Thorium isotope ratio measurements

Neder H, Heusser G, Laubenstein M (2000) Low-level y-ray germanium-spectrometer to measure veiy low primordial radionuclide concentrations. ApplRadiat Isot 53 191-195 Palacz ZA, Freedman PA, Walder AJ (1992) Thorium isotope ratio measurements at high abundance sensitivity using a VG 54-30, an energy-filtered thermal ionization mass spectrometer. Chem Geol 101 157-165... 

Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cohen AS, O Nions RK (1991) Precise determination of femtogram quantities of radium by thermal ionization mass spectrometry. Anal Chem 63 2705-2708 Cohen AS, Belshaw NS, O Nions RK (1992) High precision uranium, thorium, and radium isotope ratio measurements by high dynamic range thermal ionization mass spectrometry. Inti J Mass Spectrom Ion Processes 116 71-81... 

Platzner IT, Becker JS, Dietze H-J (1999) Stability study of isotope ratio measurements for uranium and thorium by ICP-QMS. At Spectrosc 20 6-12... 

Platzner, I. T., Becker, J. S., and Dietze, H. J. 1999. Stability study of isotope ratio measurements for uranium and thorium by ICP-QMS. Atomic Spectroscopy 20(1), 6-12. Porter, C. R. and Kahn, B. 1964. Improved determination of strontium-90 in milk by an... 

Isotope dilution mass spectrometry (ID-MS) is widely accepted as a quantification procedure of proven accuracy in elemental analysis and isotope ratio measurements [4]. Several areas of research in nuclear science, geochronology, medicinal chemistry, environmental science, and agricultural science have benefited from this technique. ID-MS is applicable to all elements that have at least two stable isotopes. Monoisotopic elements can be analyzed only if they have a long-lived natural or artificial radioisotope. For example, iodine and thorium have been determined with spikes of the long-lived isotopes 29i and 25 Th, respectively [44]. TI-MS and ICP-MS are the methods of choice for accurate ID-MS analysis. ICP-MS has the advantage that several elements can be analyzed simnltaneously under the same experimental conditions. Other ionization techniqnes discussed in this chapter have also been coupled with ID-MS. 

Thorinm-232 is the only non-radiogenic thorium isotope of the U/Th decay series. Thorinm-232 enters the ocean by continental weathering and is mostly in the particulate form. Early measurements of Th were by alpha-spectrometry and required large volume samples ca. 1000 T). Not only did this make sample collection difficult, but the signal-to-noise ratio was often low and uncertain. With the development of a neutron activation analysis " and amass spectrometry method " the quality of the data greatly improved, and the required volume for mass spectrometry was reduced to less than a liter. Surface ocean waters typically have elevated concentrations of dissolved and particulate 17,3 7,62... 

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

However, thorium has only two naturally occurring long-lived isotopes, and all Th measurements by TIMS are limited by the absence of a well-constrained isotope ratio that can be used for internal normalization purposes to correct for instrumental mass fractionation. In this regard, one of the most important advantages of MC-ICPMS over MC-TIMS is the ability to admix two elements with overlapping mass ranges and use the... 

Concentrations of uranium and thorium are also measured by other techniques (e.g., instmmental neutron activation analysis) and sometimes included in trace element analyses. In geochemical literature the ratio [Th/U] is frequently used. However, Th is the only long-lived thorium isotope and U is a fixed 99.28% of total uranium, so [Th/U] ratios are closely related (3.3% smaller) to atomic Th/ U ratios. The latter ratio has historically been termed k when used in the Pb isotope literature. To further... 

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