An analysis system for analysing the constituents of a sample of material, the system comprising: a reference supply conduit for supplying a source of a first gas; a carrier supply conduit for supplying a source of the first or a second gas; first and second reactors; a first auto-sampler for providing one or more samples of material, the first auto-sampler having an inlet for receiving gas and an outlet for providing the received gas and a sample to the first reactor; a second auto-sampler for providing one or more samples of material, the second auto-sampler having an inlet for receiving gas and an outlet for providing the received gas and a sample to the second reactor; a thermal conductivity detector having first and second channels for identifying the relative conductivity of the gases in each channel, characterised in that: the system further comprises a valve system (80, 100) arranged to control the flow of gas from the supply conduits (2,4) to the inlets (22a, 22b) of the first and second auto-samplers; the analysis system has a first mode of operation in which: • the valve system (80, 100) connects the carrier supply conduit with the first auto-sampler; • the first auto-sampler delivers the received gas and a sample to the first reactor; • the first reactor delivers at least one reaction product to one channel of the thermal conductivity detector; and • the valve system (80, 100) connects the reference supply conduit to the other channel of the thermal conductivity detector; and the analysis system has a second mode of operation in which: • the valve system connects the carrier supply conduit with the second auto-sampler; • the second auto-sampler delivers the received gas and a sample to the second reactor; • the second reactor delivers at least one reaction product to one channel of the thermal conductivity detector; and • the valve system (80, 100) connects the reference supply conduit to the other channel of the thermal conductivity detector. a reference supply conduit for supplying a source of a first gas; a carrier supply conduit for supplying a source of the first or a second gas; first and second reactors; a first auto-sampler for providing one or more samples of material, the first auto-sampler having an inlet for receiving gas and an outlet for providing the received gas and a sample to the first reactor; a second auto-sampler for providing one or more samples of material, the second auto-sampler having an inlet for receiving gas and an outlet for providing the received gas and a sample to the second reactor; a thermal conductivity detector having first and second channels for identifying the relative conductivity of the gases in each channel, the system further comprises a valve system (80, 100) arranged to control the flow of gas from the supply conduits (2,4) to the inlets (22a, 22b) of the first and second auto-samplers; the analysis system has a first mode of operation in which: • the valve system (80, 100) connects the carrier supply conduit with the first auto-sampler; • the first auto-sampler delivers the received gas and a sample to the first reactor; • the first reactor delivers at least one reaction product to one channel of the thermal conductivity detector; and • the valve system (80, 100) connects the reference supply conduit to the other channel of the thermal conductivity detector; and • the valve system (80, 100) connects the carrier supply conduit with the first auto-sampler; • the first auto-sampler delivers the received gas and a sample to the first reactor; • the first reactor delivers at least one reaction product to one channel of the thermal conductivity detector; and • the valve system (80, 100) connects the reference supply conduit to the other channel of the thermal conductivity detector; and the analysis system has a second mode of operation in which: • the valve system connects the carrier supply conduit with the second auto-sampler; • the second auto-sampler delivers the received gas and a sample to the second reactor; • the second reactor delivers at least one reaction product to one channel of the thermal conductivity detector; and • the valve system (80, 100) connects the reference supply conduit to the other channel of the thermal conductivity detector. • the valve system connects the carrier supply conduit with the second auto-sampler; • the second auto-sampler delivers the received gas and a sample to the second reactor; • the second reactor delivers at least one reaction product to one channel of the thermal conductivity detector; and • the valve system (80, 100) connects the reference supply conduit to the other channel of the thermal conductivity detector.