8
1 Introduction to Metrology
of chemical measurements depends to a great extent on the kind and composition of
the investigated objects, and on the method of its preparation before conducting the
instrumental measurements.
Example The procedure used for samples preparation: when determining DDT
(dichlorodiphenyltrichloroethane) in meat, the use of a calibrated GC-MS (Gas Chromatography Mass Spectrometry) device does not a priori ensure reliable results, since
the results depend on, among other things, the procedure and reagents used for the
extraction of DDT from the meat sample. Moreover, the conditions in which the
process was carried out (e.g., time, temperature) influence the effectiveness of the
DDT extraction.
Appropriate definition of the measurand: this belongs to the more important
aspects of metrology and is especially important in chemical measurements. The
question about the determination of cadmium in the soil can concern both the determination of the total content of the element in the soil sample delivered to the laboratory or the fraction that can be extracted in given conditions. For that reason, it is
crucial to identify the measurand and the purpose of conducting the measurements
in a given object, so that the comparison of the measurement results refers to the
same, clearly defined property.
Measurand: Quantity Intended to Be Measured
NOTE 1: The specification of a measurand requires knowledge of the kind of
quantity, description of the state of the phenomenon, body, or substance carrying
the quantity, including any relevant component, and the chemical entities involved.
[Clause 2.3; ISO/IEC Guide 99]
To be deleted Clause 2.3; International Vocabulary of Metrology—Basic and General Concepts and Associated Terms (VIM; Vocabulaire International de Métrologie)
Measuring procedures used in the chemical laboratory are usually complex processes (Fig. 1.3), beginning with the sample preparation. Various physical (e.g.,
grinding, sieving, diluting) and chemical (e.g., mineralization, extraction, derivatization) processes change the original shape of the sample in order for it to be possible
to carry out the measurement for a given property of the test object. In addition, calibration of the measuring instrumentation, carrying out the measurement in adequate
working conditions and optimised parameters of the instrument are all important factors. Considering the rules of good laboratory practice, the chemist should perform
an optimization of conditions for all steps of the measurement procedure, and then
through validation confirm their adequacy for the intended objective of the test. In
correctly carried out measurements, relevant standards should be used to enable the
traceability of the obtained result. In practice, analytical procedures include measurement of physical quantities; for example, weighing the sample, taking the temperature
or diluting the solution in the measuring vessel to a very specific volume. In such
cases, traceability is ensured through the use of calibrated tools, including scales
and thermometers. Often, the measurement traceability is documented with a relevant calibration certificate, issued by an accredited calibration laboratory. Awareness
that ensuring measurement traceability is an indispensable part of the reliable result
1 Introduction to Metrology
of chemical measurements depends to a great extent on the kind and composition of
the investigated objects, and on the method of its preparation before conducting the
instrumental measurements.
Example The procedure used for samples preparation: when determining DDT
(dichlorodiphenyltrichloroethane) in meat, the use of a calibrated GC-MS (Gas Chromatography Mass Spectrometry) device does not a priori ensure reliable results, since
the results depend on, among other things, the procedure and reagents used for the
extraction of DDT from the meat sample. Moreover, the conditions in which the
process was carried out (e.g., time, temperature) influence the effectiveness of the
DDT extraction.
Appropriate definition of the measurand: this belongs to the more important
aspects of metrology and is especially important in chemical measurements. The
question about the determination of cadmium in the soil can concern both the determination of the total content of the element in the soil sample delivered to the laboratory or the fraction that can be extracted in given conditions. For that reason, it is
crucial to identify the measurand and the purpose of conducting the measurements
in a given object, so that the comparison of the measurement results refers to the
same, clearly defined property.
Measurand: Quantity Intended to Be Measured
NOTE 1: The specification of a measurand requires knowledge of the kind of
quantity, description of the state of the phenomenon, body, or substance carrying
the quantity, including any relevant component, and the chemical entities involved.
[Clause 2.3; ISO/IEC Guide 99]
To be deleted Clause 2.3; International Vocabulary of Metrology—Basic and General Concepts and Associated Terms (VIM; Vocabulaire International de Métrologie)
Measuring procedures used in the chemical laboratory are usually complex processes (Fig. 1.3), beginning with the sample preparation. Various physical (e.g.,
grinding, sieving, diluting) and chemical (e.g., mineralization, extraction, derivatization) processes change the original shape of the sample in order for it to be possible
to carry out the measurement for a given property of the test object. In addition, calibration of the measuring instrumentation, carrying out the measurement in adequate
working conditions and optimised parameters of the instrument are all important factors. Considering the rules of good laboratory practice, the chemist should perform
an optimization of conditions for all steps of the measurement procedure, and then
through validation confirm their adequacy for the intended objective of the test. In
correctly carried out measurements, relevant standards should be used to enable the
traceability of the obtained result. In practice, analytical procedures include measurement of physical quantities; for example, weighing the sample, taking the temperature
or diluting the solution in the measuring vessel to a very specific volume. In such
cases, traceability is ensured through the use of calibrated tools, including scales
and thermometers. Often, the measurement traceability is documented with a relevant calibration certificate, issued by an accredited calibration laboratory. Awareness
that ensuring measurement traceability is an indispensable part of the reliable result