# Laboratory test sample representativity: an easily neglected aspect in consignment and its economic impact

#### Li Huachang and Zhu Mingwei

BGRIMM MTC Technology Co. Ltd, China

An unrepresentative sample is unable to reflect the true quality of materials and goods, and will eventually cause laboratory chemical analysis results that cannot substantiate trade settlements between buyers and sellers. In quality inspection of mineral products and metals, sampling errors account for ~80 % of the total error, with sample preparation errors responsible for ~15 % and analytical errors accounting for only 5 %. If sampling is poorly represented, no matter how accurate the sample preparation and chemical analysis, quality grading can be severely compromised. Ignoring primary sampling, there are still significant representativity problems arising from sample preparation causing all parties difficulty when trying to find an answer to the crucial question: “where did the money go?”. Thus, at the second and third stage sampling levels also, huge economic losses can occur for the buyer or the seller. Three cases from the international copper industry sector are presented.

## Example 1: Significant trade settlement impact from sampling and grading of high purity copper cathode material

Copper cathode material is usually divided into three categories according to the content of impurities. Impurity element concentrations of Class A cathode copper shall not exceed 0.0065 % in total, while lead (Pb) must not exceed 0.0005 % and iron (Fe) 0.0010 %. Reliable control of misrepresentation of samples used for laboratory testing has a great impact on quality grading and pricing of high pure cathode copper.

Africa is rich in non-ferrous mineral resources, especially copper mineral reserves.

Many Chinese companies have started operating in Africa, building mining plants, concentrating mills and smelters, and eventually smelting and producing copper cathode material and selling it globally. Copper cathode material is usually produced and traded directly in the original size format of 80 × 80 cm square plates with a thickness of about 1 cm, which weighs ~200 kg per piece. To ease transportation, copper cathode plates are usually strapped together using high strength steel bands into bundles suitable for loading weights of typically 1–2 tons each.

If the Class A copper cathode sampling process is contaminated by strap steel bands, as shown in Figure 1, it will lead to an excessive iron content, resulting in a grade reduction of the copper cathode products. Each quality grade class reduction results in a price reduction of ~$30 per ton. For a smelter with an annual output of 200,000 tons of copper cathode with, say, 10 % of samples contaminated, the annual output value is reduced up to$600,000, calculated as follows:

## Conclusions

These consignment examples demonstrate the economic importance of even the smallest differences in laboratory preparation and analysis approaches. Sub-sampling, sample preparation, transportation and sample storage processes may all have significant effects on the quality and representativity of samples that eventually enter the analytical instruments. Only by careful and strict control of each operation can the test samples ultimately used for analysis be qualified as representing the full, comprehensive quality of commodities and the goods—equally in the interest of both buyers and sellers. The apparent minute issues treated here for a large-volume bulk commodity, may quickly lead to surprisingly large, added or lost, values which are far too large to overlook in consignment economics.

## Between the laboratory and management

The reader is referred to two earlier Sampling Columns dealing with how to run a commercial analytical laboratory notably with, or without, the TOS on the agenda: Does management have the necessary foresight to accept the challenge of also caring for “the customers of the customer of the laboratory”? This is an exciting two-part story, both of which are just a click away.

K.H. Esbensen, “A tale of two laboratories I: the challenge”, Spectrosc. Europe 30(5), 23–28 (2018). https://doi.org/10.1255/sew.2018.a3

K.H. Esbensen, “A tale of two laboratories II: resolution”, Spectrosc. Europe 30(6), 26–28 (2018). https://doi.org/10.1255/sew.2018.a4