In order to accurately assess the elemental composition of solid samples, x-ray fluorescence (XRF), inductively coupled plasma (ICP) or atomic absorption spectroscopy (AA) analysis is used. Borate fusion is used to prepare the samples for these processes. Fusion involves dissolving an oxidized sample in a molten flux. The mixtures need to be either near-homogeneous or high-purity heterogenous mixtures to ensure good results. The composition of the flux is critical to ensure optimal dissolution and thus accurate analysis.
A flux is a chemical compound used in borate fusion. The technique involves mixing a sample with a borate flux which usually consists of one or more of the following:
The mixture is put in a platinum alloy crucible and agitated as it is heated to 1000°C until the flux melts and the sample is dissolved. Depending on the chosen method of analysis the melt is either cast in a mold and cooled (XRF analysis) or poured into an acid (ICP analysis).
Analyzing samples that haven’t been prepared using flux fusion can result in significant inaccuracy. Unprepared solid samples can lead to surface variations that interfere with an XRF spectrometer’s calibration. Unprepared liquid samples can contain trace elements (contaminants) which affect the accuracy of ICP results. Sample preparation using fusion fluxers contributes to more precise XRF reactions and plasma ionization.
Fluxes are also on account of a number of important properties. They :
Choosing the correct flux for your sample requires consideration of the acidity and alkalinity equilibrium. You want to select a flux that will neutralize the acidity or alkalinity of the sample, such that the acidity index of the prepared sample is around 1.13 (neutral). A range of flux compounds are available from acidic to basic so that the correct compound match can be found. It is important to bear in mind that the sample may contain different oxides with varying acidities and this will also affect the choice of flux.
The two most commonly used flux compounds are: lithium tetraborate (LiT or Li₂B₄O₇) and lithium metaborate (LiM or LiBO₂). As a general rule, the tetraborate reacts with basic oxides while the metaborate reacts with acidic oxides.
The choice of flux is also determined by the method of analysis that will be used. LiM flux is often best suited in sample preparation for ICP analysis. This is due to its lower melting point and easy crystallization when in contact with an acid solution.
Here are some examples of samples and the associated fluxes typically used with regard to XRF analysis:
|Cement||Fluore X65 (LT66%/LM34%)|
|Ceramics||Fluore X50 (LT50%/LM50% or Fluore X35 (LT35.3%/LM64.7%)|
|Catalysts||Fluore X65 (LT66%/LM34%) or Fluore X35 (LT35.3%/LM64.7%)|
|Limestone||Fluore X100 (LT 100%) or Fluore X65 (LT66%/LM34%)|
|Iron ores||Fluore X65 (LT66%/LM34%)|
|Alumina||Fluore X35 (LT35.3%/LM64.7%)|
There are some other flux factors to consider for both XRF and ICP analysis:
Furter more all our fluxes are fully prefused with low dust and irregular shaped granules with controlled particle size. Due to the irregular shaped particles, the flux mixes homogeneously with the samples and there is no separation during the actual fusion and formation of the glass disc.
IAM Drijfhout supplies a range of high quality fluxes from XRF Scientific and ICPH and we can meet your requirements for all sample types and analysis methods. Click here for more detailed information on our range of fluxes. If you are unsure of which flux to choose, or have a complex sample, our experts will be happy to assist you. Furthermore, our manufacturers can assist in making custom flux compounds when needed.