The Combination Detector is one of the most useful detector combinations because it allows the operator to reliably identify hydrocarbon peaks detected by the as halogenated or not.
While less sensitive than the ECD detector, the is much more selective, eliminating interferences which would complicate an ECD analysis (such as ambient oxygen). Sample preparation which might be required for ECD work is not required for the because of its total selectivity to chlorine and bromine, and because the FID pre-combusts any contaminants. In the high sensitivity mode (hydrogen off, using dry cylinder air), can detect down to the low picogram range. In this mode, the is about 100 times more sensitive than when used with the FID in low sensitivity mode.
The Combination Detector measures the ClO2 present in the FID exhaust gas. Because the FID combusts the sample upstream, all hydrocarbons are comverted to CO2 prior, thereby preventing large hydrocarbon peaks from contaminating.