Certified Reference Materials (CRMs) are ‘controls’ or standards used to check the quality and traceability of products. A reference standard for a unit of measurement is an artifact that embodies the quantity of interest in a way that ties its value to the reference base. At the highest level, a primary reference standard is assigned a value by direct comparison with the reference base. For example, mass is defined by an artifact maintained by the Bureau International des Poids et Mesures in Sevres, France. A primary standard is usually under jurisdiction of a national standards body.