5.6 Instrumental Qualitative Analysis
231
From Box 5.15 it follows that not all instruments are equally capable of providing reliable identifications; in fact, reliability in this context depends on the
selectivity and information content of the responses they produce. Instrumental
analytical techniques can be ranked in the three broad categories of Fig. 5.11 in
this respect - the ranking, however, can obviously have some exceptions.
The instruments in Group 1 provide a very general response (i. e. one shared
by many analytes). Such is the case with the mass measured by a balance or a piezoelectric sensor. This type of information is useless for qualitative analyses
unless altered in some way to increase its selectivity (e. g. in thermogravimetries,
which provide mass-temperature two-dimensional information, and in using
piezoelectric sensors coated with selective sorbent materials).
The instruments in Group 2 give a scarcely selective response (i. e. one that
can be subject to many interferences from sample components other than the
analyte). One case in point is UV-visible absorption spectroscopy (photometry),
where virtually every species exhibits absorption of incident light. The selectivity of this technique can be improved by derivatizing the analytes to products
with differential spectral features. Alternatively, more selective techniques such
as fluorimetry ensure higher reliability (fluorescence is a much more uncommon property than is molecular absorption). Even greater selectivity can be
achieved from on-line combinations of instruments in this group with continuous column chromatographic separation techniques (see Fig. 4.14); the poor
selectivity of the instrument is offset by the high selectivity of the continuous
separation, which isolates the analytes within dynamic zones.
Finally, the instruments in Group 3 provide highly selective information (e. g.
that of atomic absorption and emission spectroscopies) or information containing many well-resolved signals obtained at multiple instrumental parameter
values (e. g. those of IR spectroscopy or mass spectrometry). The use of this type
of instrument as detector in chromatographic techniques has given rise to socalled "hyphenated techniques", which provide significantly increased reliability in the identification. Reliability can also be substantially improved by the joint
use of information provided by one instrument each from Groups 2 and 3 (or
two from Group 3). One typical example is the highly reliable (>99%) identification of analytes in a sample from its combined IR and mass spectra. However,
the highest level of reliability is provided by the combination of a separation
technique and two or more instruments from Group 2 or, better, Group 3 (e. g. the
gas chromatography-mass spectrometry-IR spectroscopy tandem).
The detailed description of each individual technique used for qualitative
purposes that would be required to provide readers with a comprehensive
picture of the subject is beyond the scope of this introductory textbook. Instead,
the following sections discuss specific examples involving time-dependent and
time-independent signals that are intended to provide the reader with a clear
view of the situation.
231
From Box 5.15 it follows that not all instruments are equally capable of providing reliable identifications; in fact, reliability in this context depends on the
selectivity and information content of the responses they produce. Instrumental
analytical techniques can be ranked in the three broad categories of Fig. 5.11 in
this respect - the ranking, however, can obviously have some exceptions.
The instruments in Group 1 provide a very general response (i. e. one shared
by many analytes). Such is the case with the mass measured by a balance or a piezoelectric sensor. This type of information is useless for qualitative analyses
unless altered in some way to increase its selectivity (e. g. in thermogravimetries,
which provide mass-temperature two-dimensional information, and in using
piezoelectric sensors coated with selective sorbent materials).
The instruments in Group 2 give a scarcely selective response (i. e. one that
can be subject to many interferences from sample components other than the
analyte). One case in point is UV-visible absorption spectroscopy (photometry),
where virtually every species exhibits absorption of incident light. The selectivity of this technique can be improved by derivatizing the analytes to products
with differential spectral features. Alternatively, more selective techniques such
as fluorimetry ensure higher reliability (fluorescence is a much more uncommon property than is molecular absorption). Even greater selectivity can be
achieved from on-line combinations of instruments in this group with continuous column chromatographic separation techniques (see Fig. 4.14); the poor
selectivity of the instrument is offset by the high selectivity of the continuous
separation, which isolates the analytes within dynamic zones.
Finally, the instruments in Group 3 provide highly selective information (e. g.
that of atomic absorption and emission spectroscopies) or information containing many well-resolved signals obtained at multiple instrumental parameter
values (e. g. those of IR spectroscopy or mass spectrometry). The use of this type
of instrument as detector in chromatographic techniques has given rise to socalled "hyphenated techniques", which provide significantly increased reliability in the identification. Reliability can also be substantially improved by the joint
use of information provided by one instrument each from Groups 2 and 3 (or
two from Group 3). One typical example is the highly reliable (>99%) identification of analytes in a sample from its combined IR and mass spectra. However,
the highest level of reliability is provided by the combination of a separation
technique and two or more instruments from Group 2 or, better, Group 3 (e. g. the
gas chromatography-mass spectrometry-IR spectroscopy tandem).
The detailed description of each individual technique used for qualitative
purposes that would be required to provide readers with a comprehensive
picture of the subject is beyond the scope of this introductory textbook. Instead,
the following sections discuss specific examples involving time-dependent and
time-independent signals that are intended to provide the reader with a clear
view of the situation.