Abstract: Apparatus and method for determination of concentrations of a substance in solution in a sample by determining the time between when the substance first appears and when it disappears. The time between the appearance and the disappearance of the substance is used to determine the concentrations of the substance in a sample. The apparatus includes many elements of a flow injection analysis system but also includes a very important stirred dilution chamber that enables the apparatus to use a simplified detection system that is capable of being used over a wide range of sample concentrations. The apparatus can include a microwave digestion system and in one embodiment a continuous flow microwave digestion system.