Apparatus for determining a carbon isotropic abundance ratio from a light absorption spectrum comprises: a laser 13 emitting light in the 2pm waveband which is swept in wavelength by modulating the drive current with a frequency f, a specimen cell 2 for containing the carbon dioxide and a photodetector 14 for detecting the light absorption spectrum from the light which has passed from the laser through the specimen cell. A lock in amplifier 15, 16 is used to detect the 2f component or second differential of the light absorption spectrum. A controller is used to determine the spectral intensities of 12 CO 2 and 1 .CO* small Greek eta * from peak values at three points of each of the 2f component and determining the carbon abundance ratio. In one embodiment a lock in amplifier is used to detect a 3f component or third differential of the spectrum, the wavelength of the laser is then locked to the zero crossing points of the 3f component and the peak intensity values measured at the corresponding points of the second differential.
Apparatus for and method of analyzing carbon isotopes