Chemistry Reference
In-Depth Information
(columns) or even working techniques (e.g. isocratic or gradient elution) were tested, which was time-consuming
and involved consumption of expensive solvents.
The advent of multi-dimensional detection systems and the affordability of personal computers provides
with software that allows storage and subsequent processing of chromatographic data has fostered
developments of new experimental procedure for the characterization of unresolved peaks. There are four
basic alternatives to computer-assisted resolution of chromatographic peaks, namely [82]:
1. Fitting the chromatogram peak to known functions. There are some precedents to the use of this alternative
in gas chromatography ranging for the use of Gaussian and non-Gaussian models, to convoluted Gaussian
curves with exponential decay and fast Fourier transform techniques. Solutions involving comparison of
logarithmic spectra or the use of chemometric methods [83, 84] have been tested in liquid chromatography
as implemented with diode-array detectors.
2. Integration by tracing a line perpendicular to the baseline from the valley between two peaks or one
joining the valley and the end of the second peak (skimming) by computing the area of each separately
from the two zones thus established.
3. Use of derivative techniques. As far as practical applications, the information supplied by derivative
spectroscopy used as detection system in liquid chromatography has been exploited in two different
ways, namely:
a. By using the complete derivative obtained using the first derivative of the elution profile obtained at
the wavelength of the absorption maximum. In theory, the derivative should be zero at this point and
therefore the disappearance of the main peak may reveal the presence of other constituents with
different absorption features. This procedure is called 'null spectral derivative technique';
b. By using the complete derivative obtained by recording the elution profile. This procedure, known as
'spectral derivative mapping technique' was studied theoretically by Grant
et al
. [83], who discussed
specific cases where the spectral curves of potential impurities lay within the spectral band of the
major components. This procedure has been applied in the resolution of diverse mixtures.
4. A procedure has been developed by using the second derivative spectra of the components obtained
around the maxima signal of the overlapped chromatographic peaks. A diode-array spectrophotometer is
used with this purpose. This procedure, which has been used firstly to the analysis of active components
in insecticide formulations, makes possible the easy transformation of a chromatographic problem in a
spectrophotometric problem [84].
References
1. O'Haver, T.C. (1976) Modulation and derivative techniques in luminescence spectroscopy: Approaches to increased
analytical selectivity, in
Modern Fluorescence Spectroscopy
, Vol. 1 (ed. E.L. Wehry), pp. 65-81, Plenum Press,
New York.
2. John, P. and Soutar, I. (1976) Identification of crude oils by synchronous excitation spectrofluorimetry,
Anal. Chem
.,
48
, 520-524.
3. O'Haver, T.C. and Bagley, T. (1981) Signal-to-noise ratio in higher order derivative spectrometry,
Anal. Chem
.,
53
, 1876-1878.
4. Green, G.L. and O'Haver, T.C. (1974) Derivative luminescence spectrometry,
Anal. Chem
.,
46
, 2191-2196.
5. Schmitt, A. (1978) Derivative spectroscopy. An introduction with practical examples,
Tech. Lab
.,
5
, 1207-1215.
6. Osipov, V.M. and Borisova, N.F. (1985) Parametric reduction of a spectrum in derivative spectroscopy,
Zh. Prikl.
Spektrok
.,
42
, 603-606.
7. Fell, A.F. and O'Haver, T.C. (1982) Bioanalytical applications of derivative and multichannel spectroscopy,
Anal.
Proc
.,
19
, 398-403.
