Thermal ionization mass spectrometry, or TIMS, has long been used as a standard method for very precise determination of isotope ratios in fields such as isotope geology. TIMS is used for isotope systems that in the natural environment can be found in the solid phase (U, Pb, Sr, Os, B, and Li).

TIMS utilizes mass spectrometers fitted with thermal ion sources. Samples are deposited on specially treated filaments (usually rhenium or tantalum), then carefully dried. The filaments are heated slowly, leading to evaporation and vaporization of the sample. The resulting positively ionized atoms are accelerated from the ion source by an electric field, then mass-separated by an elecromagnet. The separated beams for each isotope are then collected in Faraday cups, which convert the beam into electric current. The ratio of one Faraday cup to another is used to calculate isotope ratios.

TIMS requires time-consuming sample preparation to ensure high quality chemical separation of the analyte. It is also important to correct for isotopic fractionation, particularly in the ion source.