impurity

An impurity is a substance that is present in a material or system in a concentration different from the principal constituent and is not part of the intended composition. Impurities may be introduced during production, processing, storage, or exposure to contaminants; they can be chemical, physical, or biological.

In metals and minerals, impurities affect properties such as hardness, ductility, electrical conductivity, corrosion resistance, and melting point. Common examples include sulfur and phosphorus in iron, which reduce ductility and weldability; oxygen in steel that forms oxides and can embrittle; and carbon in certain alloys where trace amounts influence structure.

In semiconductors, certain impurities are added deliberately as dopants to modulate electrical conductivity; otherwise impurities are undesired. Donor and acceptor dopants such as phosphorus or boron are used to control charge carriers and device performance.

In pharmaceuticals and chemicals, impurities include residual solvents, process-related impurities, and degradation products; regulatory standards limit impurity levels in drugs and require validated analytical methods to detect them. Impurity control is a core aspect of quality by design and risk management in production.

Analytical methods for impurities include chromatography, mass spectrometry, spectroscopy, and thermal analysis. Purification techniques to remove or reduce impurities include distillation, crystallization, filtration, washing, and chemical treatment, as well as advanced purification steps in manufacturing. The presence and level of impurities shape product quality, safety, and performance across industries.