Analytical Chem istry - DePauw University

308 Analytical Chemistry 2.0size and hardness. Large particles are crushed using jaw crushers capable ofreducing particles to diameters of a few millimeters. Ball mills, disk mills,and mortars and pestles are used to further reduce particle size.A significant change in the gross sample’s composition may occur duringcrushing and grinding. Decreasing particle size increases the availablesurface area, which increases the risk of losing volatile components. Thisproblem is made worse by the frictional heat that accompanies crushingand grinding. Increasing the surface area also exposes interior portions ofthe sample to the atmosphere where oxidation may alter the gross sample’scomposition. Other problems include contamination from the materialsused to crush and grind the sample, and differences in the ease with whichparticles are reduced in size. For example, softer particles are easier to reducein size and may be lost as dust before the remaining sample is processed.This is a particular problem if the analyte’s distribution between differenttypes of particles is not uniform.The gross sample is reduced to a uniform particle size by intermittentlypassing it through a sieve. Those particles not passing through the sievereceive additional processing until the entire sample is of uniform size. Theresulting material is mixed thoroughly to ensure homogeneity and a subsampleobtained with a riffle, or by coning and quartering. As shownin Figure 7.11, the gross sample is piled into a cone, flattened, and dividedinto four quarters. After discarding two diagonally opposed quarters, theremaining material is cycled through the process of coning and quarteringuntil a suitable laboratory sample remains.gather material into a coneflatten the conelaboratory samplenewcyclediscarddivide into quartersdivide in halfFigure 7.11 Illustration showing the method of coning and quartering for reducing sample size. After gatheringthe gross sample into a cone, the cone is flattened, divided in half, and then divided into quarters. Two opposingquarters are combined to form the laboratory sample, or the subsample is sent back through another cycle. Thetwo remaining quarters are discarded.