Spectrometer Sample preparation without fusion
If the sample is electrically conductive and fine-grained it can be pressed into a briquette. If it is not conductive, it is mixed with conductive powder and pressed. The analysis sample is then ready.
If the sample does not have a fine particle size, a weighed quantity is ground to particle size ≤20μm. The ground material is mixed with graphite eg. with a weighed quantity in a second container in the swing mill.
At the same time a reference oxide can be added. The second grinding process is then also a mixing process. Depending on the job, the sample mixing ratio is 1:2 to 1:10. To prevent the powder from sticking on the wall of the container a few drops of liquids with a low vaporization point eg. alcohols, can be added. These vaporize when struck by the grinders and tear the powder away. An admixture of reference oxides dose not usually lead to a usually lead to a sufficiently homogeneous distribution, particularly if there is a large difference between the densities of the constituents to be mixed. This problem is done away with, if the reference is a constituent of the sample.
Cheap grinding vessels often give rise to Fe or Cr abrasion products; expensive vessels in Mo or W carbide are then necessary. The ground mixture is then transferred, using a small brush, from the grinding vessel via a glass funnel into a press die. Containers and funnels with surface charges are useless since analysis powder will stick to them and lead to contamination. The die ? should be 20-30 mm to accommodate 3-4 burn spots. Pressures of 30-50t, and press times of 10-30s are normally used.
To facilitate removal of the briquette, paper should be placed at the top and bottom of the die. Paper prevents the ram from sticking, and the briquette does not shatter when it is ejected. The sample is ready when the paper has been peeled off and grinding is not necessary.
When using a tape machine or cup electrode in the powder method, molten fusion agent including a reference is added to the sample after it has been ground to <20μm and sieved, and it is mixed in a swing mill. Molten fusion agent plus reference will hereafter be referred to as buffer. The buffer has two functions:
to reduce inter-element effects,
to dilute the sample down to concentration at which the calibration curves are linear ie.
to reduce self-absorption.
The fusion agent contains elements with low ionization energy so that the plasma temperature is mainly determined by these elements and less by the composition of the sample. The fusion agent acts as a spectrochemical buffer. Depending on the job, the sample mixing ratio 1:5 to 1:20.
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