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9 Managing the Quality Systems
9.7 The Frequency of Internal Control
There are no explicit and general rules regarding the frequency of the use of control
samples in a given measurement series. This should always be evaluated individually,
regarding the type of routine test samples, the number of items analyzed, and previous
experience regarding the stability of the analytical system. The analyst is the one
who should decide on the proper sequence of measuring the control samples. It is
important that the measurements of control samples are executed so as to accompany
the routine test samples.
When tailoring the frequency of the internal control, it is worth taking the following under consideration:
– At least one control sample should be used in a given measurement batch;
– The order of control samples in the series of test samples should be random, bearing
in mind the recommendation that the control should be executed at the beginning
and at the end of the measurement batch;
– In cases where the content of the determined substance varies significantly for the
routine test samples, it is recommended that two or more control samples be used
for different levels of concentration;
– In routine measurements, often at least one control sample is used in the batch of
20 test samples (certainty level 5%);
– In screening tests, often one sample is used for every 40 or 50 laboratory samples.
9.8 Control Samples
As mentioned previously, items used as control samples could be of different origin,
including matrix RMs, chemical standards, laboratory RMs, routine test samples,
blanks, to list some of the most commonly used. In a given measurement procedure,
either the single control sample can be selected, or the set of control samples are
used to monitor various features of the measurement process.
Bellow, the most commonly used control samples is described.
9.8.1 Certified Reference Materials and Chemical Standards
Those are the most commonly used for checking any systematic effects as well as
for estimating the random variation of the measuring system. Chemical standards of
known purity could be prepared by laboratory or could be obtained from the suppliers
of chemicals; CRMs are usually supply by the qualified producers, both should be
of known purity and known content of the substance of interest.
9 Managing the Quality Systems
9.7 The Frequency of Internal Control
There are no explicit and general rules regarding the frequency of the use of control
samples in a given measurement series. This should always be evaluated individually,
regarding the type of routine test samples, the number of items analyzed, and previous
experience regarding the stability of the analytical system. The analyst is the one
who should decide on the proper sequence of measuring the control samples. It is
important that the measurements of control samples are executed so as to accompany
the routine test samples.
When tailoring the frequency of the internal control, it is worth taking the following under consideration:
– At least one control sample should be used in a given measurement batch;
– The order of control samples in the series of test samples should be random, bearing
in mind the recommendation that the control should be executed at the beginning
and at the end of the measurement batch;
– In cases where the content of the determined substance varies significantly for the
routine test samples, it is recommended that two or more control samples be used
for different levels of concentration;
– In routine measurements, often at least one control sample is used in the batch of
20 test samples (certainty level 5%);
– In screening tests, often one sample is used for every 40 or 50 laboratory samples.
9.8 Control Samples
As mentioned previously, items used as control samples could be of different origin,
including matrix RMs, chemical standards, laboratory RMs, routine test samples,
blanks, to list some of the most commonly used. In a given measurement procedure,
either the single control sample can be selected, or the set of control samples are
used to monitor various features of the measurement process.
Bellow, the most commonly used control samples is described.
9.8.1 Certified Reference Materials and Chemical Standards
Those are the most commonly used for checking any systematic effects as well as
for estimating the random variation of the measuring system. Chemical standards of
known purity could be prepared by laboratory or could be obtained from the suppliers
of chemicals; CRMs are usually supply by the qualified producers, both should be
of known purity and known content of the substance of interest.