214
5 Qualitative Aspects of Analytical Chemistry
I 5.4 Standards and Calibration in Qualitative Analysis
The two types of calibration depicted in Fig. 3.10 are quite applicable to Qualitative Analysis.
Equipment calibration, based on standards not containing the analyte, is
done to ensure correct functioning of the identification instrument. Thus, polyethylene film (a reference material, RM) is typically used to calibrate IR absorption spectrophotometers. The film spectrum should coincide with that supplied
by the manufacturer; otherwise, the instrument should be recalibrated. A solid
fluorophore (e.g. rhodamine) dispersed in a transparent plastic block is employed to check that the two monochromators of a fluorimeter operate to
specification.
Method calibration uses standards containing the analyte to establish an
unambiguous relationship between some physico-chemical property of the
analyte (or its reaction product) and the signals provided by a pre-calibrated
instrument under specific operating conditions. The identification involved in
qualitative analyses relies on standards that are used in two different ways for
method calibration, namely (see Box 3.13):
(a) Calibration based on the relationship between the signal and properties of
the analyte (or its product). Such is the case with identification in Classical
Qualitative Analysis: the product of a chemical reaction (a colourless
species, gas or solid) is previously identified by the operator using a
standard of pure analyte. Once the features (colour, odour, texture) of the
product have been recorded by the brain, the identification test is performed
on a sample aliquot; whether the new product is identical with that
previously obtained from the standard is decided upon by the analyst (see
Box 1.5). In Instrumental Qualitative Analysis, the standard provides two
data sets (instrument parameters and signals) for comparison (see Box 3.9).
The greater the number of data used and the more similar they are, the more
reliable will be the determination. Comparisons can be made visually or
with the aid of a computer; the latter allows the sample spectrum to be automatically checked against those for a wide range of standards included in a
so-called "spectral library". The identification can be made even more
reliable by using a derivative (first, second, etc.) of the original signal; this
expands the initially available information with new bands and diminishes
background noise.
(b) Calibration based on the behaviour of the analyte (or standard) in a dynamic instrumental system that produces signals of temporal or spatial
dimension. Thus, in a column chromatographic system (capillary electrophoresis and liquid, gas or supercritical fluid chromatography), the analyte
is identified from its retention time; by contrast, in planar chromatography
and classical electrophoresis, identification relies on the distance travelled
(migrated) by the analyte. The use of standards in this context is described
in detail in Sect. 5.6.3.
5 Qualitative Aspects of Analytical Chemistry
I 5.4 Standards and Calibration in Qualitative Analysis
The two types of calibration depicted in Fig. 3.10 are quite applicable to Qualitative Analysis.
Equipment calibration, based on standards not containing the analyte, is
done to ensure correct functioning of the identification instrument. Thus, polyethylene film (a reference material, RM) is typically used to calibrate IR absorption spectrophotometers. The film spectrum should coincide with that supplied
by the manufacturer; otherwise, the instrument should be recalibrated. A solid
fluorophore (e.g. rhodamine) dispersed in a transparent plastic block is employed to check that the two monochromators of a fluorimeter operate to
specification.
Method calibration uses standards containing the analyte to establish an
unambiguous relationship between some physico-chemical property of the
analyte (or its reaction product) and the signals provided by a pre-calibrated
instrument under specific operating conditions. The identification involved in
qualitative analyses relies on standards that are used in two different ways for
method calibration, namely (see Box 3.13):
(a) Calibration based on the relationship between the signal and properties of
the analyte (or its product). Such is the case with identification in Classical
Qualitative Analysis: the product of a chemical reaction (a colourless
species, gas or solid) is previously identified by the operator using a
standard of pure analyte. Once the features (colour, odour, texture) of the
product have been recorded by the brain, the identification test is performed
on a sample aliquot; whether the new product is identical with that
previously obtained from the standard is decided upon by the analyst (see
Box 1.5). In Instrumental Qualitative Analysis, the standard provides two
data sets (instrument parameters and signals) for comparison (see Box 3.9).
The greater the number of data used and the more similar they are, the more
reliable will be the determination. Comparisons can be made visually or
with the aid of a computer; the latter allows the sample spectrum to be automatically checked against those for a wide range of standards included in a
so-called "spectral library". The identification can be made even more
reliable by using a derivative (first, second, etc.) of the original signal; this
expands the initially available information with new bands and diminishes
background noise.
(b) Calibration based on the behaviour of the analyte (or standard) in a dynamic instrumental system that produces signals of temporal or spatial
dimension. Thus, in a column chromatographic system (capillary electrophoresis and liquid, gas or supercritical fluid chromatography), the analyte
is identified from its retention time; by contrast, in planar chromatography
and classical electrophoresis, identification relies on the distance travelled
(migrated) by the analyte. The use of standards in this context is described
in detail in Sect. 5.6.3.