In 1873 in his Elements de Toxicologie, Chapuis described a system of classifying toxic agents by their origin or nature into several categories: gases, volatile substances, corrosive agents, metals, anions and nonmetals, nonvolatile organic substances, and miscellaneous. Closely related to this descriptive classification is the method for separating a toxic agent from the matrix in which it is embedded. The agent of interest may exist in a matrix of a simple solution or may be bound to protein and other cellular constituents; before analysis can be done, the agent must be isolated.