6.15 Summary
85
Before purchasing the material, it is worth checking whether the material has the
status of a CRM or RM. Commercially available control materials are very useful in
laboratory practice but do not have the status of a CRM.
It is also important to check whether the material is of natural origin or whether
it has been prepared synthetically. One should also examine whether the analyte is
present naturally or was added during the processing of the RM. It is also worth
paying attention to the content of an analyte; often the content differs significantly
from that expected in the sample tested in the laboratory.
From the description of the material, it is important to seek out information regarding the intended use of the material and how to ensure a reference value is obtained.
Some CRMs are designed to recover the reference value through performing studies
using the well-defined and specified analytical procedure. This can include sample
preparation (e.g., the digestion of the soil or extraction using various media: aqua
regia, hydrochloric acid, acetate buffer), and it may concern specific measurement
techniques.
It is also important to known what the minimum amount of sample is for reproducing the reference value in terms of its value and uncertainty. Sometimes materials
are sought for having superior homogeneity—in order to be able to apply as smallest aliquots. However, sometimes the most suitable materials are those mimic real
samples, such as a grain size exceeding 1 mm and a homogeneity of no less than 1 g.
It is also important to evaluate how the certification is carried out. Frequently, certification is performed by statistical evaluation of the results from expert laboratories.
By contrast, there are also materials for which the reference value was determined
by the primary method (e.g., INAA or ID-MS).
Some materials requiring special storage are provided in appropriate packaging
and under suitable conditions. If the certificate contains storage conditions that must
be followed, the reference value is guaranteed only if the user follows the given
recommendations; RMPs guarantee the stability of the material and the stability of
the reference value during the validity of the material (shelf life). These properties
are guaranteed until the opening of the package; thus, after opening, the user is
required to use and store the material in accordance with guidelines provided. After
opening, the packaging manufacturer is no longer responsible for the stability of the
material. This does not mean automatically that the unused material (IMPORTANT:
that remaing in the package!) is not suitable for use. The material is useful so long
as it is kept under the appropriate conditions and used according to good laboratory
practice.
Specified by the RMPs, the uncertainty of the reference values means that the
value taken as certified value is in the range of assumed probability, usually 95%.
The reference value for the properties, along with the assigned uncertainty, is taken
as such when the CRM is used for calibration of the measuring instrument. If the
CRM is used to evaluate the bias, the uncertainty is used for the evaluation whether it
overlaps with the uncertainty associated with the value determined in the laboratory.
The structure of RM certificates (Table 6.1) is designed by the producer; however,
various essential information should be always included—namely, certified value(s)
accompanied by the uncertainty of properties in a given matrix (i.e. the type of
sample).
85
Before purchasing the material, it is worth checking whether the material has the
status of a CRM or RM. Commercially available control materials are very useful in
laboratory practice but do not have the status of a CRM.
It is also important to check whether the material is of natural origin or whether
it has been prepared synthetically. One should also examine whether the analyte is
present naturally or was added during the processing of the RM. It is also worth
paying attention to the content of an analyte; often the content differs significantly
from that expected in the sample tested in the laboratory.
From the description of the material, it is important to seek out information regarding the intended use of the material and how to ensure a reference value is obtained.
Some CRMs are designed to recover the reference value through performing studies
using the well-defined and specified analytical procedure. This can include sample
preparation (e.g., the digestion of the soil or extraction using various media: aqua
regia, hydrochloric acid, acetate buffer), and it may concern specific measurement
techniques.
It is also important to known what the minimum amount of sample is for reproducing the reference value in terms of its value and uncertainty. Sometimes materials
are sought for having superior homogeneity—in order to be able to apply as smallest aliquots. However, sometimes the most suitable materials are those mimic real
samples, such as a grain size exceeding 1 mm and a homogeneity of no less than 1 g.
It is also important to evaluate how the certification is carried out. Frequently, certification is performed by statistical evaluation of the results from expert laboratories.
By contrast, there are also materials for which the reference value was determined
by the primary method (e.g., INAA or ID-MS).
Some materials requiring special storage are provided in appropriate packaging
and under suitable conditions. If the certificate contains storage conditions that must
be followed, the reference value is guaranteed only if the user follows the given
recommendations; RMPs guarantee the stability of the material and the stability of
the reference value during the validity of the material (shelf life). These properties
are guaranteed until the opening of the package; thus, after opening, the user is
required to use and store the material in accordance with guidelines provided. After
opening, the packaging manufacturer is no longer responsible for the stability of the
material. This does not mean automatically that the unused material (IMPORTANT:
that remaing in the package!) is not suitable for use. The material is useful so long
as it is kept under the appropriate conditions and used according to good laboratory
practice.
Specified by the RMPs, the uncertainty of the reference values means that the
value taken as certified value is in the range of assumed probability, usually 95%.
The reference value for the properties, along with the assigned uncertainty, is taken
as such when the CRM is used for calibration of the measuring instrument. If the
CRM is used to evaluate the bias, the uncertainty is used for the evaluation whether it
overlaps with the uncertainty associated with the value determined in the laboratory.
The structure of RM certificates (Table 6.1) is designed by the producer; however,
various essential information should be always included—namely, certified value(s)
accompanied by the uncertainty of properties in a given matrix (i.e. the type of
sample).