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What is a Flame Ionization Detector?

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  • Written By: Andrew Kirmayer
  • Edited By: Angela B.
  • Last Modified Date: 04 December 2016
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A flame ionization detector (FID) is an instrument used for detecting the presence of hydrocarbons, specifically butane, hexane, and other carbon-containing compounds that might be present in the measured sample. The instrument is connected to a gas chromatograph by a tube called a capillary, and features a chamber with a flame. Gases are injected into this chamber from one source, while hydrogen and oxygen are added from another. An electric ignition component is used to ignite the flame inside; the subsequent combustion of hydrogen and oxygen creates a charged current between the flame jet, which acts as an electrode, and another electrode in the chamber.

The capillary column inserted into the instrument is connected to the gas chromatograph, a device used to analyze the chemical composition of a sample of gas. This is the instrument with which the response is measured. The cylindrical electrode that surrounds the flame collects ions formed from the combustion process when a voltage is applied between the two. A current is generated and then amplified while the output is collected by electronic data collectors. Each gas type has a specific base current and flow rate, and when this is charted, operators can determine the gas present by consulting a guide that shows the flow rates that different gases typically have.

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When the fuel is added to a flame ionization detector, it is activated at a predetermined flow rate. After the air is pumped in, the flame is ignited and left for an hour to stabilize and burn continuously for the most accurate results. Shutting off the flow for the fuel turns off the flame, and then the inflow for other gases is turned off. A portable detector works in a similar way and is commonly used to monitor volatile organic compounds (VOCs). With these, the sensitivity can be adversely affected if the temperature changes quickly or there are intense electrical fields present in the environment.

A flame ionization detector is only able to detect compounds that are organic. The instrument is used commonly for pharmaceutical production and analyzing pesticides. It is possible to measure methane or even toxic compounds, such as hydrogen cyanide, because they contain carbon molecules. Inorganic gases are difficult to detect with an FID. Ammonia, for example, does not have carbon in its molecular structure, so it may go unnoticed.

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anon275674
Post 3

You are correct miriam98. There are other methods utilized to detect the presence of gas, not only for leaks, but also for accumulation. Some of the different methods used are Combustible Gas Indicators and Bubble Testing.

Also, gas companies are required to place odorants within natural gas which makes it easier to detect if gas is accumulating within a building or residence. If you hear hissing, smell gas, or suspect that there may be a leak or accumulation of gas within your home, or anywhere, it would be wise to contact your local gas company to have a field technician come out and inspect for the presence of gas.

Charred
Post 2

@miriam98 - Well I think it’s clear that they use these detectors for only certain types of gases. Near where we live there are large landfills that can create methane gas emissions.

I am sure (or I hope) that people working with those landfills use methane detectors to detect the presence of methane. In principle if it’s out somewhere that’s not close to residential neighborhoods it should be safe, but even then I think the EPA imposes certain limits on the amount of methane that can be released.

Also methane can seep into the home as well from what I hear. What makes it dangerous is that it’s odorless and so you really would need a detector of some kind to know if it’s there or not.

miriam98
Post 1

If flame ionization gas detectors can detect only organic compounds and not inorganic compounds, then I assume they would not be the only detectors you would use to filter out the presence of certain gases.

If you want a clear picture of what’s present in gases or pollutants, I think that you would have to use a variety of detectors, with the FID detector being only one of them.

Even then according to the article you can get different results based on certain environmental factors like changes in temperature. So I wouldn’t rely on a flame ionization detector if I wanted to know everything that was going on really.

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