Laser microprobe stable isotope measurements on geological materials: Some experimental considerations (with special reference toδ³⁴S in sulphides)

A laser, especially when focused through a petrographic microscope, can provide a localised heat source for extraction of light elements from solids prior to stable isotope ratio measurements by gas-source mass spectrometry. In certain cases, such as conversion of sulphides to SO₂ by combustion in an oxygen atmosphere, it may be necessary to choose operating characteristics which allow the chemistry at the solid target surface to be controlled. Experimental considerations such as wavelength, laser mode and irradiance are discussed and it is shown that several successful system work at very similar irradiances of∼ 10⁹W m^-2. It is suggested, for those circumstances where high spatial resolution is required and where deep penetration into the surface cannot be tolerated, that a suitable modus operandi is to use a relatively low power but narrowly focused beam to excavate trenches perpendicular to the direction in which resolution is demanded. This allows a spatial resolution of< 100 μm even for porous pyrite witha poor quality of surface.