Abstract: The invention provides apparatus and methods for determining the isotope ratio of a sample. The apparatus comprises a dynamically heated chamber (1); a reactor (4), wherein an outlet of the dynamically heated chamber is coupled to a reactor inlet; an isotope ratio spectrometer (6), wherein an outlet of the reactor is coupled to a spectrometer inlet; such that a gas flow path is provided from the dynamically heated chamber to the isotope ratio spectrometer; wherein the apparatus includes at least one selective gas trap (3,5) in the gas flow path, the gas trap being configured to selectively and reversibly trap one or more gases present in the gas flow in use.