Which fluxes should be used for (XRF/ICP) analyses?

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.

What is flux?

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:

  • lithium tetraborate (Li2B4O7)
  • metraborate (LiBO2)
  • lithium bromide (LiBr)
  • lithium iodide (Lil)

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).

See also the mixture of ICP fluxes here and XRF fluxes.

ICPH Fluore X

Why are fluxes used?

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 :

  • are solvents for a wide range of sample compositions
  • facilitate rapid dissolution of the sample
  • have a low melting point, minimizing volatilization
  • reduce power costs
  • help to extend crucible life
  • have low viscosity which aids mixing
  • are non-hygroscopic which aids weighing
  • have high density particle size that contributes to homogeneous mixing, rapid melting and longevity of platinum crucibles

How to select the right flux for your test?

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:

CementFluore X65 (LT66%/LM34%)
CeramicsFluore X50 (LT50%/LM50% or Fluore X35 (LT35.3%/LM64.7%)
CatalystsFluore X65 (LT66%/LM34%) or Fluore X35 (LT35.3%/LM64.7%)
LimestoneFluore X100 (LT 100%) or Fluore X65 (LT66%/LM34%)
Iron oresFluore X65 (LT66%/LM34%)
AluminaFluore X35 (LT35.3%/LM64.7%)

There are some other flux factors to consider for both XRF and ICP analysis:

  • The melting point of the flux should be high enough for complete dissolution of the sample, yet not too high otherwise it will cause volatilization. This is particularly important for samples containing sodium (Na), potassium (K) or sulfur (S).
  • The purity of the flux is essential for reducing sample contamination that can cause interference during analysis. This is particularly important in ICP if trace level accuracy is required.
  • Hygroscopy (the level of moisture retention) in the flux affects weighing accuracy as well as the fusion process. A flux needs to be as dry as possible.
  • Additives can be included in a flux compound to improve specific flux functions. For example:
    • Added oxidants ensure full oxidation of compounds during fusion. This reduces damage to platinum labware and helps with good bead formation.
    • Adding a releasing agent (non-wetting agent) helps prevent particular sample types from sticking to the mold.
  • The surface area varies when fluxes are manufactured using different processes into a beaded, granular or powdered product. Surface area variations can influence hygroscopy, weighing accuracy and melting times. As a general rule:
    • a pre-fused flux with low dust and controlled particle size will provide optimum results for borate fusion
    • the flux must have a physical structure similar to the sample to ensure good mixing

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.