Open University Uranium-Series Laboratory    
  Earth and Environmental Sciences, The Open University, Milton Keynes, UK    
  Analytical performance and Quality Assurance  


Uranium-series determinations, i.e. 234U/238U, 230Th/232Th ratios, U, Th, Pa and Ra concentrations by isotope dilution, are performed routinely on a number of laboratory shelf standards and this has allowed us to build up long-term QA files for our laboratory standards. An example for a uranium standard is shown below.
U-456 chart

Long term reproducibility of the 235U/236U ratio for our U-456 standard. This standard has been prepared gravimetrically by mixing dilutions of CRM U112a natural uranium and BC772D 236U spike. ‘Whiskers’ indicate the ±2sm uncertainty in individual measurements and ‘tram-lines’ indicate the mean and 2s.d. variation over the whole period (2 rsd=0.30%). Over the three year period of analyses there has been detector changes and laboratory down-time, visible as  minor discontinuities in the data.

All shelf standards are mixtures and dilutions of internationally recognised Certified Reference Materials and spikes. We maintain QA files for reagent blanks and Total Procedure Blanks. Precision of our shelf standards is at the 1-2‰ level and allows overall uncertainty on calculated ages to be at the 0.5-1% level, depending, of course, on the age and U concentration of the samples and assuming closed system conditions.
For comparison with other U-series laboratories we analyse ‘rock’ standards, including internationally recognised TML, ATHO and our own ThITS, a trachyte and YS, a homogenised young stalagmite. We routinely analyse the OU Th’U’ standard that was prepared for: van Calsteren and Schwieters (1995). This solution has acquired the status of an informal international standard and has been analysed by many other U-series laboratories.  Figures below are based on compilations from Sims et al, in prep.
TML labs
OU Th'U'std
Th isotope ratio data for Table Mountain Latite rock standard from a compilation by Sims et al., in prep. ‘Whiskers’ indicate reproducibility at 2 standard deviation.
Th isotope ratios for the OU Th’U’ standard from a compilation by Sims et al., in prep. ‘Whiskers’ indicate reproducibility at 2 standard deviation.

Both graphs show that results from the different laboratories are indistinguishable, despite different chemical separation methods and different types of mass spectrometer from different vintage and manufacturers.

Thorium isotope ratios in samples containing less then 0.5 pg 230Th can be measured with a precision better than ±1% (2σm) and Ra abundances, as low as 0.2 pg, can be determined to ±1%(2σm) .
OUUSF participates in inter-laboratory ‘round robin’ uranium isotope ratio measurement comparisons such as the recent REIMEP 18, organised by IRMM. REIMEP-18 consisted of 4 artificial isotope mixtures to match typical sample requirements of environmental and nuclear sciences. Our results are within the expected range for all environmental isotope ratios, Richter et al., 2006.


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© Peter van Calsteren
Last updated: date December 23, 2011 11:36