Chemistry Reference
In-Depth Information
is characteristic of all the odd-order derivatives. The most characteristic feature of the second-order derivative
is a negative band with the minimum at the same wavelength as the maximum on the zero-order band. It also
shows two additional positive satellite bands on either side of the main band. The fourth derivative shows a
positive band. The presence of a strong negative or positive band, with the minimum or maximum at the same
wavelength as
λ
max
of the absorbance band, is characteristic of the even-order derivatives. Note that the
number of bands observed is equal to the derivative order plus one.
Derivative UV spectroscopy has been widely used as a tool for quantitative analysis, characterization; and
quality control in the agricultural, pharmaceutical and biomedical fields. This outstanding feature coupled
with zero-crossing, least-square deconvolution, or Fourier transform data-processing techniques, has received
increasing attention in single and multi-component quantitative analysis, especially in UV-absorbing
matrices.
A wide study has been published in a book by Talsky [15], in which the instrumentation used was described,
as well as numerous methodologies and applications. Diverse reviews published from 1986 to 2009 [16-19]
exposed the different aspects of derivative spectrometry: theoretical, instrumental devices and analytical
applications. Other interesting reviews are published in 2004 by Karpinska about various aspects of application
in chemical analysis [20] and in 2005 by El-Sayed and El-Salem about developments and analytical
applications [21].
13.2.1
Strategies to greener derivative spectrophotometry
Miniaturization of analytical methods and instrumentation has resulted in precisely controlled trace level
analyses, and their high environmental compatibility owing to insignificant amount of solvent and reagent
consumption, low waste production, and low operational cost. Most of the current analytical procedures
consist of a sample preparation step, and quantitation by chromatography and spectrometry.
Solvent extraction has been most widely used method for sample preparation. However, the classical mode
of solvent extraction performed in a separatory funnel has inconveniences of unfavourable partition equilibria,
formation of emulsions, necessity to use large volume of hazardous and ozone depleting organic solvents and
difficulty in waste disposal. Miniaturization of extraction process by using microlitre volumes of the organic
solvent has avoided many of these problems, allowed new ways in which the sample pre-treatment can be
performed [22-24].
UV-Vis spectrophotometry is a mature analytical technique applied to many thousands of determinations
owing to its simplicity, flexibility, low cost and convenience. Due to the widespread use of UV-Vis
spectrophotometers for routine analysis and as a result of the great demand to decrease the sample volume
needed to perform a measurement, UV-Vis micro-spectrophotometry have been developed.
13.2.1.1
Micro-scale derivative spectrophotometry batch procedures by use of internal standard
The feasibility of UV-Vis spectrophotometry for multi-component analysis has encouraged us to use the
method of internal standard in micro-scale multi-step spectrophotometric procedures and thus, enhance their
analytical performance and reduce adverse environmental impact.
The analytical signal is defined as the ratio between the signals of analyte and of internal standard. The
most important feature of internal standard (IS) in UV-Vis spectrophotometry is that less rigorous procedures
become possible and, from the green chemistry point of view, the micro-scale batch procedures can be used
without sacrificing the results quality.
The well-known limitation of separation/preconcentration procedures is the increasing risk of analytical
errors. Such source of errors can be controlled by using a method of IS, which is common in chromatographic
and in some spectrometric techniques.
