There exists three types of gas detectors that respond in different conditions such that: specific detectors, selective detectors and non-selective detectors.
The specific detectors have a suggestive name that indicates the substances they are able to detect. One example of specific detector is nitrogen phosphorous detector that is able to identify compounds that contain phosphorous or nitrogen.
The selective detectors are able to identify the compounds that share the same chemical properties (fluorescence, heat of combustion) or physical properties (dielectric constant).
The non-selective detectors (katharometer) are able to identify the vapors from all compounds, whose specific heats or thermal conductivities are not the same with the one of carrier gas.