XRF Laboratory Tips – Sample Preparation
Many laboratories spend a considerable amount of time and money setting up a XRF laboratory but sometimes fail to realise the full benefits. This is often due to poor sample preparation maintenance practices. Sample preparation equipment tends to be purchased, put into service and then forgotten about. Somehow it is expected to run forever. It is often not until a mill stops running, fused beads shatter repeatedly or the QC samples go out of control that action is taken. I can not emphasise too strongly that the results obtained from an XRF spectrometer are only as good as the sample presented to it. In this article I would like to look at a number steps in the sample preparation process and suggest some simple maintenance actions that will greatly assist the quality of results.
Primary Sample Crushing and SplittingThe greatest potential threat to analysis quality at this stage of the operation is sample contamination. This is especially so in automated grinding systems. A strict protocol needs to be in place to ensure that mills and sample dividers are cleaned between samples. For automated systems a maintenance regime that includes ensuring that drains are not blocked, cleaning reservoirs are full and that the cleaning cycle operates correctly are required. Mills and grinding elements should be replaced or refurbished if they become badly pitted (i.e. can retain traces of the previous sample).
XRF Pressed Powder PelletsTo obtain good XRF results using the pressed powder technique, control of particle size is absolutely critical. Particle size can be controlled by closely controlling grinding time and the mass of sample introduced to the mill. Initially grinding times are determined by experimentally constructing grinding curves. These are plots of XRF intensity versus grinding time. XRF intensity initially rises sharply with grinding time and then tends to plateau. The chosen grinding time should be in the “plateau” region. Grinding time should be set by means of a timer connected to the mill power supply. Control of mass can often be achieved by using scoops of a suitable volume.
In production or when rapid analysis results are needed there is often a temptation to shorten the grinding time. Operators need to be given an understanding of why grinding times must not be altered.
A similar set of operational and maintenance protocols are required as for Primary Crushing to minimise the risk of sample contamination.
XRF FusionsXRF bead quality is critical for good XRF results and yet many operators will tolerate poor quality (chipped, scratched, not flat) or cracked beads. Factors that effect bead quality include:
Ken Turner
Ken Turner Consulting
E-Mail: [email protected]
Primary Sample Crushing and SplittingThe greatest potential threat to analysis quality at this stage of the operation is sample contamination. This is especially so in automated grinding systems. A strict protocol needs to be in place to ensure that mills and sample dividers are cleaned between samples. For automated systems a maintenance regime that includes ensuring that drains are not blocked, cleaning reservoirs are full and that the cleaning cycle operates correctly are required. Mills and grinding elements should be replaced or refurbished if they become badly pitted (i.e. can retain traces of the previous sample).
XRF Pressed Powder PelletsTo obtain good XRF results using the pressed powder technique, control of particle size is absolutely critical. Particle size can be controlled by closely controlling grinding time and the mass of sample introduced to the mill. Initially grinding times are determined by experimentally constructing grinding curves. These are plots of XRF intensity versus grinding time. XRF intensity initially rises sharply with grinding time and then tends to plateau. The chosen grinding time should be in the “plateau” region. Grinding time should be set by means of a timer connected to the mill power supply. Control of mass can often be achieved by using scoops of a suitable volume.
In production or when rapid analysis results are needed there is often a temptation to shorten the grinding time. Operators need to be given an understanding of why grinding times must not be altered.
A similar set of operational and maintenance protocols are required as for Primary Crushing to minimise the risk of sample contamination.
XRF FusionsXRF bead quality is critical for good XRF results and yet many operators will tolerate poor quality (chipped, scratched, not flat) or cracked beads. Factors that effect bead quality include:
- Fusion temperature
- Mould temperature
- Quality of the platinum ware surface
- Mixing during fusion
- Cooling rate
- Sample flux ratio
- Flux composition
- Sample composition
- Use of a release agent
- Fusion and mould temperatures can be checked with a thermocouple, optical pyrometer (especially useful with gas fusion equipment) or observing the melting points of specific compounds.
- The mixing action of automated systems should be regularly checked for correct operation.
- Refractory furnace linings should be checked to ensure that they are not spalling. Potentially particles of lining could contaminate the fusion sample.
- Gas burners should be visually inspected to ensure that jets are not blocked.
- On gas systems, gas pressures and flow rates should be maintained within the correct operating range.
- Where forced air cooling of the beads after fusion is used, flow rates must be maintained within the correct operating range.
- Platinum ware, especially moulds, should be in good condition. Crucibles and moulds should be free from pits and scratches, moulds should be flat and should have a high polish. Moulds should be re-polished regularly and crucibles keep in good shape. Moulds that is in poor condition will invariable cause XRF beads to stick to the mould and/or crack. Commercial crucible reshaping tools and polishing equipment is available.
Ken Turner
Ken Turner Consulting
E-Mail: [email protected]