Chemistry Reference
In-Depth Information
temperature control is needed to generate effective analytical signals. The thermal energy that is commonly
used is non-specific to the process, that is, it is not targeted at the chemical bond. Much of the energy is
'wasted' in heating up reactors, solvent and even the surrounding environment, instead of targeting only the
molecules undergoing reaction. In many cases, it is possible to use other sources of energy, such as light,
microwaves, or sound, on the place of action.
15.3.1
Using microwaves in place of thermal heating
The most common way to prevent heating the surrounding environment is to incorporate microwave heating
into the process.
Microwaves are 1 m to 1 mm (0.3-300 GHz) in length electromagnetic radiation, and therefore have
frequencies similar to radar and telecommunication devices. Household and industrial appliances are
regulated and operate on fixed frequencies (commonly 2.45 GHz). Microwave energy is a non-ionizing type
of electromagnetic radiation that causes molecular motion through the migration of ions and the rotation of
dipoles without altering the molecular structure. Because the mechanism by which microwave energy is
absorbed is complex and varies for different substances, it is not a universal substitute. The absorbed energy
is transferred to molecular kinetic energy and the sample heats up almost instantaneously. Substances that do
not have a dipole moment (or in which one cannot be induced) cannot be directly heated by microwaves.
Because of the large distances between molecules, gases also cannot be heated by microwave radiation.
Nonetheless, microwaves are a more efficient source of heating than conventional thermal heating because
energy is imparted directly to the medium in which the process takes place, rather than through the walls of
a vessel.
The use of microwave ovens in laboratories is common and microwave-assisted sample preparation
techniques are used in analytical laboratories all over the world [7, 8]. The latest advances in the application
of microwave techniques to various fields of analytical chemistry include: sample digestion for elemental
analysis, solvent extraction, sample drying, measurement of moisture, analyte desorption and adsorption,
sample clean up, chromogenic reaction, speciation and nebulization of sample solutions [9].
15.3.1.1 Accelerated extraction
Elevated temperatures produce high efficiency extraction as a result of increased diffusivity of the solvent
into the interior of the matrix and enhanced desorption of the components from its active areas. The effect of
microwave energy is greatly dependent on the nature of both the solvent and the matrix. Usually, a solvent is
chosen that strongly absorbs microwave energy. However, in some cases (for thermolabile compounds), the
microwaves are absorbed only by the matrix, which heats the sample and releases the solutes into the cold
solvent.
Analytical equipment for microwave-assisted sample preparation can be classified according to: the type
of microwave energy (multi-mode in cavity or focused by waveguide) applied to the sample, the subjection
or lack of subjection to overpressure (from open atmospheric to closed pressure vessels of more than 80 atm),
or the approach - dynamic (a flowing stream of mixture passes through a microwave field) or static (the
stream of mixture is stopped when it is contained within a microwave field). Closed vessel systems (under
controlled pressure and temperature) are generally of the multi-mode type (the microwave radiation is
dispersed into a cavity where the sample is located, so that it is randomly irradiated) and open-vessel systems
(under atmospheric pressure, open or closed to the atmosphere) are of the focused type (the microwave
radiation is focused on a restricted zone where the sample is placed).
The unique feature of MW extraction is that components can be extracted selectively either by selective
heating of a phase or single component of a system (for example, free water molecules in vegetable cells), or
