Abstract: An apparatus to measure the concentration of a constituent of a solution containing dissolved gas includes a conduit connectable to a source of the solution. Flow controllers are provided on the conduit, selectively to permit the flow of the solution in the conduit. Pressure regulators are also provided on the conduit to control the pressure of the solution in the conduit, and maintain the pressure in the conduit means at a level sufficiently high to prevent gassification of dissolved gas in the solution, thereby to prevent foaming in the conduit. A gas chromatographic analyser (GCA) is in communication with the conduit via a sampling valve, to permit the flow of discrete samples of solution into the GCA for analysis. The interior of the GCA is maintainable at the same pressure as in the conduit.