A technique of determination of nitrate-nitrite content of a test sample, without also determining additional nitrogen content. The technique is highly selective and sensitive, to the parts per billion level. The nitrate-nitrite content of a sample is reduced to nitric oxide which is determined via its chemiluminescence reaction with ozone. Nitrite is selectively reduced under mild conditions and the total nitrate-nitrite content is determined by stronger reduction conditions. Specific reducing agents to be used in combination with the chemiluminescence reaction are also provided.