8.19 Evaluation of Uncertainty by Mathematical Model
145
Determination of nitrates (standard procedure ISO 6777:1984)
Description of the measuring procedure: after adding the reagents, in 20 min,
a colorful complex is created, with a durability of 2 h, which means that the
measurement can be conducted about 1.5 h after a stable color is obtained.
In laboratory practice: depending on the components of the matrix, a gradual
decrease of the absorbance can be observed, of about 15% of the value noted
directly after the color stabilizes. Moreover, in the first 15 min (when the
measurements are most often conducted), the absorbance decreased by almost
6%.
Example 4 In chemical measurements, an essential part of the analytical procedure
is the sample preparation. In most cases, it is not possible to describe the influence
of each of the step of the sample preparation process in the form of a mathematical
equation (modeling). It is also not easy to assign numerical values to the uncertainty
introduced by those steps.
In practice, to assess the influence of sample preparation, the recovery test is used.
This can be executed by use of the reference materials, similar in their properties
to the tested samples, or by adding a known amount of analyte to the investigated
sample. Thus, the uncertainty of recovery should be defined, which is not always
easy, and does not always reflect the real behavior of the tested object.
Factors influencing uncertainty of recovery
When using reference materials
– Disparity between the form (physical state, granulation, uniformity) of the
test sample and the reference material;
– The chemical and physical form of the analyte in the reference material;
When using test samples with the addition of the standard
– Disparity between the chemical form in which the addition is introduced to
the test sample, and in which it is present in the sample;
– The step of the analytical procedure in which the standard is added;
– The time used for establishing the chemical equilibrium between the analyte
added and originally being present in test sample.
BE AWARE! The uncertainty of the recovery for test samples with the addition
of the standard can be much higher in comparison with the recovery of the reference
material.
145
Determination of nitrates (standard procedure ISO 6777:1984)
Description of the measuring procedure: after adding the reagents, in 20 min,
a colorful complex is created, with a durability of 2 h, which means that the
measurement can be conducted about 1.5 h after a stable color is obtained.
In laboratory practice: depending on the components of the matrix, a gradual
decrease of the absorbance can be observed, of about 15% of the value noted
directly after the color stabilizes. Moreover, in the first 15 min (when the
measurements are most often conducted), the absorbance decreased by almost
6%.
Example 4 In chemical measurements, an essential part of the analytical procedure
is the sample preparation. In most cases, it is not possible to describe the influence
of each of the step of the sample preparation process in the form of a mathematical
equation (modeling). It is also not easy to assign numerical values to the uncertainty
introduced by those steps.
In practice, to assess the influence of sample preparation, the recovery test is used.
This can be executed by use of the reference materials, similar in their properties
to the tested samples, or by adding a known amount of analyte to the investigated
sample. Thus, the uncertainty of recovery should be defined, which is not always
easy, and does not always reflect the real behavior of the tested object.
Factors influencing uncertainty of recovery
When using reference materials
– Disparity between the form (physical state, granulation, uniformity) of the
test sample and the reference material;
– The chemical and physical form of the analyte in the reference material;
When using test samples with the addition of the standard
– Disparity between the chemical form in which the addition is introduced to
the test sample, and in which it is present in the sample;
– The step of the analytical procedure in which the standard is added;
– The time used for establishing the chemical equilibrium between the analyte
added and originally being present in test sample.
BE AWARE! The uncertainty of the recovery for test samples with the addition
of the standard can be much higher in comparison with the recovery of the reference
material.