Introduction to Sample Preparation: Homogeneity

Sample preparation is the process where a representative piece of material, chemical or substance is extracted from a larger amount, bulk or batch for subsequent analysis. Representative samples are selected to accurately reflect the larger group and represent the characteristics of the whole material. Ideally representative samples are homogeneous or similar in nature, but when that is not possible, the best attempts must be made to achieve samples which represent the majority of the characteristics of the larger grouping.

The preparation of samples is one of the most important steps in analytical methods for many reasons, including the fact that some materials cannot be analyzed in an in-situ condition (such as proteins, DNA and RNA). Some samples have interfering substances and species that can produce faulty results. Sample preparation can include many processes, from reactions or treatment with chemical agents, to filtration, dilution, and extraction.

Many physical samples need particle size reduction to create representative samples, usually due to their overall heterogeneous nature or state when laboratory or testing samples require a certain level of homogeneity. Homogeneity is the state of being of uniform composition or character, whereas heterogeneity lacks uniformity in one or more characteristics. Homogeneity and heterogeneity often depend on perspective and context where the smaller the sampling frame, the less homogeneous a material or substance can appear. For some samples, the measure of homogeneity can be accomplished with a process that creates large size reduction (crushing) where other samples for other processes will require reduction to fine particles (milling). Smaller particles and homogeneous materials are most often needed for many further laboratory sample preparation techniques.

Laboratory or analytical samples must be processed into a form which allows for extraction or digestion ultimately for an analytical instrument or chemical testing. Sample processing involves reducing the material size to ensure samples for homogeneity and extraction into a suitable matrix for analysis. The most common method for obtaining a homogeneous sample is grinding or comminution. Grinding samples allows for a reduced sample size in order to increase accuracy and decrease uncertainty. In a study by Thiex et al., it was shown that the smaller the particle size, the less sample was needed to achieve a lower amount of uncertainty in a sample (Table 1).

Table 1. Effect of particle size on amount of material, in grams, needed to ensure various uncertainty levels for representative samples.