Open University Uranium-Series Laboratory
Earth and Environmental Sciences, The Open University, Milton Keynes, UK

Instruments
OUUSL operates the following analytical equipment:
Finnigan MAT262-RPQ-II TIMS, recently refurbished by Spectromat with new solid-state power supplies, focussing system, magnet control system, parallel processor, fast interface and software.
Nu Instruments Multi-Collector Plasma Ionisation Mass Spectrometer MC-ICPMS with triple ion counting and ‘RPQ’, jointly development between Open University and Oxford University Earth Sciences Departments, and Nu Instruments
A γ spectrometry system consisting of an ORTEC GEM detector with DSPEC digital signal processor, mechanical cooling and GammaVision software.
NewWave 193nm excimer laser ablation system.

The MAT262-RPQ-II and the Gamma Spectrometer are dedicated to U-series, the Nu Instruments MC-ICPMS and the NewWave laser ablation system are operated by OUUSF but also used for other research.

The current generation of solid source mass spectrometers (TIMS) have ion detection capability close to the theoretical limits of analytical sensitivity, and measurement uncertainty is therefore determined primarily by counting statistics (van Calsteren and Schwieters, 1995). For elements such as Ra with relatively high ionisation efficiencies, major improvements are not foreseen. Th and U have much lower thermal ionisation efficiencies of ~1 ion in 10,000 atoms.

The MC-ICPMS is a double-focussing 'forward geometry' mass spectrometers with a deceleration stage (RPQ) for high abundance sensitivity to eliminate the tail of a high intensity peak on adjacent low intensity peaks.

The Nu Instruments has variable dispersion ion optics to focus the ion beams into a fixed collector array with 12 Faraday cups and three 16-stage discrete-dynode Secondary Electron Multipliers.
Sensitivity >200V/ppm is routine with the DSN-100 desolvating nebuliser
Analysis strategy
TIMS and MC-ICPMS both have strong points and disadvantages. MC-ICPMS has better ionisation efficiency and is therefore more sensitive than TIMS and the preferred option for small, young, low uranium samples, but mass-dependent isotope fractionation and instrument drift are both an order of magnitude larger than with TIMS over the whole concentration range. TIMS remains the preferred instrument where concentration and availability are not limiting factors and where high precision (for high age resolution) is a requirement.
TIMS is also essential for the high-precision characterisation of MC-ICPMS standards that are used in the standard-sample-standard analysis strategy to correct for instrument fractionation and drift. This way we also ensure direct compatibility between the data from both instruments.
Fractionation and drift
For MC-ICPMS sample utilisation, a parameter similar to ionisation efficiency in TIMS, can be as high as 1 ion detected per 100 atoms in a sample when using desolvation apparatus. However, the disadvantage is that masspdependant isotope fractionation in plasma ionisation is an order of magnitude larger than for TIMS but it is fairly constant to within a few ‰ in an analysis period. Mass-dependant isotope fractionation can be minimised by careful tuning of the instrument using the same standard solutions, using an automated cleaning procedure between samples and regular maintenance of the plasma interface. Mass-dependant isotope fractionation can be corrected in various ways: using an internal invariant isotope ratio, by the addition of a spike of known isotope ratio, or by bracketing sample analysis with standard analysis.
 
Th and Ra do not have internal invariant isotope ratios and the 235U/238U ratio is rather larger than optimal. Addition of a mixed 233U/236U spike such as IRMM3636 is possible but the amount of radioactive material that has to be added to the sample to significantly improve precision is too large to be practical for routine use. We have opted for the bracketing approach where a standard of comparable composition and concentration is analysed between samples. The standards that are interspersed with the samples are characterised using TIMS and results from the two instruments are fully compatible. Use of the 233U/236U mixed spike is impossible for samples that are also analysed for 231Pa because we use a 233Pa spike and 233U is indistinguishable from 233Pa by mass spectrometric analysis.
 
Laser ablation
Extensive testing of our UV excimer laser ablation system hyphenated to the Nu Instruments MC-ICPMS has been carried out with artificial standards. The standards are comparatively rich in U and Th and data analysis has shown additional isotope fractionation compared with Aridus sample introduction. It is likely that the Laser-Induced Plasma at the ablation site causes additional fractionation which decreases precision. For in situ measurement of (234U/238U) and (230Th/232Th) it is essential to precisely correct for all fractionation effects resulting in unacceptably large uncertainties for samples with low U concentrations such as most speleothems. However, laser ablation can be used to determine ages on coral because unaltered marine coral contains 3.3ppm U (Potter et al., 2005).
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© Peter van Calsteren
Last updated: 23 December, 2011 11:31