Why does a thermal conductivity detector respond to all analytes except the carrier gas? Why isn't the flame ionization detector universal?

The thermal conductivity detector responds to all analytes except the carrier gas because the thermal conductivity of the carrier gas is very different from that of the analytes. The most commonly used carrier gas is He, which has the second highest thermal conductivity next to$H_2$ . Thus, any analyte mixed with He lowers the overall thermal conductivity of the gas stream.

Flame ionization detector is not universal because the mechanism of detection depends on the production of ${\mathrm{CHO}}^{+}$from CH radicals that are generated by carbon atoms (except carbonyl and carboxyl carbons). The reaction is shown below:

$\mathrm{CH}+\mathrm{O}\to {\mathrm{CHO}}^{+}+{\mathrm{e}}^{-}$

Thus, flame ionization detectors respond to most hydrocarbons with near constant response per carbon atom but is not sensitive to non hydrocarbons.