A method for controlling a liquid volume flowing onto a reactive surface which includes a sample application port, two systems of capillary channels, a reaction chamber, and an overflow chamber. The liquid sample to be analyzed is applied to the sample application port which gives the liquid entry to a capillary channel leading to the reaction chamber which contains a material capable of detecting the component of interest in the liquid. When the reaction chamber is filled, the remaining liquid will flow through another capillary channel to an overflow chamber. The capillary channel leading to the overflow chamber is controlled so that liquid cannot flow back into the reaction chamber.