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The Cause of Sampling Errors

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Published Date Written by Sharon Banks

Be sure a representative analytical sample is analysed by the laboratory.
Sampling errors arise both in the initial sample taking (the owner) and by reducing this bulk to a small representative sub-sample (the laboratory analyst) for analysis.
The taking of the original sample is usually not under the analyst’s control, but taking of a representative sub-sample certainly is, and it is important not to introduce unnecessary errors at this point.

In many geological samples and other materials, certain elements (eg P; Cr; Sn or Au) may be present in only a single mineral grain within the sample. For example, if a sample contains 0.10% P2O5 which is present entirely as the mineral apatite Ca3(PO4)2, then the total amount of Ca3(PO4)2 in the sample is only 0.22%. So it is important that the sampling technique does not result in loss of this mineral.
In general the finer the particle size of the sample the smaller the sampling errors will be for a given mass of sub-sample and given component concentration.
UIS Analytical Services is equipped with a jaw crusher and a number of vibratory feed rotary splitters to ensure representative sub-sampling. These sample dividers are fitted with a hopper to accommodate a wide range of sample volumes.
It is important that progressive reduction in particle size precede any reduction in sample weight before further processing. Under no circumstances is the analytical sample sieved to separate coarse from fine particles caused by differential grinding – this is dangerous as the sample composition would in all likelihood be altered by such a procedure.
The single pot mills utilized at UIS Analytical Services are equipped with variable speed motors (rpm). This allows us to optimize the milling parameters for varying sample matter, as well as for different types of milling vessels (tungsten carbide, chrome steel, corundum, zirconia etc).
Be sure that the laboratory performing your analysis is equipped to perform correct sampling and sample preparation procedures, as sampling errors, are probably among the most neglected aspects of any analytical technique.

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