fine action limits and warning limits at appropriate distances fiom the true value. 6.2 The role of certified reference materials Certified reference materials (CRM) as defined in Section 2 (i.e., with a statement of uncertainty and traceability), when available and of suitable composition, are ideal control materials in that they can be regarded for traceability purposes as ultimate standards of trueness(2O). In the past CRMs were regarded as being for reference purposes only and not for routine use. A more modem approach is to treat CRMs as consumable and therefore suitable for IQC. The use of CRMs in this way is, however, subject to a number of constraints. 0 1995 IUPAC, Pure and Applied Chemistry, 67,4
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