THC Measurement with a Flame Ionization Detector (FID)

How does an FID work?

A flame ionization detector (FID) burns the organic compounds in a sample in a hydrogen flame. Molecules containing carbon-hydrogen bonds ionize in the flame and create a measurable ion current between two electrodes. The current is approximately proportional to the number of carbon atoms passing through the flame, so the result is generally given in ppmC, as a methane or propane equivalent.

The detector uses hydrogen as fuel and combustion air. With direct sample injection, the instrument measures total hydrocarbons (THC). When a gas chromatography column is placed before the detector, methane and non-methane hydrocarbons (NMHC) can be measured separately.

What FID measures and where it is used

An FID is sensitive to hydrocarbons and volatile organic compounds (VOC). Main areas of use:

• Emission monitoring and total organic carbon/THC measurement in flue gas
• Methane, NMHC and VOC monitoring in ambient air
• Measurements at landfill sites, biogas plants and around refineries
• Fugitive emission (LDAR) surveys with portable instruments

Where FID works well and where it struggles

An FID has a wide linear range and a fast response; response factors are close to one another for many hydrocarbons. It is almost insensitive to water vapor, CO₂ and inert gases, which makes measurement in humid samples easier.

Limitations: the detector requires a flammable fuel such as hydrogen, which calls for planning in terms of site safety and gas supply. Because the response factor varies by component, results should be interpreted as equivalents. Large changes in oxygen content can affect the response. Heavy hydrocarbons tend to condense, so heated sample lines and heated detectors are used in high-temperature applications. An FID can respond less to partially oxidized and halogenated compounds; for such target components, a photoionization (PID) detector may be preferred.

Choosing an instrument for FID

When selecting, the type of target components (total hydrocarbons, methane separation or toxic VOCs), measurement range and sample temperature are assessed. Gas supply for fuel hydrogen and combustion air should be planned in advance for safe installation in the plant.

Tetra and FID analyzers

In Türkiye, Tetra carries out the supply, commissioning, calibration and service of FID-based analyzers. The product range includes the Thermo Scientific 51i total hydrocarbon analyzer (with a high-temperature option), the 55i methane/non-methane hydrocarbon analyzer, the TVA2020 portable toxic vapor analyzer (FID and PID together) and Synspec Alpha series FID/PID VOC analyzers. Synspec GC955 gas chromatographs can also be configured with an FID detector according to the application.