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

Spikes

• 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

Analytical facilities

Laboratories

• 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

• 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

• Finnigan Triton TIMS (Installed March 2005) Equipped with RPQ filter, SEM and oil-free scroll pump.
• Ra samples loaded onto outgassed, zone-refined Re filaments using a Ta-HF-H₃PO₄ activator solution

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



Cycle 1

• ²³⁴U on Ion Counter 0 (IC0)
• ²³⁵U on Faraday cup L2
• ²³⁸U on Faraday cup H1



Cycle 2

• ²³⁶U on IC0
• ²³⁸U on Faraday cup L1

Th isotope analysis by MC-ICP-MS

• A solution of 20ppb Th yields a beam of and ∼1V of ²³²Th, 100s of cps of ²³⁰Th
• ²³⁰Th is collected on an ion counter preceded by a RPQ deceleration lens. Typical abundance sensitivity is ∼450ppb at 1 a.m.u. for a transmission of 95-98% (190ppb at 2 a.m.u. but transmission not determined)


• Th analysis using CRM145 or CRM960 and Th 'U' standards
• 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 Th 'U' analyses to get βTh

Cycle 1

• ²³⁰Th on IC0
• ²³²Th on Faraday cup L1

Cycle 2

• ²²⁹Th on IC0
• ²³²Th on axial Faraday cup (Ax)

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%)

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%