Several embodiments of gas chromatography systems are disclosed incorporating mechanical sample injection valves. The injection valves incorporate a delivery tube having an outlet which is swept past the inlet of a separation column to provide a short duration inlet plug into the separation column. Carrier gas and sample gas flow is managed through the valve to prevent inadvertent injection after an injection is desired and to assure that the sample is not contaminated by past sample flow. Embodiments of injections valve are described for providing inputs into several gas chromatography columns. In addition, a gas chromatography system is described in which multiple columns are used which are time multiplexed with a single detector to produce a complex combined chromatogram from the plural columns. By appropriate selection of column characteristics and injection sequence, the resulting chromatogram can provide an overlay of chromatograms which provide good resolution of all the constituents of the mixture of interest within a short analysis time.