The authenticity of natural and synthetic matter can be checked by measurement of the isotope ratios of C, N and S. Controlled substances like drugs of abuse (cocaine, heroin) and explosives (TNT) or, simply, traces of paint can hold information in their isotope pattern. Total combustion of samples in an elemental analyzer followed by on-line determination of the isotopes of the combustion products (CO2, N-2, SO2) in an isotope ratio mass spectrometer (EA-IRMS) provides high sample throughout with a minimum of sample preparation.