A specimen pyrolysis reactor (1) has two nested tubes surrounded by a heater (4) for high temperature heating of a reactive carbon and a catalyst layer (6) applied mid-way along the inner tube (3). A specimen pyrolysis reactor (1) has: (a) an outer ceramic tube (2) surrounding an inner vitreous carbon tube (3) which contains carbon for the pyrolysis and which is centered within the outer tube (2) by upper and lower heat resistant funnels (5a, b) such that it is out of contact with the atmosphere and the outer tube, the funnels being sealed to the upper and lower edges of the outer tube; (b) a horizontal layer (6) of reactive carbon and catalyst applied mid-way along the inner tube (3); and (c) a heater (4) which surrounds the outer tube (2) and which produces temperatures of 1300-1600 deg C in the inner tube at the level of the layer (6). Independent claims are also included for the following: (i) an elementary analyzer for on-line coupling to an isotopic ratio mass spectrometer (IRMS) and consisting of the above pyrolysis reactor (1) connected to a gas chromatography column for separation of carbon monoxide and one or both of nitrogen and hydrogen; and (ii) a method of on-line mass spectrometric determination of the oxygen isotopic composition of organic and inorganic specimens by coupling a pyrolysis reactor via a gas chromatography column to an IRMS, in which the specimen is completely pyrolyzed to CO, N2 and H2 in the reactor at 1300-1600 deg C in the presence of carbon and a catalyst, the gaseous pyrolysis products are entrained by a helium flow into the chromatography column for CO separation from the N2 and H2, and the separated pyrolysis products are passed by the helium flow into the IRMS for determination of the delta 18O value and thus the oxygen isotopic ratio.
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