At any one time, the effluent from one column is directed to a flame ionisation detector (FID) while the other column is directed to the mass spectrometer via a capillary combustion interface. A high temperature, zero dead volume,
automated 4-port valve, housed in a separate isothermal oven, controls this function. The use of 0.53 mm columns permits high sample loading at flows sufficient to avoid
the use of make up gas elsewhere in the interface.