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.