Dissolution of crystals and determination of U, Th & Sm content by ICP-MS

After the extraction of helium (which is the product of radiocative decay) we have to determine the amount of alpha-emitting (so called parent) elements. Thus the crystals are dissolved and the U, Th and Sm contents are determined by solution ICP-MS technique. The uranium and thorium are present usually in sub-nanogram amount in single acessory crystals. For the precise determination of such mall masses we have to keep the volume of the solution relatively smal to achieve high signall and we have to apply sotope dilution method. In spite of these efforts the U & Th measurements are typically biased the He age by more uncertanity than the detection of helium.

Apatite: After degassing the Pt capsule is gently opened under a binocular microscope and immersed in 2% nitric acid (+ 0.05% HF) that is spiked with known amount of 233U and 230Th. The volume of solution is usually 0.6 to 1.5 mL. The teflon vial is placed in an ultrasonic bath for 10 to 15 minutes and the solution is measured after an overnight equilibration time, always in the following day.

Opening of Pt capsule

Zircon: The degassed crystals carefully removed from the Pt tube or from the Nb foil and transferred in a teflon vial. The vials are spiked by 233U and 230Th and this spike solution is dried carefully in a clean fume-cupboard. The vials filled with ultrapure, distilled 48% HF - 65% HNO3 mixture. The individual vials are closed in a bigger teflon 'mother' bomb that contains also HF-HNO3 mixture for pressure compensation. The zircons are bombed for 6 days at 220 °C. After dissolution the acid mixture is evaporated and the fluoride salts are converted to nitrate by gently heating by 500 µL cc. HNO3. The dried nitrate salt is stored until the ICP measurement and ca. 24 hours before the analysis dissolved by 2% nitric acid.

Different types of teflon vials and mother bombs used for zircon digestion..

Spiked solutions were analyzed by a Perkin Elmer Elan DRC II ICP-MS mass spectrometer with a APEX micro-flow nebulizer. U and Th concentrations were determined by isotope dilution, while Sm by external calibration. Matrix elements (Ca, Zr, Ti and Fe) and platinum are also determined. The oxide formation rate is monitored by detecting 238U16O and 232Th16O masses.

Table shows the sequence of ICP-MS measurements of solutions.

The registration of the signal is performed always in time-resolved mode. We prefer this, slightly more labour-intensive method in order to control properply the processes in the icp.

 
Time-resolved record of analytes during a drift-solution and an apatite sample. Off-line calculations of isotope ratios, drift correction, outlier and other tests done by PepiAGE.

The uranium and thorium oxide formation ratio is usually very low, below 1%. The dissolved amount of Pt (from the capsules) creates insignificant PtAr isobaric interference.

Acknowledgement: At the construction and during the running of the He-labor Volker Karius, Klaus Simon and Reini Wolff have contributed significantly to the development of the chemical protocol, at the preparation of spike solutions and at the icp-ms measurements.