5.5 Classical Qualitative Analysis
217
The qualitative CMP of choice differs widely depending on whether a single
analyte (e.g. clenbuterol in beef for human consumption), a compound family
(e.g. atmospheric aromatic hydrocarbons), a small group of species (e.g. pesticides in oranges) or a wide range of analytes (e.g. metals and non-metals in
rocks) is to be identified. The process grows in complexity from white to grey to
black samples (see Sect. 5.1). In fact, some black samples (i. e. samples of absolutely
unknown composition) may contain scores of analytes.
I 5.5.2 Types of Reagents
The CMP of choice for a classical qualitative analysis is also dictated by the nature
of the analytes involved (inorganic, organic, biochemical), which is thus one other
possible classification criterion for Classical Qualitative Analysis (see Fig. 5.7).
Analytes can be identified by using reagents of varied nature. Selectivity in a
qualitative CMP increases and the ideal situation of Fig. 5.8 is approached in the
following sequence: inorganic < organic < biochemical < immune reagents. The
many possible combinations of analyte and reagent types result in a wide range
of situations of variable complexity (the more similar the analytes in a mixture
are the more complicated will be their identification). In fact, such combinations
dictate the type of identification reaction to be used in each case. Thus, inorganic
analyte-inorganic reagent combinations typically involve a precipitation, redox
or complex-formation reaction; inorganic analyte-organic reagent couples
usually require a coloured or fluorescent chelate formation reaction; in the
organic analyte-organic reagent and biochemical analyte-biochemical reagent
combinations, the identification reaction is invariably of the organic and
biochemical type, respectively.
The specific purpose of a reagent in Classical Qualitative Analysis can be
widely variable. Figure 5.7 shows the three principal types of reagents used in
this context.
Group reagents are intended to effect the separations described in Fig. 5.8.
Most often, they rely on separation by precipitation (the reagent causes the
formation of insoluble products), liquid-liquid extraction (the reagent gives
hydrophobic products that can be readily extracted into organic solvents) or ion
exchange (the reagent itself acts as the exchange material and selectively retains
one or more analytes under specific experimental conditions). The purpose of
group reagents is to isolate species in groups in such a way that each individual
analyte can be ultimately identified with a high reliability.
Group reagents must meet some general requirements, namely:
(a) They should isolate every single analyte belonging to each group. Also, the
reaction used to identify each individual analyte within the group should be
adequately sensitive and selective.
(b) The separation should be selective: species not belonging to the particular
group should remain in the original phase.
(c) Excess reagent and the second (added) phase should not interfere with
subsequent identification reactions.
217
The qualitative CMP of choice differs widely depending on whether a single
analyte (e.g. clenbuterol in beef for human consumption), a compound family
(e.g. atmospheric aromatic hydrocarbons), a small group of species (e.g. pesticides in oranges) or a wide range of analytes (e.g. metals and non-metals in
rocks) is to be identified. The process grows in complexity from white to grey to
black samples (see Sect. 5.1). In fact, some black samples (i. e. samples of absolutely
unknown composition) may contain scores of analytes.
I 5.5.2 Types of Reagents
The CMP of choice for a classical qualitative analysis is also dictated by the nature
of the analytes involved (inorganic, organic, biochemical), which is thus one other
possible classification criterion for Classical Qualitative Analysis (see Fig. 5.7).
Analytes can be identified by using reagents of varied nature. Selectivity in a
qualitative CMP increases and the ideal situation of Fig. 5.8 is approached in the
following sequence: inorganic < organic < biochemical < immune reagents. The
many possible combinations of analyte and reagent types result in a wide range
of situations of variable complexity (the more similar the analytes in a mixture
are the more complicated will be their identification). In fact, such combinations
dictate the type of identification reaction to be used in each case. Thus, inorganic
analyte-inorganic reagent combinations typically involve a precipitation, redox
or complex-formation reaction; inorganic analyte-organic reagent couples
usually require a coloured or fluorescent chelate formation reaction; in the
organic analyte-organic reagent and biochemical analyte-biochemical reagent
combinations, the identification reaction is invariably of the organic and
biochemical type, respectively.
The specific purpose of a reagent in Classical Qualitative Analysis can be
widely variable. Figure 5.7 shows the three principal types of reagents used in
this context.
Group reagents are intended to effect the separations described in Fig. 5.8.
Most often, they rely on separation by precipitation (the reagent causes the
formation of insoluble products), liquid-liquid extraction (the reagent gives
hydrophobic products that can be readily extracted into organic solvents) or ion
exchange (the reagent itself acts as the exchange material and selectively retains
one or more analytes under specific experimental conditions). The purpose of
group reagents is to isolate species in groups in such a way that each individual
analyte can be ultimately identified with a high reliability.
Group reagents must meet some general requirements, namely:
(a) They should isolate every single analyte belonging to each group. Also, the
reaction used to identify each individual analyte within the group should be
adequately sensitive and selective.
(b) The separation should be selective: species not belonging to the particular
group should remain in the original phase.
(c) Excess reagent and the second (added) phase should not interfere with
subsequent identification reactions.