Thermal ionisation mass spectrometry (TIMS)
Thermal ionisation mass spectrometry has seen spectacular progress over the past 30 years, brought about by automation, advances in solid state electronics, multi-collection techniques and filtering, not to mention advances in the ‘savoir faire’ of those using these instruments. Consequently, it is now possible to measure isotope ratios for elements such as Sr or Nd with a precision better than 10-5 on sample sizes of a nanogram or less using TIMS. For those elements that are not abundant in nature, such as Os, one of the fundamental limitations for analysis is how many of the atoms present in the sample are effectively measured. With the development of negative-TIMS techniques for Os it is now possible to measure some 25% of the atoms present in the sample, and since that time progress in low-blank chemistry has enabled the measurement of exceptionally small samples. As an illustration it is now possible to measure the isotope composition of Os in 50 mls of seawater with a concentration of ca. 10-14 g of Os per gram of seawater.
These advances have been paralleled for a number of other elements of interest, such as Sr, Nd, Pb and Th-U series isotopes. Thus, TIMS instruments, particularly in the field of negative thermal ionisation, and for particularly small samples play a fundamental role in modern isotope geochemistry. With the advent of isotope double-spike techniques, these advances can now be extended to explore the stable isotope variations of elements previously only studied for their radiogenic isotope compositions, such as Sr.
At present the Arthur Holmes laboratories have two Thermo Fisher Triton instruments. One is largely dedicated to the measurement of Nd, Sr and Pb isotopes, and negative ionisation (N-TIMS) analysis of Os isotopes. The second Triton is dedicated to ultra high-precision isotope ratio measurements on small samples.