A Flame-Ionization Detector (FID) measures real-time concentrations of organic vapors in low levels from parts-per-billion (ppb) up to 50,000 parts-per-million (ppm). A FID is a very sensitive instrument used in a wide variety of applications such as:

• Hazardous waste sites
• Clean-up of spills
• Protecting workers from toxic vapors
• Refineries
• Leak detection for compliance
• Pulp and paper plants
• Hazardous materials response
• Terrorist chemical attacks

How Does an FID Work?
FIDs, like PIDs, are ionization detectors. The difference between them is the way ions are produced. In our "introduction to PIDs" section you learned that PIDs use high energy Ultra Violet light to ionize molecules. FIDs do not use ultra violet light, instead the compounds in the sample stream are burned in a hydrogen fueled-flame. The hydrogen flame can ionize any organic material with an ionization potential (IP) of 15.4 or less. Like the PID, ions formed in the sample stream are driven in one direction by a bias electrode and accumulated at a collecting electrode. The ion current is then amplified and converted to a meter readout (analog or digital, depending on the model FID) in parts-per-million.

view a diagram

When to Use an FID
FID's just like PID's are primarily screening devices. They will detect any volatile organic compound (VOC) with an Ionization Potential below 15.4 eV. Simply put, a FID tells you that something is there, never what it is (besides that it is a VOC), and only sometimes how much. For example, the standard calibration for a FID is 100ppm Isobutylene, if isobutylene is the VOC in the air the ppm reading on the instrument is an accurate representation of how much Isobutylene is present. However, if there is or can be other VOC's in the air, you can only be certain that something is there, not what or how much. Sometimes FID's can be calibrated to other gases, like Methane. Some FID's contain built in correction factors that allow the instrument to be calibrated to Isobutylene but have the reading adjusted for other gases.

PID or FID?
PID's are susceptible to humidity and cannot detect VOC's with an ionization potential above 11.8 eV. FID's are not susceptible to humidity and can detect VOC's up to 15.4 eV. FID's are desirable in situations of high humidity, or when detecting gases with high ionization potentials like Methane. This comes at the cost of having to have a not only a battery, but a Hydrogen cylinder to fuel the flame.