) combustible/organic compounds, and 2) toxic/inorganic compounds. And there is some considerable overlap between these broad categories. Organic (hydrocarbon) compounds are generally also toxic as well as being combustible. And a few inorganic (non-hydrocarbon) compounds may also be combustible as well as being toxic.

This article will focus on organic compounds that are usually combustible and may also be toxic. We will exclude inert compounds and, generally, inorganic compounds.

Sensor technologies

Popular sensing technologies for combustible/ organics are catalytic combustion, metal oxide, photo ionization and flame ionization. There are a few others, but these are the most popular for safety monitoring, hygiene use and basic gas analysis.

Catalytic combustion is very popular for safety applications and produces a reliable and easy-to-use sensor. Because it responds based on thermal changes from combustion of the gas it is sensing, the catalytic sensor tends to be more uniform in its response to a wide range of flammable gases and vapors. This characteristic makes it the preferred method for flammability detection in the Lower Explosive Limit or Lower Flammable Limit scale. Its primary weaknesses compared to the other technologies mentioned are a relatively high, lower detectable limit and susceptibility to serious sensor poisoning, which can degrade its sensing performance.

The metal oxide semiconductor technology has better lower detectable limit capabilities compared to the catalytic combustion sensors, but not as good as the photo ionization or flame ionization technologies. Popular applications for MOS sensors are in a relatively high parts per million (ppm) detection range such as a few hundred to a few thousand ppm. These ranges are suitable for the toxic levels of some organic compounds. The MOS, PID and FID technologies, then, are not oriented toward flammability safety, but rather toxicity safety with reference to exposure standards such as the TLV - TWA/STEL/C/ PEL/IDLH values for toxic organic compounds. (Please note that some MOS and PID sensors are also used to detect some inorganic compounds as well. The common denominator is, again, protection against toxic exposure.)

Since MOS sensors are considered to be broadband sensors, some limitations include its lack of accuracy, responsiveness to moisture and a wide variation in response characteristics to compounds other than the target gas.

The photo ionization detector (PID) affords a low detectable limit capability, often as low as 100 parts per billion (ppb) or lower. Some weaknesses of the PID include relatively small monitoring ranges, limited detectable compounds (PIDs will not respond to methane, for example), and its susceptibility to interference from water vapor. Also a PID lamp will gradually lose power even if it’s not in use, and many PID lamps have a life expectancy of less than one year.

Whether or not a PID can detect a compound depends upon the energy required to remove an electron from the compound (its ionization potential). If the PID’s lamp energy is greater than the compound’s ionization potential, then the PID will detect it. Often different lamps are used to purposely expand detection or decrease interferences depending on the desired or unwanted ionization potentials. Again however, a PID will not respond to methane and also has a “small dynamic range” or full-scale range of approximately 0-2,000 ppm.

Basically, a PID works by using an ultraviolet (UV) lamp of a specific energy and an ionization chamber. Compounds pass through the chamber and are excited by photons of UV energy and are ionized: R + hv = R+ + e- (where R = most organic/inorganic compounds).

A flame ionization detector (FID) is also capable of a relatively low detectable limit, albeit, slightly higher than a PID, for many compounds, 200 ppb or less. The FID further offers a wider dynamic range (0-50,000 ppm or so) and is highly sensitive to virtually all hydrocarbon vapors, including methane. It is also extremely stable and repeatable with fast response and recovery times. Also, FIDs are virtually unaffected by ambient levels of CO, CO2 or water vapor.

Essentially, a FID measures compounds by utilizing a flame produced by the combustion of hydrogen and air. When hydrocarbons in the sample are brought into the detection zone, ions are produced: RH + O = RHO+ + e- = H2O + CO2 (where R = carbon compound).

A collector electrode with a polarized voltage is also located inside the detector chamber and the ions produced by the reaction are attracted to it. As the ions migrate to the collector, a current is produced that is directly proportional to the concentrations of hydrocarbons presented to the flame. Some FIDs have dilution devices that can introduce an air sample to the mixture, thereby increasing its already wide dynamic range, and also serving to introduce oxygen into oxygen-deficient samples to maintain the integrity of the flame and thus the unit.

Applications for FIDs

From this information, we can conclude that the flame ionization detector is a very versatile, accurate and reliable instrument well suited to a variety of important applications including fugitive emissions monitoring, emergency response analysis, hazardous waste evaluation, underground storage tanks, industrial hygiene and natural gas leak detection.

With the increasing stringency of governmental regulations for compound exposure and accountability, the flame ionization detector is playing an increasingly vital role in meeting these requirements now and will continue to do so in the future.

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