Certificate of - European Reference Materials

Policy for the Statement of Metrological Traceability
on Certificates of ERM ® Certified Reference Materials
05 May 2008
Metrological traceability of measurement results is a key requirement for the comparability of
measurement results in time and space with other data, e.g., other measurement results, legal
limit values or product specifications. Similarly, traceability of the certified values of a CRM
(certified reference material) is a prerequisite to be able to compare a measurement result with
the certified value. Consequently, the statement on metrological traceability of the certified value
of the specified property is a requirement of the ISO Guide series on Reference Materials. The
related terms such as 'metrological traceability' are defined in documents such as ISO Guide 99
[1].
The certified (property) value is attributed to a quantity representing a property of the CRM (see
ISO Guide 35 [2]). Following the terminology of ISO Guide 99, a 'quantity' is "a property of a
phenomenon/body/substance, to which a number can be assigned with respect to a reference.
This reference can be a measurement unit, a measurement procedure or a reference material".
Consequently, a quantity (e.g., amount-of-substance concentration of ethanol in a wine sample)
would be the combination of the identification/description of the property (ethanol) of a
body/item (wine) and the 'base (or derived) quantity' (amount-of-substance concentration). The
IUPAC Provisional Recommendations on "Metrological Traceability of Measurement Results in
Chemistry" [3] describe that combination in form of a sequence
(see Table 2.1-1 in [3]).
system ⇒ component/analyte ⇒ kind of quantity
(wine) (ethanol) (amount-of-substance concentration)
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A certified value on a CRM certificate belongs to a specified quantity and is the combination of a
number (with its uncertainty) and the measurement unit.
Therefore, the key information on page 1 of an ERM CRM certificate is actually a combination
of 5 attributes:
• Identification of the body/matrix (e.g., wine)
• Identification/description of the property/component (e.g., ethanol)
• Description of the certified base or derived quantity/kind-of-quantity (e.g., amount-ofsubstance
concentration)
• A number (e.g., 3.8 – with its corresponding uncertainty (e.g., ± 0.1)
• The measurement unit (e.g., mol/L)
The combination of these attributes has to be covered by the "traceability statement". When
preparing a traceability statement, one should take into consideration (according to ISO Guide
99) that 'metrological traceability' sets a requirement on the 'measurement result', which itself is
defined as "quantity value(s) being attributed to a measurand" (with 'measurand' as the "quantity
intended to be measured"). The 'quantity value' is the product of a number (with its uncertainty)
and a measurement unit.
The measurement result has to be related to a stated reference and (as described in ISO Guide
99) such a stated reference can be:
− a value defined by the definition of a measurement unit or
− a value realized by a measurement procedure (including the measurement unit for a nonordinal
quantity) or
− a value carried by a measurement standard (i.e., a certified reference material).
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For most of the quantities described on CRM certificates, one (or both) of the following cases
have to be considered:
− a measurand (quantity) which is defined by its structure alone (e.g., a chemical entity
such as a specific ion, atom, molecule) or
− a measurand (quantity) which is operationally defined by a described measurement
procedure.
Obviously the certified value as the mathematical product of a number and the measurement unit
has to be derived via properly calibrated measurement systems and it is this calibration hierarchy
that needs to be described. As this is usually too much information to be inserted on the first
page of a CRM certificate (and possibly even on the certificate at all), this description of the
traceability hierarchy should be abbreviated in a footnote accompanying the term "certified
value" on page 1 of the ERM CRM certificate as indicated here. Although not mandatory for an
ERM CRM, the certification report is the best place to deliver all information concerning
metrological traceability and calibration hierarchies.
The following principles for the traceability statements on ERM CRM certificates (and ERM
certification reports) should be followed:
1. For "structurally defined" measurands quantified in total:
− Matrix (e.g., wine): no footnote required
− Certified property (e.g., ethanol): no footnote required
− Certified (kind of) quantity (e.g., amount-of-substance concentration): no footnote
required
− Certified value (e.g., 3.8 mol/L): footnotes stating the statistical parameters used for the
certified value(s) (e.g., unweighted mean of…) and stating the metrological traceability of
these values, typically as "It is traceable to the International System of Units (SI)."
− Uncertainty (e.g., 0.1 mol/L): footnote stating the type of uncertainty value given (e.g.,
expanded uncertainty estimated in accordance with the GUM with a coverage factor 2
corresponding to a level of confidence of about 95 %).
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The calibration hierarchy should be described in the certification report (or on subsequent pages
of the certificate), whereby the principle, including reference to the most important calibrants,
should be given and a reasonable level of detail is recommended.
2. For "operationally/method defined" measurands:
− Matrix (e.g., sandy soil): no footnote required
− Certified property (e.g., extractable PCB 153): Footnote required stating the definition of
the property, typically like "as defined by the procedure according to ISO 12345" or:
"as obtained by Soxhlet extraction and subsequent quantification by GC-MS"
− Certified (kind of) quantity (e.g., mass fraction): no footnote required
− Certified value (e.g., 5.3 μg/kg): footnotes stating the statistical parameters used for the
certified value(s) (e.g., unweighted mean of…) and stating the metrological traceability of
these values, typically as "It is traceable to the International System of Units (SI)."
− Uncertainty (e.g., 0.1 μg/kg): footnote stating the type of uncertainty value given (e.g.,
expanded uncertainty estimated in accordance with the GUM with a coverage factor 2
corresponding to a level of confidence of about 95 %).
Again the applied calibration hierarchies should be described in the certification report (or on
subsequent pages of the certificate), whereby the principle, including reference to the most
important calibrants, should be given and a reasonable level of detail is recommended.
References:
[1] ISO/IEC Guide 99 "International vocabulary of metrology – basic and general concepts and
associated terms (VIM), ISO, Geneva (2007)
[2] ISO Guide 35 "Reference materials – general and statistical principles for certification",
3 rd edition, ISO, Geneva (2006)
[3] http://www.iupac.org/reports/provisional/abstrac07/fajgelji_draft_2007-09-18.pdf
Hendrik Emons
On behalf of the ERM cooperation
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