5.5 Limitations of the Standard Addition Method
61
– The chemical form or the behaviour of the added analyte are not always identical
to that of the analyte present originally in the test sample.
– The standard addition method does not eliminate the errors resulting from the
presence of additive interferences, causing the parallel shift of the analytical graph
as well as those caused by human error.
5.6 The Method of the Internal Standard
In analytical practice, results depend largely on the measurement’s parameters as
well as on the composition of the sample. For the calibration approaches described
above, the assumption was made that the quantity (mass or volume) of standard or test
sample was accurately known and the parameters that affect the overall measurement
procedure were constant. There are, hoverer several analytical procedures where
those assumption are not valid, it is not possible to ensure the quantity of the sample
taken for measurements or it is not possible to guarantee the stability of all parameters
involves.
In such cases, the methodology that could overcome these problems is the method
of internal standard. In this method, the response of the detector (analytical signal)
given by the analyte of interest is compared with that given by another element or
compound of known concentration (named the ‘internal standard’), which is present
in the sample. Although the internal standard may already be present in the sample in its original constitution, it is often added to the sample before performing
measurements.
The method of internal standard offers very high accuracy and precision, mainly
due to the fact that the analytical signal for analyte and standard are both
measured in the same portion of the sample and at the same time.
In fact, the method of internal standard involves tracking the signal of the internal
standard, simultaneously with the signal of the analyte, which allows for compensation of random errors of the measuring system and/or its variation over time and
changes in the composition and/or physical properties of the sample.
As an internal standard, a substance is chosen that does not react chemically with
the sample (in case it is added to the sample), nor interfere in any way with the
analyte. In contrast to the standard addition method, knowledge of the content of the
substance acting as the internal standard is not essential. During the measurements, a
monitored signal is obtained for the internal standard, and assuming its concentration
is constant, it is believed that any change in the value of the signal indicates a change in
the measurement conditions. Another assumption is that the change in the conditions
affects to the same extent the signal of the substance acting as an internal standard,
and the signal from the presence of the analyte. The calibration dependence, in this
61
– The chemical form or the behaviour of the added analyte are not always identical
to that of the analyte present originally in the test sample.
– The standard addition method does not eliminate the errors resulting from the
presence of additive interferences, causing the parallel shift of the analytical graph
as well as those caused by human error.
5.6 The Method of the Internal Standard
In analytical practice, results depend largely on the measurement’s parameters as
well as on the composition of the sample. For the calibration approaches described
above, the assumption was made that the quantity (mass or volume) of standard or test
sample was accurately known and the parameters that affect the overall measurement
procedure were constant. There are, hoverer several analytical procedures where
those assumption are not valid, it is not possible to ensure the quantity of the sample
taken for measurements or it is not possible to guarantee the stability of all parameters
involves.
In such cases, the methodology that could overcome these problems is the method
of internal standard. In this method, the response of the detector (analytical signal)
given by the analyte of interest is compared with that given by another element or
compound of known concentration (named the ‘internal standard’), which is present
in the sample. Although the internal standard may already be present in the sample in its original constitution, it is often added to the sample before performing
measurements.
The method of internal standard offers very high accuracy and precision, mainly
due to the fact that the analytical signal for analyte and standard are both
measured in the same portion of the sample and at the same time.
In fact, the method of internal standard involves tracking the signal of the internal
standard, simultaneously with the signal of the analyte, which allows for compensation of random errors of the measuring system and/or its variation over time and
changes in the composition and/or physical properties of the sample.
As an internal standard, a substance is chosen that does not react chemically with
the sample (in case it is added to the sample), nor interfere in any way with the
analyte. In contrast to the standard addition method, knowledge of the content of the
substance acting as the internal standard is not essential. During the measurements, a
monitored signal is obtained for the internal standard, and assuming its concentration
is constant, it is believed that any change in the value of the signal indicates a change in
the measurement conditions. Another assumption is that the change in the conditions
affects to the same extent the signal of the substance acting as an internal standard,
and the signal from the presence of the analyte. The calibration dependence, in this