Questions related to use of reference materials

What is a nominal value?

A nominal value is a provisional value, usually the target one wants to achieve with the CRM. It is usually different from the certified value and should therefore not be used.

How should I use the data obtained on measurements of a CRM?

There are different possibilities to use the data obtained on a measurement of a CRM. It can be used to help to
  • validate a method of measurement. This is to see, if the observed value and the certified value match within the uncertainty.
  • verify a specific measurement procedure. This is to see, if the observed value and the certified value match within the uncertainty under the specific laboratory conditions.
  • control the accuracy of measurements within the uncertainty.
  • calibrate a measurement device for a specific type of measurement. This is to establish the relation between a ‘signal’ and the quantity intended to be measured (usually a concentration). In the simple case, a calibration factor and its uncertainty is calculated from the observed value and the certified value.
  • establish proficiency testing. The use of a well characterised (i.e. certified) material in form of a ‘blind sample’ is advantageous, in order to monitoring against a stated reference, in particular the inalterable SI being stable in time and space.
  • develop new methods of measurement.
  • estimate measurement uncertainty.

How reliable is my measurement result?

The reliability of the method applied has to be demonstrated by means such as method validation, proper use of quality assurance tools (including the use of CRMs) and the participation in proficiency testing.

Which value do I use for calibration?

The value mentioned on the certificate is the certified value for the reference material and consequently the value to be used for calibration. But the uncertainty of the certified value has also to be included for the estimation of the measurement uncertainty.

How do I compare my result with the certified values?

The underlying principle is that one has to check whether the difference between the measured result and the certified value is larger than the expanded combined uncertainty of measurement and certified value. This is done as follows:
  1. Calculate the standard uncertainty of the certified value (uCRM). This is obtained by dividing the expanded uncertainty given on the certificate by the expansion factor (also stated on the certificate).
  2. Estimate the measurement uncertainty (um) of the result. As a very rough approximation, the reproducibility standard deviation can be used
  3. Combine the two uncertainties:
  4. Check whether 2*uc is larger than the difference between the certified and the measurement value. If this is the case, the measurement result agrees within the limits of the respective uncertainties with the certified values
For example: Certified reference material BCR-605 (road dust) with a trimethyllead-content of 7.9 ± 1.2 µg/kg has been used. The analytical result was 11.10 µg/kg and the standard measurement uncertainty as determined in the method validation (um) is 15 %, i.e. 1.67 µg/kg. The difference between analytical result and certified value is therefore 3.2 µg/kg. Prior to the determination of the combined uncertainty, the standard uncertainty of the certified value of the reference material must be determined. The uncertainty of the certified value (UCRM) is given as 95 % confidence interval with 6 degrees of freedom as stated on the certificate. The factor of the t-distribution (t95, 6) is 2.447. The confidence interval must be divided by this factor to obtain the standard uncertainty of the certified value. This standard uncertainty (uCRM) is therefore


The combined uncertainty (uc) of measurement and certified value is the square root of the quadratic sum of the individual uncertainties


The results show that the uncertainty of the certified value has almost no influence on the combined uncertainty. This combined uncertainty is now multiplied with a coverage factor of 2 to obtain the expanded uncertainty (here 3.48 µg/kg). This expanded uncertainty is bigger than the difference between analytical results and certified values (3.2 µg/kg). Within the limits of the uncertainties, no method bias is visible.

How do I know whether the certified value is valid for my method?

There are two CRM properties that describe the usefulness of a material for a certain method, namely traceability and commutability. Traceability describes what the certified value actually refers to including the definition of the measurand: is it about total or extractable element content, Kjeldahl or Dumans N? Commutability describes whether a material behaves similarly as a certain routine sample for a given method.

Therefore, one should check whether the certified value of a CRM is traceable to the same reference as one’s method. If this is not the case, the material is unsuitable.

Example: Certified values for dietary fibre traceable to a certain ISO method are only valid, if exactly this method is used.

Additionally, one should check whether commutability is ensured. If this is not the case, the material may be unsuitable.

Which method do I have to use?

Any method that is traceable to the same reference as the certified value can be used.


Where can I get additional information on my material?

IRMM publishes for each material an extensive report available on this web site. This report gives valuable information on methods used by the participants and potential problems besides giving more detailed information on the production of the material.



Last Update 24/04/2006