Department of Earth and Planetary Sciences - USRG
U-Series Research Group
U-series isotope analysis
Analytical issues and challenges
- traditional α-counting yields a precision of ∼4-10%
- Th and Pa have thermal ionisation efficiency <0.01% (PIMS)
- Th and Pa are highly insoluble
- Ra has a thermal ionisation efficiency ∼5-14% (TIMS)
- Ra and Pa only present at fg/g levels
- 234U/238U ratios ∼10-5, 230Th/232Th ratios ∼10-6
- 238U and 232Th "tail" onto 234U and 230Th, respectively
- Mixed 236U-229Th spike for U-Th analysis (Oak Ridge)
- 228Ra spike milked from equilibrium 232Th solution (NIST3159)
- 233Pa spike milked from 237Np or derived by irradiation of 232Th
- 209Po spike available from NIST
- Calibration against SRM solution standards
- Cross-check with analysis of secular equilibrium material (TML)
- Th and Pa spikes need ∼0.01M HF to keep in solution
Decay chains of the U-series isotopes (activity = λN). Click to enlarge
- Newly constructed clean lab facilities with four self-contained laboratories, including two dedicated solely for U-series work.
The clean lab area includes 7 laminar flow workstations and two separate balance rooms
U-series clean lab facilities
- Nu Plasma high resolution multi-collector ICPMS, equipped with an RPQ filter (Installed Nov. 2003)
- Sample in solution of 2% HNO3, sprayed into the plasma using a DSN 100
Nu Plasma MC-ICP-MS
- Finnigan Triton TIMS (Installed March 2005) Equipped with RPQ filter, SEM and oil-free scroll pump.
- Ra samples loaded onto outgassed, zone-refined Re filamentsusing a Ta-HF-H3PO4 activator solution
Finnigan Triton TIMS
U isotope analysis by MC-ICP-MS
- U analysis using CRM145 or CRM960 standard
- Comparison of measured 238U/235U ratio to the true ratio (137.88) yields mass bias fractionation.
- Comparison of mass fractionation-corrected 234U/238U to the true ratio (5.286x10-5) yields ion counter gain
- Intersperse samples with CRM145 analyses
- 234U on Ion Counter 0 (IC0)
- 235U on Faraday cup L2
- 238U on Faraday cup H1
- 236U on IC0
- 238U on Faraday cup L1
234U/238U standard measurements of TML. True value= 1. Click to enlarge.
Th isotope analysis by MC-ICP-MS
- 230Th on IC0
- 232Th on Faraday cup L1
- 229Th on IC0
- 232Th on axial Faraday cup (Ax)
TML-230Th/238U bracketed with Th'U'. True value= 1. Click to enlarge
Ra isotope analysis by TIMS
- Slow heating to 1200°C then use 138Ba to focus
- Scan masses 224-230 to check for Ra and hydrocarbons
- Build Ra beam (100-1000 cps @ 1250-1350°C)
- Dynamic collection on SEM ∼80 ratios with 8 second integration times (precision <0.5%)
TML standard measurements for 226Ra/230Th. True value= 1. Click to enlarge.
210Pb isotope analysis by α-counting
- 210Pb concentrations are determined from 210Po on separate 1-2 g sample dissolutions which are spiked with a 209Po tracer calibrated against a value of 7.95 dpm/g for TML
- Samples are loaded onto a 1 ml anionic column in 1N HCl containing H3BO3 where most major and trace elements were washed off in 1N and 0.5N HCl after which Po is eluted in warm 7.5N HNO3
- Po is autoplated onto a Ag disc above a magnetic stirrer for 6-8 hours in 150 ml of warm 0.5N HCl which contained ascorbic acid. 210Po and 209Po were counted for ∼1 week to achieve a precision of <4%
Ortec Octete Plus α-counter.
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