The most common method for obtaining a homogeneous sample is grinding comminution.
When performing sample prep, you may need to reduce the size. If you don’t have the right equipment, it may be difficult to obtain what you need. The most common method for obtaining a homogeneous sample is grinding of comminution. Grinding samples has many benefits for sample preparation since it increases homogeneity, increases surface area and decreases particle size which can improve extraction efficiency. Some of the negatives regarding grinding samples are: potential contamination, increase in moisture, evaporation, loss or alteration of volatile compounds or labile constituents, and safety issues regarding grinding.
Sample size reduction
Sample size reduction is accomplished by either crushing or grinding using forces of impact, attrition, shearing, or compression (Figure 1).
• Impact Force: the striking of one object or material against another. One object may either be stationary or both may be in motion.
• Attrition Force: created by materials rubbing against each other usually in opposite directions or planes.
• Shearing force: cleaving or cutting of a material by some cutting implement or blade.
• Compression force: slow application of a force against a solid to crush it into smaller pieces usually between two solid surfaces.
Particle reduction of solids occurs in multiple stages starting with the accumulation of defects or stresses in a concentrated location increasing the strain on a solid or particle. The stress forms microcracks and in crystal lattices, it will disrupt the crystal lattice in several cells or locations. The microcracks then join to form a larger major disruption or crack which ultimately divides the solid into pieces (Figure 2).
Types of grinding applications
Different applications, quantity of throughput and final end products often designate the method employed to grind materials for your sample prep (Figure 3). Crushers are commonly shearing or compression disruptors and are used to create larger particles in the 50 to 100 mm range. These particles are often just a primary step in some processing schemes. Most crushers are able to either process a large continuous stream of materials or larger batches. Grinders often produce smaller particles in smaller scales. There are many types of grinders based on the method of grinding and the force used to grind materials.
Different applications, quantity of throughput and size of final products often designate the method employed to grind materials from largescale crushers to finer impact mills. Table 1 gives the relative reduction of particle size from original material and the equipment needed.
Table 1. Particle size and equipment needed.
Learn more about milling and grinding.