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5 Qualitative Aspects of Analytical Chemistry
formation required is purely qualitative (e.g. the presence or absence of certain
pesticides in lemons). On the other hand, the most complicated situation is that
where the nature and origin of the sample are unknown and a comprehensive
qualitative report of its composition is required.
The primary purpose of analytical schemes is to ensure maximal reliability in
the qualitative test conducted on each analyte, which entails assuring adequate
sensitivity and selectivity. No doubt, separation techniques playa central role in
these processes. Below are briefly discussed the two most widely used types of
analytical schemes.
I 5.5.3.1 Schemes without Group Separation
In this type of scheme, each qualitative test is applied to a separate aliquot of the
original sample. Identification thus relies in the sequential use of direct tests
- some, however, may include a specific separation if needed. They lie in between
the direct and mixed options depicted in Fig. 5.8.
This type of analytical scheme has three salient technical features, namely:
(a) It uses highly sensitive and selective reagents.
(b) It is conducted in a strict operational sequence: from tests involving reagents
of a high sensitivity and/or selectivity (a near-ideal situation) to others
based on moderately sensitive and selective reagents.
(c) It applies separation techniques to individual sample aliquots when the
information provided by previous qualitative tests suggests the presence of
interferences with that being carried out. Obviously, previously identified
analytes should not interfere with those to be tackled later in the sequence.
Figure 5.9 illustrates the operating procedure for an analytical scheme without
group separation that possesses the above technical features. The first two tests
are based on two ideal reagents (R 1 and R2 ) that allow the first two analytes to be
reliably identified in a direct manner. The third test is interfered by one or both
of the previous analytes, so it requires the addition of masking ligand (ML 3 )
prior to the identification reagent proper (R 3 ). The fourth test entails the prior
separation of the first three analytes by using an appropriate reagent (SR 4 ) before the new identification reagent (R 4 ) is added to detect the fourth analyte.
Finally, the fifth test requires both separation (SR s) and the use of a masking
reagent (MLs) in order to avoid disturbances to the identification reaction with
reagent Rs .
As can be seen, the qualitative tests performed grow in complexity as the
scheme progresses, to an extent that depends on the number of species present
in the sample. Thus, the separation in the fourth test will be unnecessary if the
first, second and third tests are negative.
These schemes are labour-intensive, time-consuming and complex when the
number of analytes is relatively large. However, they are technically efficient and
affordable for white and grey samples containing a limited number of analytes,
and also in those cases where highly sensitive and selective reagents are available.
5 Qualitative Aspects of Analytical Chemistry
formation required is purely qualitative (e.g. the presence or absence of certain
pesticides in lemons). On the other hand, the most complicated situation is that
where the nature and origin of the sample are unknown and a comprehensive
qualitative report of its composition is required.
The primary purpose of analytical schemes is to ensure maximal reliability in
the qualitative test conducted on each analyte, which entails assuring adequate
sensitivity and selectivity. No doubt, separation techniques playa central role in
these processes. Below are briefly discussed the two most widely used types of
analytical schemes.
I 5.5.3.1 Schemes without Group Separation
In this type of scheme, each qualitative test is applied to a separate aliquot of the
original sample. Identification thus relies in the sequential use of direct tests
- some, however, may include a specific separation if needed. They lie in between
the direct and mixed options depicted in Fig. 5.8.
This type of analytical scheme has three salient technical features, namely:
(a) It uses highly sensitive and selective reagents.
(b) It is conducted in a strict operational sequence: from tests involving reagents
of a high sensitivity and/or selectivity (a near-ideal situation) to others
based on moderately sensitive and selective reagents.
(c) It applies separation techniques to individual sample aliquots when the
information provided by previous qualitative tests suggests the presence of
interferences with that being carried out. Obviously, previously identified
analytes should not interfere with those to be tackled later in the sequence.
Figure 5.9 illustrates the operating procedure for an analytical scheme without
group separation that possesses the above technical features. The first two tests
are based on two ideal reagents (R 1 and R2 ) that allow the first two analytes to be
reliably identified in a direct manner. The third test is interfered by one or both
of the previous analytes, so it requires the addition of masking ligand (ML 3 )
prior to the identification reagent proper (R 3 ). The fourth test entails the prior
separation of the first three analytes by using an appropriate reagent (SR 4 ) before the new identification reagent (R 4 ) is added to detect the fourth analyte.
Finally, the fifth test requires both separation (SR s) and the use of a masking
reagent (MLs) in order to avoid disturbances to the identification reaction with
reagent Rs .
As can be seen, the qualitative tests performed grow in complexity as the
scheme progresses, to an extent that depends on the number of species present
in the sample. Thus, the separation in the fourth test will be unnecessary if the
first, second and third tests are negative.
These schemes are labour-intensive, time-consuming and complex when the
number of analytes is relatively large. However, they are technically efficient and
affordable for white and grey samples containing a limited number of analytes,
and also in those cases where highly sensitive and selective reagents are available.