Biology Forums - Study Force

Helium is the carrier gas of choice for a gas chromatograph containing a thermal conductivity
  detector, and nitrogen gas is preferred for a gas chromatograph having a flame ionization
  detector. Suggest a reason for this.

Helium has a very high thermal conductivity, thus maximizing the response when compounds
of low thermal conductivity pass the detector. In addition, all organic compounds show a peak
response that rises above (rather than falls below) the baseline. Nitrogen has a lower thermal
conductivity than helium, but is much less expensive. In a flame ionization detector, thermal
conductivity is of no consequence; therefore, nitrogen is preferred.