Sampling and physical state

from this test it has been shown that at least 41 samples are required if the specifications given in the above example are to be satisfied. 

The other major problem concerned with sampling is that of the sample size. The size of the sample taken from a heterogeneous material is determined by the variation in particle size, and the precision needed in the results of the analysis. 

Many of the problems occurring during sampling arise from the physical nature of the materials to be studied.9 Although gases and liquids can, and do, present difficulties, the greatest problems of adequate sampling undoubtedly arise with solids. 

Few problems arise over homogeneity of gas mixtures where the storage vessel is not subjected to temperature or pressure variations. Difficulties may arise if precautions are not taken to clear valves, taps and connecting lines of any other gas prior to passage of the sample. Similarly care must be taken that no gaseous components will react with the sampling and analytical devices. 

Solids. It is with solids that real difficulties over homogeneity arise. Even materials that superficially have every appearance of being homogeneous in fact may have localised concentrations of impurities and vary in composition. The procedure adopted to obtain as representative a sample as possible will depend greatly upon the type of solid. This process is of great importance since, if it is not satisfactorily done, the labour and time spent in making a careful analysis 

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