102
7 Validation of the Measurement Procedure
The sensitivity of the measuring system is understand as the ration of change of
the signal and the corresponding change in measured values (Clause 4.12, VIM 3).
The definition of sensitivity given in VIM 3 is supplemented with two notes: (1)
sensitivity of the measuring system can depend on the value of the quantity being
measured; and (2) the change considered in a value of a quantity being measured
must be large compared with the resolution. The sensitivity may be represented
as the slope of the calibration curve, which may be described by the least squares
mathematical function or calculated from the signal values for the two solutions of
different concentrations of analyte.
Detection limit determines the amount/concentration of analyte that corresponds
to the signal calculated from the value of the blank plus three times the standard
deviation at a level close to the blank sample. This is the signal that, with a certain
probability, can be distinguished from the blank.
The definition of the term ‘limit of detection’ is also given in VIM 3 (Clause
4.18), according to which it is the measured quantity value, obtained by a given
measurement procedure, for which the probability of falsely claiming the absence
of a component in a material is β, given a probability ´
α of a falsely claiming its
presence.
The definition is supplemented with three notes: (1) IUPAC recommends default
values for ´
α and β equal to 0.05; (2) The abbreviation LOD is sometimes used; (3)
The term ‘sensitivity’ is discouraged for ‘detection limit.’
Quantitation limit is the lowest amount/concentration of the analyte that may be
determined by the given measuring procedure with the specified accuracy and precision. The value of the limit of quantitation should be evaluated by appropriate
standard solutions. In practice, the solution of concentration corresponding to the
detection limit is the lowest, except the blank sample, a point on the calibration
curve.
Ruggedness/robustness, sometimes referred to as method tolerance, is a parameter
characterizing the effect of small changes in procedures on the stability of the obtained
analytical results. When using the method by different laboratories, as well as in the
same laboratory over the time, one can expect some minor variations to occur. During
method validation, it is important to determine which parameters of the analytical
procedure are most influenced by other factors—meaning which are the most critical
steps. Detailed information about how to validate the robustness of the method are
given later in the text.
Accuracy is defined in the VIM 3 (Clause 2.13) as the “closeness of agreement
between a measured quantity value and a true quantity value of a measurand.” Attention should be given to the fact that the concept of ‘measurement accuracy’ is not
of qualitative nature and is not expressed in a numerical quantity value. The smaller
the measurement error, the more accurate the measurement.
Trueness, understood as correctness of the measurement was defined in the VIM
3 (Clause 2.14) as the closeness of agreement between the average of an infinite
number of replicate measured quantity values and the reference quantity values.
Warning! The terms, ‘accuracy’ and ‘trueness’ should not be used interchangeably.
7 Validation of the Measurement Procedure
The sensitivity of the measuring system is understand as the ration of change of
the signal and the corresponding change in measured values (Clause 4.12, VIM 3).
The definition of sensitivity given in VIM 3 is supplemented with two notes: (1)
sensitivity of the measuring system can depend on the value of the quantity being
measured; and (2) the change considered in a value of a quantity being measured
must be large compared with the resolution. The sensitivity may be represented
as the slope of the calibration curve, which may be described by the least squares
mathematical function or calculated from the signal values for the two solutions of
different concentrations of analyte.
Detection limit determines the amount/concentration of analyte that corresponds
to the signal calculated from the value of the blank plus three times the standard
deviation at a level close to the blank sample. This is the signal that, with a certain
probability, can be distinguished from the blank.
The definition of the term ‘limit of detection’ is also given in VIM 3 (Clause
4.18), according to which it is the measured quantity value, obtained by a given
measurement procedure, for which the probability of falsely claiming the absence
of a component in a material is β, given a probability ´
α of a falsely claiming its
presence.
The definition is supplemented with three notes: (1) IUPAC recommends default
values for ´
α and β equal to 0.05; (2) The abbreviation LOD is sometimes used; (3)
The term ‘sensitivity’ is discouraged for ‘detection limit.’
Quantitation limit is the lowest amount/concentration of the analyte that may be
determined by the given measuring procedure with the specified accuracy and precision. The value of the limit of quantitation should be evaluated by appropriate
standard solutions. In practice, the solution of concentration corresponding to the
detection limit is the lowest, except the blank sample, a point on the calibration
curve.
Ruggedness/robustness, sometimes referred to as method tolerance, is a parameter
characterizing the effect of small changes in procedures on the stability of the obtained
analytical results. When using the method by different laboratories, as well as in the
same laboratory over the time, one can expect some minor variations to occur. During
method validation, it is important to determine which parameters of the analytical
procedure are most influenced by other factors—meaning which are the most critical
steps. Detailed information about how to validate the robustness of the method are
given later in the text.
Accuracy is defined in the VIM 3 (Clause 2.13) as the “closeness of agreement
between a measured quantity value and a true quantity value of a measurand.” Attention should be given to the fact that the concept of ‘measurement accuracy’ is not
of qualitative nature and is not expressed in a numerical quantity value. The smaller
the measurement error, the more accurate the measurement.
Trueness, understood as correctness of the measurement was defined in the VIM
3 (Clause 2.14) as the closeness of agreement between the average of an infinite
number of replicate measured quantity values and the reference quantity values.
Warning! The terms, ‘accuracy’ and ‘trueness’ should not be used interchangeably.