A liquid analyser has a reactor portion and an associated measurement portion. A sample pump is operable to deliver a liquid sample to a reactor vessel. A base pump supplys a base solution to the reactor vessel. An ozone generator supplies ozone to the reactor vessel. The liquid sample is oxidised in the reactor vessel by means of hydroxyl radicals which are generated using the base solution and ozone to reduce complex components of the liquid sample to their lowest state in solution. The oxidised sample solution is delivered to an optical detector in the measurement portion to determine the concentration of one or more selected materials such as nitrogen, phosphorous or a heavy metal in the oxidised sample solution.