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A double proportional injector is described, and an improvement of the continuous flow-injection method is discussed. Sample and reagent are both injected into an inert carrier stream; the reagent is consumed only in the presence of the sample, and can otherwise be recovered continuously. The main characteristics of the flow injection system, e.g. high sampling rate and good accuracy, precision and sensitivity, are maintained. The proposed system is compared with other flow-injection systems suggested earlier. The determination of phosphate in plant material by the molybdenum blue flowinjection method was chosen to demonstrate the feasibility of the system; the consumption of ascorbic acid is only 9% of that in the original method.