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

Sample preparation
 

All samples need some form of preparation before U-series isotopes can be analysed. For most samples this means dissolution and chemical purification to eliminate interfering elements and matrix. Sometimes laser ablation of solid material, cut to a suitable size, is preferable.

In the laboratory we adhere to detailed procedures but those are beyond the scope of this Site.

 
Preparation
 

Rocks may be crushed and ground to a fine powder for Whole Rock analysis, or lightly crushed for mineral separation. Flat slabs of material can be sampled using tungsten-carbide drill bits. We can carry out sub-sampling but it is generally assumed that collaborators do their preparation work themselves, possibly using our equipment. The ultimate success of a project depends crucially on the quality of the sample and discussion of the sampling requirements can make all the difference. It is, of course, essential that sample are not contaminated during preparation and we are happy to discuss those pitfalls as well.

 
Dissolution and purification
 

Some materials, notably carbonates can be dissolved in weak mineral acids, sometimes mild heat and pressure are required and stong acids, including HF. For many silicates we use Monel/PTFE autoclaves or Microwave dissolution (see below).

U, Th, Pa and Ra are purified using ion exchange chromatography.

Samples for TIMS are dried onto Rhenium filaments and samples for MC-ICP-MS are taken up in 3% HNO3 for nebulisation.

 
CEM MARS-5 (Microwave Activated Digestion System)
 

The MARS-5 is a closed vessel microwave system made by CEM. The vessels we use (HP-500) can withstand temperatures up to 260°C and pressures of 500 PSI. This massively reduces the time needed to dissolve samples and we have also successfully dissolved 'difficult' samples, such as coals, which were not properly digested by conventional methods.

 
A New Wave UP193HE laser ablation system,
 

The New Wave UP193HE laser ablation system is based on a TuiLaser Excistar M-100 ArF 193 nm Excimer laser is coupled to the Nu Instruments plasma mass spectrometer.

This ablation system gives high spatial resolution for precise sampling of minute amaounts of sample. A single pulse of the 193 nm laser at 10 mJ can ablate a 50 micron diameter pit and the weight of the ablated sample is about 1.2 nanogram. A 2000 year-old sample of stalagtite carbonate with a uranium concentrations of 0.15 ppm contains about 140 atoms of 230Th in that amount. The efficiency of the mass spectrometer is such that about 1 ion of 230Th per pulse is detected. At 10Hz it takes about 16 minutes to collect 10.000 ions to give a 1% uncertainty in the measurement. The total mount of ablated material is then around 12 microgram.

 
   
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© Peter van Calsteren
Last updated: 23 December, 2011 11:35
 
December 23, 2011 11:35December 23, 2011 11:35