4.1 Traceability and the Comparability of Results
43
of the measurement of physical quantities, metrological traceability can be ensured
through the use of an appropriate measuring device containing in itself information
that is consistent with generally accepted unit of measurement. In most cases, the
measurement of physical quantities is independent of the type of the object being
measured. For example, no matter whether we consider the mass of the sugar, salt
or grits, the measurement result depends on the used measuring instrument (that is,
the balance) and the used standard (that is, the weights). In this case, the key part
of the measuring procedure is the calibration of the measuring device—the balance
and weights. One should proceed similarly with the measurement of other physical
quantities; for example, the length and temperature.
Directly ensuring measurement traceability in such a way is not possible in chemical measurements. The determination of the content of iron in serum consists of
determining the amount of iron present in a specific chemical form in a sample comprising a plurality of other components that may affect the response of the detector.
Thus, the measurement procedure comprises the often-complex physicochemical
operations; for example, separation of iron ions from the matrix, converting them
into a colored complex after the addition of a suitable complexing agent, and then
measuring the absorbance of that complex. Obviously, the measuring instrument, in
this case, UV–Vis spectrometer, must undergo the calibration. In such a case, the
calibration is carried out using the most appropriate standard solutions containing
an increasing amount of the analyte (iron compound). In other words, we ‘teach’ the
measuring equipment to respond to the presence of a chemical quantity. In a real
sample, beside the substance of interest other chemical species are also present that
may affect the behavior of the substance to be determined (e.g., iron ions) during the
preparatory step, especially when adding different reagents. A good solution would
be to separate the analyte from a matrix of real sample, but here arises another problem: how efficiently are we able to separate it in a quantitative way. All these aspects
make the traceability of the chemical measurements much more difficult to achieve.
This is namely due to the lack of available reference standards for all possible cases
of chemical measurements, which is not the case in the measurement of physical
quantities.
If we consider the analytical procedure as a set of successive steps of processing the
sample, then for each of these steps we should established the appropriate reference
standard, to ensure measurement traceability of the entire procedure. Therefore, the
determination and demonstration of traceability in chemical measurements requires
consideration of several aspects. How can this be achieved?
First of all, we should:
– Clearly define the purpose of measurements and select the appropriate measurement procedure;
– Describe the measurement procedure in the form of a mathematical equation;
– On the basis of a validation process, demonstrate that all the factors that may affect
the final result were accounted for;
– Select proper reference standards for all steps of the measuring procedure;
– Determine the uncertainty that can be attributed to the result of measurement,
taking into account the uncertainty of standards and/or calibrations.
43
of the measurement of physical quantities, metrological traceability can be ensured
through the use of an appropriate measuring device containing in itself information
that is consistent with generally accepted unit of measurement. In most cases, the
measurement of physical quantities is independent of the type of the object being
measured. For example, no matter whether we consider the mass of the sugar, salt
or grits, the measurement result depends on the used measuring instrument (that is,
the balance) and the used standard (that is, the weights). In this case, the key part
of the measuring procedure is the calibration of the measuring device—the balance
and weights. One should proceed similarly with the measurement of other physical
quantities; for example, the length and temperature.
Directly ensuring measurement traceability in such a way is not possible in chemical measurements. The determination of the content of iron in serum consists of
determining the amount of iron present in a specific chemical form in a sample comprising a plurality of other components that may affect the response of the detector.
Thus, the measurement procedure comprises the often-complex physicochemical
operations; for example, separation of iron ions from the matrix, converting them
into a colored complex after the addition of a suitable complexing agent, and then
measuring the absorbance of that complex. Obviously, the measuring instrument, in
this case, UV–Vis spectrometer, must undergo the calibration. In such a case, the
calibration is carried out using the most appropriate standard solutions containing
an increasing amount of the analyte (iron compound). In other words, we ‘teach’ the
measuring equipment to respond to the presence of a chemical quantity. In a real
sample, beside the substance of interest other chemical species are also present that
may affect the behavior of the substance to be determined (e.g., iron ions) during the
preparatory step, especially when adding different reagents. A good solution would
be to separate the analyte from a matrix of real sample, but here arises another problem: how efficiently are we able to separate it in a quantitative way. All these aspects
make the traceability of the chemical measurements much more difficult to achieve.
This is namely due to the lack of available reference standards for all possible cases
of chemical measurements, which is not the case in the measurement of physical
quantities.
If we consider the analytical procedure as a set of successive steps of processing the
sample, then for each of these steps we should established the appropriate reference
standard, to ensure measurement traceability of the entire procedure. Therefore, the
determination and demonstration of traceability in chemical measurements requires
consideration of several aspects. How can this be achieved?
First of all, we should:
– Clearly define the purpose of measurements and select the appropriate measurement procedure;
– Describe the measurement procedure in the form of a mathematical equation;
– On the basis of a validation process, demonstrate that all the factors that may affect
the final result were accounted for;
– Select proper reference standards for all steps of the measuring procedure;
– Determine the uncertainty that can be attributed to the result of measurement,
taking into account the uncertainty of standards and/or calibrations.