6.8 The Type of Reference Material Versus the Type of Sample
77
soil taken from nature and that which undergoes specific processing so as to meet the
requirement of the RM runs with the same efficiency. In practise it is not a case, thus
this should be also considered, and if possible another RM (better fit to the original
soil samples) should be taken. If this is not possible, the appropriate procedure for
sample preparation of natural soil should be used (e.g., through its fragmentation to
a form similar to the reference material). Another solution may be a recovery test
carried out not only for a reference material but also for test samples.
In the case of clinical specimens, commonly available materials are usually in a
lyophilized form (e.g., blood serum), as the test samples obtained from patients are
usually delivered without being lyophilized. Of course, the freeze-dried material of
the RM is subjected to a reconstruction of the liquid state by adding an appropriate
volume of water, but certainly, afterwards the behavior of serum components could be
different. The potential impact of the freeze-drying on the result should be considered.
A useful reference material should be as close as possible in terms of chemical
composition to the matrix as well as in terms of the physical form of the test
samples.
6.9 The Content of the Substance to Be Determined
in the Reference Material and the Sample
The basic requirement is for the concentration of the substance in the reference
material to be similar to the concentration that is present in the analyzed test samples.
In practice, this is not always possible; often the available RMs have a much higher
content of the substance and rarely much lower. Some manufacturers recommend
the dilution of the prepared solution in such cases, but this will decrease not only
the concentration of the substance to be determined but also the concentration of the
matrix components, which can seriously affect the measurement result.
Another problem is the chemical form of the substance to be determined. This
is due to two reasons: first, for different chemical forms, extraction efficiency may
vary; second, in a number of measuring techniques, the detector response depends on
the chemical form of the analyte. For example, for the determination of arsenic and
selenium, by atomic absorption spectrometry with hydride generation, the efficiency
of the reduction depends on the original oxidation state of the element (e.g., As(III)
and As(V), and Se(IV) and Se(VI)). Another example is the determination of mercury
by ICP MS (Inductively Coupled Plasma Mass Spectrometry); apart from a very high
temperature of plasma, efficiency of the ionization of mercury present in the form of
phenylmercury is significantly lower than the efficiency of ionization of the mercury
present in the sample in the form of an inorganic ions, and this affects the number of
ions reaching the mass detector. Such examples are numerous.
77
soil taken from nature and that which undergoes specific processing so as to meet the
requirement of the RM runs with the same efficiency. In practise it is not a case, thus
this should be also considered, and if possible another RM (better fit to the original
soil samples) should be taken. If this is not possible, the appropriate procedure for
sample preparation of natural soil should be used (e.g., through its fragmentation to
a form similar to the reference material). Another solution may be a recovery test
carried out not only for a reference material but also for test samples.
In the case of clinical specimens, commonly available materials are usually in a
lyophilized form (e.g., blood serum), as the test samples obtained from patients are
usually delivered without being lyophilized. Of course, the freeze-dried material of
the RM is subjected to a reconstruction of the liquid state by adding an appropriate
volume of water, but certainly, afterwards the behavior of serum components could be
different. The potential impact of the freeze-drying on the result should be considered.
A useful reference material should be as close as possible in terms of chemical
composition to the matrix as well as in terms of the physical form of the test
samples.
6.9 The Content of the Substance to Be Determined
in the Reference Material and the Sample
The basic requirement is for the concentration of the substance in the reference
material to be similar to the concentration that is present in the analyzed test samples.
In practice, this is not always possible; often the available RMs have a much higher
content of the substance and rarely much lower. Some manufacturers recommend
the dilution of the prepared solution in such cases, but this will decrease not only
the concentration of the substance to be determined but also the concentration of the
matrix components, which can seriously affect the measurement result.
Another problem is the chemical form of the substance to be determined. This
is due to two reasons: first, for different chemical forms, extraction efficiency may
vary; second, in a number of measuring techniques, the detector response depends on
the chemical form of the analyte. For example, for the determination of arsenic and
selenium, by atomic absorption spectrometry with hydride generation, the efficiency
of the reduction depends on the original oxidation state of the element (e.g., As(III)
and As(V), and Se(IV) and Se(VI)). Another example is the determination of mercury
by ICP MS (Inductively Coupled Plasma Mass Spectrometry); apart from a very high
temperature of plasma, efficiency of the ionization of mercury present in the form of
phenylmercury is significantly lower than the efficiency of ionization of the mercury
present in the sample in the form of an inorganic ions, and this affects the number of
ions reaching the mass detector. Such examples are numerous.