6.6 Applicability of Reference Materials
75
In the case of the given CRM, for which several properties have associated reference values, the certificate contains expanded uncertainties values for each property.
The certificate of the RM should also contain information on the procedure used for
the evaluation of the uncertainty and information on the coverage factor k, applied.
The standard uncertainty of the reference value is obtained by dividing the stated
expanded uncertainty by that coverage factor.
In some cases, the uncertainty refers to 95% of the confidence interval of the mean
value obtained with using the mean values delivered by laboratories participating in
the evaluation of the reference value. In this case, the Student t-value for the 95%
confidence level for n-1 degrees of freedom (n is the number of laboratories) is
determined from the statistical tables for the t-distribution, which means that the
coverage factor may not be equally 2. Then the standard uncertainty of the reference
value is determined by dividing the expanded uncertainty by the coverage factor
(varying from 2) specified in given certificate.
In many cases, the uncertainty assigned to the reference value is symmetrically
allocated around this value. However, there are situations where the uncertainty is
asymmetrically allocated. For example, for corn powder (ERM
® BF418c; maize
powder), the contents of genetically modified maize 1507 is given as 9.9 g/kg with
assigned uncertainty of (−0.6 ÷ +0.8) g/kg. This means that the certified reference
value has been assigned an asymmetric uncertainty range and the following approach
should be applied when comparing the reference value with the value obtained in the
laboratory:
– Positive uncertainty range (0.8 g/kg) should be used when the average value
obtained in the laboratory is higher than the reference value;
– Negative uncertainty range (0.6 g/kg) should be used when the average value
obtained in the laboratory is lower than the reference value.
6.7 Selection of Reference Materials
An important aspect of the laboratory practice is the proper selection of a suitable
reference material, depending on the intended use. First, the commonsense principle,
says that the best RM is one that meets the requirements regarding the use of the
results of measurements for a given purpose.
An important criterion for assessing the suitability of a RM is to decide whether
the material is to be used to calibrate the measuring instrument, or to assess the
behavior of the analyte during the entire measurement procedure, including both the
preparation of the test sample and the measurement itself. In the first case (calibration
of measurement instrument), pure chemical substances are most frequently used,
which allow the response of the measuring system (signal) to be assigned to a given
quantity of the substance to be determined. The uncertainty of the reference value
and the precision of the measurements a contribute to the uncertainty budget of the
calibration process. In the second case (evaluation of the measurement procedure),
the RM is used to evaluate bias of the results when using a selected measurement
procedure, or the value (in percentage or as a fraction of) of the difference between the
75
In the case of the given CRM, for which several properties have associated reference values, the certificate contains expanded uncertainties values for each property.
The certificate of the RM should also contain information on the procedure used for
the evaluation of the uncertainty and information on the coverage factor k, applied.
The standard uncertainty of the reference value is obtained by dividing the stated
expanded uncertainty by that coverage factor.
In some cases, the uncertainty refers to 95% of the confidence interval of the mean
value obtained with using the mean values delivered by laboratories participating in
the evaluation of the reference value. In this case, the Student t-value for the 95%
confidence level for n-1 degrees of freedom (n is the number of laboratories) is
determined from the statistical tables for the t-distribution, which means that the
coverage factor may not be equally 2. Then the standard uncertainty of the reference
value is determined by dividing the expanded uncertainty by the coverage factor
(varying from 2) specified in given certificate.
In many cases, the uncertainty assigned to the reference value is symmetrically
allocated around this value. However, there are situations where the uncertainty is
asymmetrically allocated. For example, for corn powder (ERM
® BF418c; maize
powder), the contents of genetically modified maize 1507 is given as 9.9 g/kg with
assigned uncertainty of (−0.6 ÷ +0.8) g/kg. This means that the certified reference
value has been assigned an asymmetric uncertainty range and the following approach
should be applied when comparing the reference value with the value obtained in the
laboratory:
– Positive uncertainty range (0.8 g/kg) should be used when the average value
obtained in the laboratory is higher than the reference value;
– Negative uncertainty range (0.6 g/kg) should be used when the average value
obtained in the laboratory is lower than the reference value.
6.7 Selection of Reference Materials
An important aspect of the laboratory practice is the proper selection of a suitable
reference material, depending on the intended use. First, the commonsense principle,
says that the best RM is one that meets the requirements regarding the use of the
results of measurements for a given purpose.
An important criterion for assessing the suitability of a RM is to decide whether
the material is to be used to calibrate the measuring instrument, or to assess the
behavior of the analyte during the entire measurement procedure, including both the
preparation of the test sample and the measurement itself. In the first case (calibration
of measurement instrument), pure chemical substances are most frequently used,
which allow the response of the measuring system (signal) to be assigned to a given
quantity of the substance to be determined. The uncertainty of the reference value
and the precision of the measurements a contribute to the uncertainty budget of the
calibration process. In the second case (evaluation of the measurement procedure),
the RM is used to evaluate bias of the results when using a selected measurement
procedure, or the value (in percentage or as a fraction of) of the difference between the