Metrological traceability is the property that links a measurement result to a stated reference through a documented unbroken chain of calibrations, each with a stated uncertainty. In practice that chain ends at one of the seven SI base units (kilogram, metre, second, ampere, kelvin, mole, candela) realised by a national metrology institute and inter-compared through the Bureau International des Poids et Mesures (BIPM) in Sèvres.
In India the national metrology institute is the National Physical Laboratory (NPL) at Pusa Road, New Delhi, a constituent unit of the CSIR. NPL maintains India's primary standards for mass, length, time, temperature, electrical units, photometry and select chemical reference materials, and Indian calibration labs traceably link their working standards to NPL through certified calibration certificates. The lab's analytical balance is calibrated against a working-standard mass that is calibrated against a secondary mass that traces to NPL's primary kilogram standard. The chromatograph's column-oven temperature traces through a calibrated platinum-resistance thermometer to NPL's temperature scale. The volumetric pipettes trace through gravimetric calibration to the same kilogram.
Reference materials carry the chemistry side of the chain. A reference material (RM) is any material with property values sufficiently homogeneous and well established to use for calibration or method validation. A certified reference material (CRM) is an RM with a certificate stating property values, their uncertainty and a traceability statement. NIST SRM 1577c bovine liver (certified for As, Pb, Hg, Cd and trace elements) is the canonical example. A primary CRM is measured by a fully described primary method (NIST SRM 3149 mercury, IRMM-014 boron). A secondary CRM is measured against a primary: commercial ampoules from Cerilliant, Restek, Sigma-Aldrich and Cayman Chemical fall here.
The Indian Pharmacopoeia Reference Standards (IPRS) deserve a special mention. Issued by the Indian Pharmacopoeia Commission at Ghaziabad under the Ministry of Health and Family Welfare, IPRS are the official Indian reference standards for pharmacopoeial assays and the preferred materials for ayurvedic-product authentication, herbal-drug forensic work and FSSAI compliance testing. The catalogue covers roughly 350 active pharmaceutical ingredients and 200 herbal markers, extended annually.
What the standard requires of CRM use is straightforward. Run one every batch where available, verify the result lies within its certified uncertainty, document the lot and certificate version on the batch record and the analytical certificate, replace expired CRMs and document the change. The chain is the audit trail the defence counsel will follow if the case is contested.