Certificates of analysis

A certificate of analysis (CoA) reports results for identified material. A lot or batch ID helps establish which material the results represent. The method and reporting basis determine what the numbers mean. Sigma-Aldrich lot-linked certificate lookup

Use the certificate for the attributes and material it identifies. A technical data sheet describes product properties and application conditions. A safety data sheet covers hazards and handling.

Original unit and conformity example

Assume an invented material specification limits impurity X to 0.050% by mass maximum. A laboratory reports 420 mg/kg, with expanded uncertainty 60 mg/kg, and explicitly states its coverage basis. This is a numerical illustration, not a real certificate or service recommendation.

• 1% by mass = 10,000 mg/kg, so the limit is 500 mg/kg.
• The reported result is 0.0420% by mass.
• The expanded uncertainty is 0.0060 percentage points by mass.
• The symmetric interval is 360–480 mg/kg, or 0.0360–0.0480% by mass.

Under an assumed simple rule result ≤ limit, 420 passes. Under a different illustrative upper-limit rule result + U ≤ limit, 420 + 60 = 480 mg/kg, so it also passes.

If a second hypothetical sample reports 470 ± 60 mg/kg, the simple rule passes it, but the second rule does not meet its acceptance condition because 530 > 500 mg/kg. That does not prove the true impurity content exceeds the limit; it means this evidence does not satisfy that stated guarded-acceptance rule. Neither rule is prescribed here, and no general probability of conformity follows without the uncertainty model and decision assumptions.

The numerical uncertainty is not added to the material's impurity content or subtracted from the purchase specification. It changes the decision's evidence basis, not the substance itself.

Identity determines the scope of the result

A product name can cover multiple grades, formulations, lots and package sizes. A lot identifier connects the reported evidence to a defined production population, while a sample identifier may connect it only to the material submitted for examination. The link between a tested sample and a shipment depends on the sampling and identification arrangement.

This table explains dimensions of evidence rather than a universal mandatory template. Required contents differ with the material, purpose, contractual terms and applicable sector rules. A missing field limits interpretation; it does not by itself establish a jurisdiction-independent legal violation.

Sampling is particularly important when one result is used to describe a heterogeneous batch. Eurachem/CITAC's compliance guide notes that uncertainty from sampling can matter when the measurand concerns a lot rather than only a laboratory sample. A precise result on an unrepresentative test portion does not establish the entire lot's uniformity. Eurachem/CITAC, Use of uncertainty information in compliance assessment, second edition 2021, section 2

Result, specification and decision are different statements

A numerical result estimates a property under the stated method and conditions. A specification is a requirement or comparison limit. A statement such as “conforms” is a decision made by applying an acceptance rule to the evidence. These three statements are related but not interchangeable.

For example, a maximum impurity limit of 500 mg/kg does not mean that the measured impurity content is 500 mg/kg. A result of 420 mg/kg does not describe how much uncertainty is associated with that estimate. A pass statement alone may not disclose the numerical margin to the limit.

Methods also define what a value means. Mass fraction, mass concentration, dry-basis content and as-received content are different quantities. A percentage without its basis can be ambiguous. “Not detected” is not a numerical proof of zero; it depends on the method's detection capability and reporting convention. A value reported below a quantification threshold has a different evidential meaning from a fully quantified result.

Uncertainty belongs to the measurement

Measurement uncertainty characterizes the dispersion associated with the reported value; it is not an extra product tolerance. A specification band defines permitted product behavior, while an uncertainty interval expresses a limitation of knowledge about the measured quantity.

Eurachem/CITAC describes decision rules connecting measured values, uncertainty and specification limits. Its discussion distinguishes simple acceptance from rules using guard bands and relates those choices to the risk of an incorrect decision. The same measured value can therefore support different declared outcomes under different agreed rules. Eurachem/CITAC, second edition 2021, sections 2–4

Expanded uncertainty also needs its stated coverage basis. An uncertainty associated with a certified reference value is not automatically the uncertainty of a later analysis performed using that reference material. Sample preparation, calibration, instrument behavior and the later measurement procedure contribute their own effects.

Named example: a reference-material CoA

NIST's SRM 3161a Tin Standard Solution, lot 230104, has a CoA issued April 11, 2025. It reports tin mass fraction as 10.004 ± 0.018 mg/g, with a stated 95% confidence interval and coverage factor 1.997. The certified value is intended as a primary calibration standard for quantitative tin determination and is traceable to the SI unit for mass fraction. NIST, SRM 3161a lot 230104, page 1

The same certificate gives validity through May 31, 2031, conditional on proper material storage and use; damage, contamination or modification nullifies the certified value. Those are conditions for this named material, not a universal CoA shelf life. NIST, SRM 3161a, validity statement

A reference-material certificate has a different purpose from a routine production-batch certificate. Sigma-Aldrich distinguishes certified reference materials, analytical standards and reagent grades with different intended uses. As a synthesis of those distinctions, a reagent's CoA alone does not establish the characterization and traceability needed for a reference material. Sigma-Aldrich, Calibration, qualification and validation

Traceability, validity and evidential limits

Metrological traceability concerns the measurement result and its reference chain. NIST explicitly states that its certification does not automatically extend to results later obtained with an instrument it calibrated or an SRM it supplied. A statement that a laboratory used a NIST material is therefore narrower than a demonstration that every later result is metrologically traceable with suitable uncertainty. NIST, Metrological Traceability FAQ

A certificate's issue date, test date, retest date and material expiry can describe different events. Later contamination, moisture uptake, segregation or processing can change the material, while a revised certificate may change the document's description of its original state. Neither document age alone nor a familiar supplier name resolves those possibilities.

Explanatory synthesis: a CoA is evidence for its stated properties, population, methods and conditions. It does not independently establish shipment identity, suitability for a particular manufacturing process, compliance with untested requirements or authorization to use the material. These limits are properties of the evidence, not a universal rejection rule.

Source versions

Reviewed 2026-11-02 PST. The NIST certificate's actual header identifies lot 230104, version April 11, 2025. The Eurachem/CITAC PDF cover and publication pages identify second edition 2021. No separately verified technical issue date is assigned to the cited live Sigma-Aldrich or NIST FAQ pages. The numerical examples and explicitly labeled synthesis are original, bounded explanations.

Related reference

Supplier documents and evidence