5.3 Types of Qualitative Identification
213
(b) The signals provided by the analytical techniques used in a qualitative CMP
can be produced by the analyte itself or the product of a chemical, enzymatic or immune reaction, for example (Fig. 5.6, 3). This substep of the CMP
boosts sensitivity and selectivity in the identification. For example, the
bluish colour of the cupric ammine, Cu(NH3W, is too light to be easily
detected and can be masked by other, stronger colours. The addition of a
ligand (cuproine) to form a bright-red, soluble chelate (CuLi), dramatically
increases the sensitivity and selectivity of the identification, which is thus
made much more reliable.
(c) The need to obtain an acceptably reliable binary response has led to including a separation method in many qualitative CMPs. The characteristics
and uses of separation methods are discussed in Sect. 4.3.4. Classical
Qualitative Analysis uses non-chromatographic methodologies such as
precipitation, liquid-liquid extraction or ion exchange. On the other hand,
Instrumental Qualitative Analysis usually employs chromatographic approaches (gas, liquid and supercritical fluid chromatography) involving the
use of a detector (instrument) to continuously monitor the signal produced
by the eluent that emerges from the separation column (Fig. 5.6,4).
(d) Qualitative CMPs can be conducted manually (e. g. by visually inspecting the
formation of a precipitate and/or color, by comparing IR spectra), semiautomatically (e.g. when locating the retention time for an analyte in a
chromatogram with the aid of an electronic integrator or computer) or fully
automatically (e.g. in spectral searches of computer libraries) (see Fig. 5.6, 5).
(e) The reliability of the binary response additionally depends on the dimension of the information supplied by the CMP (Fig. 5.6,6). Some instruments
provide the values of one, two, three or even more parameters; also, signals
can be single or multiple in number. The more raw data (parameter values
and signals) are available and the better discriminated they are, the more
reliable will be the identification. Thus, Classical Qualitative Analysis uses a
multidetection system (the human senses and brain) and one, two or more
signals for identification (e.g. the formation of a precipitate, this and the
precipitate color, the previous two and the precipitate texture, etc.). On the
other hand, the identification of a metal ion present in trace amounts in
water entails using atomic absorption spectrometry: a signal at a preset
wavelength (i.e. an instrumental parameter). A fluorescent analyte (or its
reaction product) is identified fluorimetrically, i. e. from a fluorescence
intensity signal obtained at two different instrumental parameter values
(the excitation and emission wavelengths). The body of spectra provided by
instrumental techniques (IR spectroscopy, mass spectrometry, nuclear
magnetic resonance) for a given species - or a suitable combination of the
information thus gathered - makes a powerful, highly reliable identification
tool on account of the many signals and instrumental parameters it encompasses.
213
(b) The signals provided by the analytical techniques used in a qualitative CMP
can be produced by the analyte itself or the product of a chemical, enzymatic or immune reaction, for example (Fig. 5.6, 3). This substep of the CMP
boosts sensitivity and selectivity in the identification. For example, the
bluish colour of the cupric ammine, Cu(NH3W, is too light to be easily
detected and can be masked by other, stronger colours. The addition of a
ligand (cuproine) to form a bright-red, soluble chelate (CuLi), dramatically
increases the sensitivity and selectivity of the identification, which is thus
made much more reliable.
(c) The need to obtain an acceptably reliable binary response has led to including a separation method in many qualitative CMPs. The characteristics
and uses of separation methods are discussed in Sect. 4.3.4. Classical
Qualitative Analysis uses non-chromatographic methodologies such as
precipitation, liquid-liquid extraction or ion exchange. On the other hand,
Instrumental Qualitative Analysis usually employs chromatographic approaches (gas, liquid and supercritical fluid chromatography) involving the
use of a detector (instrument) to continuously monitor the signal produced
by the eluent that emerges from the separation column (Fig. 5.6,4).
(d) Qualitative CMPs can be conducted manually (e. g. by visually inspecting the
formation of a precipitate and/or color, by comparing IR spectra), semiautomatically (e.g. when locating the retention time for an analyte in a
chromatogram with the aid of an electronic integrator or computer) or fully
automatically (e.g. in spectral searches of computer libraries) (see Fig. 5.6, 5).
(e) The reliability of the binary response additionally depends on the dimension of the information supplied by the CMP (Fig. 5.6,6). Some instruments
provide the values of one, two, three or even more parameters; also, signals
can be single or multiple in number. The more raw data (parameter values
and signals) are available and the better discriminated they are, the more
reliable will be the identification. Thus, Classical Qualitative Analysis uses a
multidetection system (the human senses and brain) and one, two or more
signals for identification (e.g. the formation of a precipitate, this and the
precipitate color, the previous two and the precipitate texture, etc.). On the
other hand, the identification of a metal ion present in trace amounts in
water entails using atomic absorption spectrometry: a signal at a preset
wavelength (i.e. an instrumental parameter). A fluorescent analyte (or its
reaction product) is identified fluorimetrically, i. e. from a fluorescence
intensity signal obtained at two different instrumental parameter values
(the excitation and emission wavelengths). The body of spectra provided by
instrumental techniques (IR spectroscopy, mass spectrometry, nuclear
magnetic resonance) for a given species - or a suitable combination of the
information thus gathered - makes a powerful, highly reliable identification
tool on account of the many signals and instrumental parameters it encompasses.