Laminated wafers with channels formed therein define a plurality of gas chromatographic columns in a body, with substantially minimal connecting channels between an injector and a detector. One or more of the columns at a time is selected by valve to be receptive of sample gas. Detector and injector cavities have surfaces with the adsorbent phase utilized in the column being further coated on the surfaces, whereby the detector and injector constitute an integral portion of the gas chromatographic column. A channel is formed between wafers each with grooves that have semicircular cross sections so as to form the channel with a circular cross section. A digital gas injector includes a plurality of chambers with respective measured volumes. Valves select one or more of the chambers at a time to be receptive of sample gas from the common inlet. A detector comprises a body having a passage therein receptive of a first flow consisting of a sample gas mixed into a carrier gas, and alternatively receptive of a second flow consisting of a reference gas. Preferably the first and second flows are oppositely directed by valves alternatively through the passage. Boron or silicon nitride film is utilized in cavities in the body so as to provide a structural component for membranes of the valves and for hot wire detectors.