|
The accuracy and precision of data that is reported by an XRF laboratory reflects care taken by the analyst to ensure that appropriate techniques are adopted to prepare the sample and calibrate the spectrometer. Many laboratories have systems in place to compensate for short and long-term drift in the spectrometer with analysts following quality procedures for calibration maintenance. However, the maintenance of sample preparation equipment must not be overlooked and where fusion techniques are adopted, the care of platinum ware is vital. Good analytical practice should minimise cross contamination between samples but in a busy laboratory fusing a wide range of powdered materials, careful handling and cleaning regimes will reduce errors and extend the life-time of the platinum ware.
Platinum ware for XRF analysis
The advantages of presenting a sample for analysis in the form of a fused bead are well documented. However, it is perhaps important to highlight situations that are harmful to the platinum / 5% gold alloy
There are some key precautions to be taken when handling platinum ware
- Handle ony with platinum tipped tongs
- Avoid prolonged unnecessary heating
- Fuse only under oxidising conditions
- Ensure that metals such as Cr, Fe, Ni, Cu, Zn & Pb do not come into direct contact with the platinum ware
- Fine metal powders should be pre-oxidised before fusion
- Minimise cross contamination using a thorough cleaning regime for all platinum ware
Cleaning platinum ware
A crucible or casting dish should never be hit or tapped with any implement to remove residual fused sample stuck to the surface. Any residual material should be removed by one of the following methods:
- Soak in 10% solution of citric acid at 50ºC in an ultrasonic bath
- To shorten the cleaning cycle, use 20% w/v citric acid in a beaker at 80ºC
- For a more aggressive method use 10% HCl
- If all else fails, boil in Cl free HNO3
- After acid cleaning, wash thoroughly with clean water and dry
WARNING: Never us Aqua Regia.
Polishing platinum ware
Spectroscopists using semi-quantitative programmes for the analysis of fused beads will be familiar with the presence of trace levels of both Pt & Au in a reporting list. Gold lost from the surface will reduce the non-wetting properties of the ware. The sheen from the surface of new platinum ware will have been replaced by a dull appearance. The crucibles will be more difficult to clean and deformations in the casting dish may cause difficulty in releasing the bead and will increase errors in the reported result.
The user can carry out some degree of maintenance on the platinum ware. With a suitable die and hydraulic press, casting dishes can be re-pressed to keep them perfectly flat. Formers are available from some platinum manufacturers or customised moulds can be ordered through analysco.
The most effective form of maintenance however is polishing. A diamond paste of the correct grade can be used to polish the surface of the platinum alloy. This will remove a small amount of the surface to get back to the original alloy. It will also have the effect of smoothing out any minor scratches or imperfections on the surface. For more serious scratches and surface damage, a coarse diamond paste can be used, prior to final polishing with a fine paste
The life of platinum ware can be considerably extended by careful use and polishing but it should be noted that polishing will reduce the weight of the platinum, subsequently reducing the amount of credit on replacement. In time, the platinum will crystallise and crack to which the only solution is replacement. Polishing should primarily be considered as a means of maintaining the quality of results and not as a means of artificially extending the life of the alloy.
|
