Spectrometer sample preparation with fusion
If calibration curves with unacceptable SR appear even when using a reference from the sample, the main reason for this will be difference forms of bond.
Precision is often only slightly improved but accuracy is considerably improved by fusion. Fusion causes dilution of the raw sample. Calibration must be carried out with concentrations of the elements in the raw sample. The form of bond is often not known, so that a procedure is often followed which assumes that the analyte concentrations, those converted to the oxide are plotted on a co-ordinate.
The buffer is not a mixture of the fusion agent with one or more references but is present in molten state, for the following reasons:
a) Water-free borate or carbonate is hygroscopic and cannot be easily stored.
b) The reference oxide can be added more accurately in larger quantities.
c) Weighing of fusion agent and reference oxides is eliminated in routine operation.
d)Samples mixed with buffer wet the crucible only slightly after fusion. The melt is poured out without leaving a residue. Surface tension is increased by pre-melting.
e)The fusion times are shorter
f) The buffer can be weighted and mixed easily because it does not stick.
Buffer production: (if it is not to be purchased but produced in-plant).
Mix the fusion agent and reference oxides for some min in the mixer. Place the mixture in a graphite crucible and melt it for 30 min in a muffle furnace at 1000 ℃. Pour the melt into a Cu mould. Close the filled mould with the mould cover. The inside of the mould is similar to a waffle iron, so that the solidified melt breaks easily into small pieces. These are stored in sealable glass bottles from which the quantities required each day are removed and crushed for a few s in the swing mill before starting work.
The fusion agents used are lithium-tetraborate, sodium-tetraborate or carbonate. Sodium-tetraborate is better than lithium-tetraborate. It is available in purer form; it is cheaper, needs shorter fusion times and has less tendency to crystal formation; fusions are always as clear as glass.
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