Traceability in Chemical Measurement

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Traceability in Chemical Measurement
Why Traceability?

• Comparable results are needed in order to avoid
  duplicating measurements which cost time and
  money.

Comparable results can only be achieved by
anchoring them to a common base.
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ELEMENTS OF TRACEABILITY

• an unbroken chain of comparisons going back to a
  standard acceptable to the parties, usually a
  national or international standard

• measurement uncertainty

• documentation

• competence

• reference to SI units

• recalibrations

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How is traceability of results best achieved?

• stated and/or internationally agreed suitable
  references

• an uncertainty statement for the measurement
  whose key role is to enable to judge the "fitness
  for purpose" of a result
• Guide to the Expression of Uncertainty in
  Measurement, ISBN 92-67-10188-9, International
  Organization for Standardization, Genève 1993 (
  draft updates, respectively)

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Technical Elements of Traceability

• A laboratory finds itself typically at the end of
  the traceability chain

• it must be able to rely on all the references
  necessary in the measurement process, as well as
  on method validation

• values of all references are themselves traceable
  to stated references and are accompanied by a
  reliable uncertainty statement.

5
Technical Elements of Traceability

• Determination of amount of substance often
  requires measurements of different properties,
  e.g.

• sample mass, on a balance compared to a mass
  reference

• analyte identity by comparison to a reference

• analyte quantitation by comparison to a different
  reference

Each property of the result should be traceable,
and each may contribute uncertainty to the
reported result.
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Traceability of Values Carried by RM

• traceability supplied from outside the laboratory
  is the traceability of values carried by
  references, especially by certified reference
  materials

• producers of reference materials must be aware
  that the values they supply are invariably an
  indispensable link in the traceability chain.

Reference ILAC
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LEVELS IN THE CALIBRATION HIERARCHY

• International
• The Bureau International des Poids et Mesures
  (BIPM) is in charge with coordinating the
  development and maintenance of primary standards
  and organizes intercomparisons on the highest
  level.

• National Metrology Institutes
• National Metrology Institutes are the highest
  authorities in metrology, maintaining national
  standards, ensuring that the primary standards
  themselves are internationally comparable.

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TERMINOLOGY IN THE HIERARCHY OF STANDARDS

• Primary standard
• A standard that is designated or widely
  acknowledged as having the highest metrological
  qualities

• International standard
• A standard recognized by an international
  agreement to serve internationally as the basis
  for assigning values to other standards

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TERMINOLOGY IN THE HIERARCHY OF STANDARDS

• National standard
• A standard recognized by a national decision to
  serve as the basis for assigning values to other
  standards

• Reference standard
• A standard, generally having the highest
  metrological quality available at a given
  location or in a given organization

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TERMINOLOGY IN THE HIERARCHY OF STANDARDS

• Transfer standard
• A standard used as an intermediary to compare
  standards.

• Travelling standard
• A standard, sometimes of special construction,
  intended for transport between different
  locations.

• Working standard
• Standard calibrated against a reference standard
  and used routinely to calibrate or check material
  or measuring instruments.

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Calibration hierarchy