Research School of Earth Sciences
A facility for very high precision isotopic analysis for geological time measurement and isotopic tracing (Sensitive and Precise Isotopic Dating of Earth’s and Extraterrestrial Rocks, SPIDE2R) is currently under construction at the Research School of Earth Sciences, the Australian National University.
It is based on thermal ionisation mass spectrometry, and includes the most promising, but thus far little explored, analytical developments: thermal cavity source and ion beam measurement by charge collection. This unique facility will increase the sensitivity and precision of isotopic analysis by an order of magnitude or more compared to the best currently available instruments, enabling frontier research in the Earth and planetary sciences.
Limits on precision of the isotopic ratio (142Nd/144Nd is used as an example) based on counting statistics, and their dependence on the sample size and the total ion yield (ions detected per atoms consumed). Detector noise and possible signal aberrations are not included in the error estimate.
Understanding the evolution of the Earth and the Solar System relies on our ability to measure natural isotopic variations of elements spanning the range of the periodic table, with ever higher precision from ever smaller samples. Presently, three techniques for isotopic analysis of non-volatile elements are used in modern earth and planetary sciences, each technique having its unique strength and area of application. Secondary ionization mass spectrometry (SIMS, also known as ion microprobe) is best suited for in situ analysis of minerals with unsurpassed spatial and depth resolution. Inductively coupled plasma mass spectrometry (ICPMS) provides fast analyses of samples (either dissolved, or ablated with a laser), and is applicable to a wide range of elements due to the potency of the ~10,000°K plasma to ionise almost all elements with nearly 100% efficiency. Thermal ionisation mass spectrometry (TIMS) is the technique of choice in cases where the highest possible precision of isotopic ratios is required.
Precision of isotopic ratios depends on counting statistics (number of ions detected), and on the sources of the noise and signal aberration in the mass spectrometer. Given the same level of the instrument noise, a mass spectrometer with higher ion yield (the ratio of the number of ions detected to the number of atoms consumed for analysis) is capable of more precise measurements, or can perform analysis at a specified precision level from a smaller sample. Ion yield in the modern SIMS and MC-ICPMS (multiple collector ICPMS – the most sensitive and precise version of ICPMS) optimised for maximum sensitivity is between 0.1 - 1.0% (Fig).
Despite nearly 100% ionisation efficiency, the ion yield in MC-ICPMS is limited to 1% or less by efficiency of ion extraction from the plasma. In contrast, in modern TIMS 40-50% or more ions are extracted from the source and transmitted to the detectors, and the ion yield is limited by the efficiency of ionisation. For most elements and "conventional" surface ionization, only 0.1-1.0% atoms are converted to ions, thus putting the "conventional" TIMS in the same sensitivity range with MC-ICPMS and SIMS. The efficiency of thermal ionisation can be enhanced to 10-50%, making TIMS by far the most sensitive method of isotopic analysis. This can be achieved by using emission activators, from which the analysed elements are evaporated mainly in ionic, rather than atomic, form, or by replacing the conventional flat ribbon filament with a high efficiency thermal cavity ion source. With the SPIDE2R facility, we will explore and optimise both emission activators and the cavity source, as well as their combination; this approach has not been attempted before.
Precision of isotopic ratios deteriorates if additional sources of noise are present in the mass spectrometer. In particular, the measurements of the ion beam are compromised by the thermal noise of feedback resistors, which seriously undermines precision for ion beams of 10-12-10-13A using "conventional" electrometers with high value feedback resistors, connected to Faraday cups, and makes these devices impractical for measuring ion beams smaller than ~10-14 A. Small ion beams are usually measured using ion counting devices, such as secondary electron multipliers, but these devices are poorly suited for measuring ion beams larger than 10-13 A, because the aberrations due to non-linearity and non-zero dead time increase as the ion beam grows. The "charge collection" technique allows the measurement range to be extended below 10-14 A, while preserving the advantages of the Faraday cup arrays.
The SPIDE2R facility will combine the reputable quality of the best commercial TIMS with the high efficiency thermal cavity ion source and the low noise multi-channel detector system based on the principle of charge collection. With this combination, we expect to achieve a 3-10 fold increase in the sensitivity in analysis of refractory elements, and elements with high ionisation potentials, such as Hf, Nd, U, Th, Pu, Ra, Os and Pb, and to gain similar improvement in precision for medium and small quantities of these elements. This new level of precision and sensitivity will offer the opportunity to carry out isotopic analyses in geochemistry, cosmochemistry, geochronology, astrophysics and other disciplines, which are now considered unattainable because of the limitations of existing mass spectrometers.