TM 07 X-Ray Fluorescence

This instrumental technique allows for relatively quick and accurate chemical analysis of elements for sodium and above in the periodic table.

Methods for testing elements which are difficult to carry out by XRF:-

·         Boron compounds can be tested using inductively coupled Plasma (ICP) techniques.

·         Sulphur can now be determined by XRF; however, it is usually analysed by using an induction furnace and measuring the amount of gas given off using infra red techniques.

·         Carbon can not be determined by XRF and is usually analysed by using an induction furnace and measuring the amount of gas given off using infra red techniques.

·         Lithium compounds can be analysed by atomic absorption (AA) or inductively coupled Plasma (ICP).

·         Fluorine is now possible by XRF using instruments special developed for the lighter elements, however it is usually analysed by a ion selective electrode technique. Initially a liquor for analysis is collected by pyrohydrolysis. Steam being passed over a  heated ( ~ 920^oC)  mixture of sample and a catalyst (such as vanadium pentoxide).

·         Selenium compounds can be analysed by atomic absorption (AA) or inductively coupled Plasma (ICP).

Sample preparation techniques for XRF analysis.  

The method of sample preparation depends on the type of analysis required or the type of XRF instrument used

Semi-Quantitative analysis on unknown materials can be carried out by a simple scan of the sample in a powder, liquid or cut specimen. Used predominantly on Energy Dispersive Instruments. 

Quantitative  analysis 

Pressed Disk analysis by grinding the sample to less than 53 micron and pressing the sample into a tablet or cup usually with some inert binder. Generally a loss on ignition at 1020^oC is carried initially and the disc prepared from the ashed sample. Used on XRF instruments that have Wave Length and or  Energy Dispersive capabilities.

Fused Bead analysis. The sample is ground and then mixed with fusion mixture ( A Lithium Borate compound). The fused bead is formed after heating in platinum/gold crucibles and pouring on to a platinum/gold mould. Once cooled the fused bead is analysed on the XRF machine.

XRF Analysis 

There are two types of XRF spectrometers:simultaneous  and sequential 

Simultaneous are simple to use. Once the spectrometer has been set up and calibrations carried out, samples can be easily processed. Each channel has its own separate crystal,collimator and detector. This restricts the analysis  to only those elements for which there are fixed channels.