74
6 Reference Materials in Chemical Measurements
– The development (optimization) of new measurement procedures;
– The process of validation of the measurement procedure;
– The verification of the measurement procedure;
– The evaluation of the accuracy of measurements;
– The calibration of the measuring instrument for a particular type of measurement.
This allows the relationship between the received signal value and the content of
the analyte in the sample to be determined;
– Proficiency testing: the use of a well-characterized material, with a reference value
hidden from participants, allows the competence and proficiency of laboratories to
be assessed in terms of the defined type of analysis, in relation to an independently
determined reference value (rather than relative to the average value obtained on
the basis of the results provided by the participants).
In all cases, the RM should be treated as a test sample for which measurements
are made in the given laboratory conditions. Subsequently, the results (a value with
assigned uncertainty) for the RM are compared with the certified value and its uncertainty. In everyday practice, quality terms are often used: stating that the results ‘are
consistent’ or ‘inconsistent’ with the certified value. It is, however, recommended
to apply a quantitative assessment of compliance, which involves comparison of
the reference value with its assigned uncertainty of the mean value obtained in the
laboratory and the respective uncertainty.
The certified value of a given property of CRM is to be used together with the
uncertainty u CRM stated on the certificate of CRM material. The absolute difference
C between the measured mean value of C and reference value C CRM is calculated
according to the formula:
C |C CRM − C|
(6.1)
Uncertainties are usually expressed as standard deviations, and their variances are
additive. Uncertainty of C is thus u, which is calculated as the propagation of
uncertainty of the reference value u CRM and the uncertainty of the value determined
in the laboratory u C .
u
√
u
2
CRM + u
2
c
(6.2)
The expanded uncertainty U Δ corresponding to a confidence interval of approximately 95% is obtained by multiplying the standard uncertainty by a coverage factor
of k 2.
To assess the accuracy of the measurement procedure used in the laboratory,
the value of ΔC must be compared with U Δ ; if ΔC ≤ U Δ , it is considered that
there is no statistically significant difference between the measurement result
and the reference value within the uncertainty.
6 Reference Materials in Chemical Measurements
– The development (optimization) of new measurement procedures;
– The process of validation of the measurement procedure;
– The verification of the measurement procedure;
– The evaluation of the accuracy of measurements;
– The calibration of the measuring instrument for a particular type of measurement.
This allows the relationship between the received signal value and the content of
the analyte in the sample to be determined;
– Proficiency testing: the use of a well-characterized material, with a reference value
hidden from participants, allows the competence and proficiency of laboratories to
be assessed in terms of the defined type of analysis, in relation to an independently
determined reference value (rather than relative to the average value obtained on
the basis of the results provided by the participants).
In all cases, the RM should be treated as a test sample for which measurements
are made in the given laboratory conditions. Subsequently, the results (a value with
assigned uncertainty) for the RM are compared with the certified value and its uncertainty. In everyday practice, quality terms are often used: stating that the results ‘are
consistent’ or ‘inconsistent’ with the certified value. It is, however, recommended
to apply a quantitative assessment of compliance, which involves comparison of
the reference value with its assigned uncertainty of the mean value obtained in the
laboratory and the respective uncertainty.
The certified value of a given property of CRM is to be used together with the
uncertainty u CRM stated on the certificate of CRM material. The absolute difference
C between the measured mean value of C and reference value C CRM is calculated
according to the formula:
C |C CRM − C|
(6.1)
Uncertainties are usually expressed as standard deviations, and their variances are
additive. Uncertainty of C is thus u, which is calculated as the propagation of
uncertainty of the reference value u CRM and the uncertainty of the value determined
in the laboratory u C .
u
√
u
2
CRM + u
2
c
(6.2)
The expanded uncertainty U Δ corresponding to a confidence interval of approximately 95% is obtained by multiplying the standard uncertainty by a coverage factor
of k 2.
To assess the accuracy of the measurement procedure used in the laboratory,
the value of ΔC must be compared with U Δ ; if ΔC ≤ U Δ , it is considered that
there is no statistically significant difference between the measurement result
and the reference value within the uncertainty.