In principle, the Flame Photometric Detector (FPD) is identical to the FID with the addition of an S or P filter and a photomultiplier tube.
Because also other atoms and (partial) molecules emit light, specific optical filters are used that only allow the light with specific sulphur or phosphorus wavelengths to pass.

In the FPD, the column effluent is mixed with air and burnt in a hydrogen rich atmosphere, producing a small flame. S and P containing sample molecules are decomposed into S₂ and HPO. The decomposed sample then enters into a second flame that supplies the energy for exciting these fragments. Upon relaxation light is emmited.

The specific emission that results after application of an S or P filter, is converted into a current and amplified by the photomultiplier tube. This current is the detector signal.

Some FID's use a dual stacked jet arrangement providing two vertically positioned flames:

• The lower flame produces highly reduced species from all the organic material eluting from the column, many of which emit at wavelengths that would either obscure or quench the required emissions from the P or S compounds.
• The second flame is an optimized flame which excites P or S containing radicals to HPO* and S2* species respectively. These produce very characteristic emission spectra, giving maximum emission intensities at wavelengths of 526 nm for phosphorus and 394 nm for sulphur. The emitted radiation is monitored by the photomultiplier and the resulting current is amplified. As with the NPD, the FPD does not normally require a separate make-up gas flow, as the jet volume is rapidly purged by the air supply which is premixed with the carrier gas.

Other FPD designs use single flames where the two zones are created by the supply of additional fuel gas higher in the detector compartment.

The sensitivity of the detector is a little higher for phosphorus-containing compounds than for sulphur-containing compounds, but a significant drawback in the sulphur mode is its inherent non-linearity. This is due to the formation of the S2 radical upon which the selective response is based. The detector output is theoretically proportional to the square of the sulphur mass flow rate.

The FPD is widely used for the trace determination of sulphur compounds, particularly when they are in the presence of much larger amounts of other types of compounds such as hydrocarbons, etc.
 
The FPD detects many hetero-atoms, including metallic elements. In fact photo-emissive properties have been utilized in GC for the selective detection of elements such as halogens, nitrogen, boron, selenium. The measurement of photo-emission at selected wavelengths is a very attractive way to achieve selectivity, as it involves more clearly understood principles than other flame detectors. Despite this, the use of the FPD is now almost exclusively limited to the selective detection of phosphorus and sulphur-containing compounds.