Answer :
Final answer:
Gas sensing detectors commonly use either optical or thermal sensors to detect the presence or concentration of various gases, with specific detectors like flame ionization and thermal conductivity detectors employed in applications such as gas chromatography.
Explanation:
Most gas sensing detectors are based on either optical sensors or thermal sensors. Gaseous ionization detectors, for example, utilize the ionizing effect of radiation on gas-filled sensors, leading to a measurable current flow. Optical sensors like the flame photometric detector rely on the emission from elements like phosphorous and sulfur to selectively detect compounds containing these substances. Thermal sensors, on the other hand, can be found in devices like thermionic detectors that respond to the presence of nitrogen or phosphorous, or thermal conductivity detectors (TCD) that depend on differences in thermal conductivity between gases.
In regard to the specifics of the choices provided, optical sensors operate by detecting light emitted or absorbed by substances, while thermal sensors detect changes in temperature or thermal properties. For instance, flame ionization detectors (FID) and TCDs are commonly used in gas chromatography with FIDs being primarily sensitive to hydrocarbons and TCDs being essentially universal, barring the carrier gas. Applications of these sensors extend to various fields including robotics and biomedical imaging systems, where they form an integral part of the detection apparatus.
