Chemistry Reference
In-Depth Information
not involve matrix dissolution, except in the case of biological samples. More efficient lixiviation schemes
can be implemented with the support of ultrasounds, microwaves or increased pressure, as well as using
enzymatic approaches or nanomaterials.
11.2.1.1 Ultrasounds
In recent years, use of ultrasounds has proved useful for decreasing the amount of harmful reactants
involved, as well as simplifying experimental procedures. This is a clean technology with numerous
analytical applications such as surface cleaning, extraction, digestion, homogenization, and so on. One
work that exemplifies well the benefits that can be achieved from the point of view of the application of
Green Analytical Chemistry to atomic spectrometry has been published by Gil
et al.
[33]. In this work, a
new cold vapor technique is described for the determination of mercury by means of an ultrasound-assisted
reduction/volatilization process, followed by atomic absorption spectrometry for Hg detection and
quantification. In this method, the use of a chemical reducing agent or of any other type of chemical is
avoided, consequently eliminating production of those hazardous wastes typically related to the use of such
reducing agents.
11.2.1.2 Microwaves
Microwave-assisted sample pretreatment/digestion is practically the method of choice in atomic spectroscopy.
From a green point of view, the rapid heating achieved as a consequence of the microwave irradiation and the
use of closed vessels under controlled conditions of pressure and temperature are some of the main advantages
offered. In addition to this, one of the most important improvements to date was first reported by Lorentzen
and Kingston [34], who developed a method that permitted to achieve total digestion of a sample using
diluted acids only, instead of concentrated reactants as usual. Recent work carried out by Flores
et al.
[35]
based on digestion by microwave-induced combustion is also noteworthy. In this work, traces of metals of
catalyst residues are determined in carbon nanotubes using oxygen for this combustion, and absorbing
the resulting products in diluted solutions of acids, prior to flame or graphite furnace atomic spectroscopy,
ICP-OES or ICP-MS monitoring [35].
11.2.1.3
Pressurized liquid extraction
Pressurized liquid extraction, a well-known technique for the extraction of organic compounds, is based on
the use of solvents at a high pressure and/or high temperature without reaching the critical point [36]. In this
situation, the properties of the solvents are modified and the solubility of the analytes in the extracting
solution is increased. Due to this increased solubility, the extraction can be performed with smaller amounts
of solvents, and the generation of waste is subsequently reduced. In addition to this, it is possible to use water
as extracting solvent, which further enhances the green side of this approach [37].
This method has been proposed as an environmental friendly alternative to acid digestion procedures for
metal extraction, using home-made or commercially available devices developed for this purpose. There are
a high number of applications based on this strategy [38-41].
11.2.1.4 Enzymatic approaches
It is possible to combine all the methods of lixiviation described before with the use of enzymes, resulting in
a group of procedures that can be applied to biological samples working under mild conditions, without the
use of any toxic reagents. However, in the end, depending of the type of enzyme used, it is still necessary to
deal with the residues.
