Chemistry Reference
In-Depth Information
transporting it to detectors (e.g. with US nebulization, levitation or slurry formation) [13]. Ultrasonic
equipment operating in the range of 20-100 kHz is relatively inexpensive and readily available. Some high
frequency (500 kHz) equipment has also been used for more fundamental mechanistic investigations.
Ultrasound energy causes acoustic cavitation - bubble formation and implosion - in solutions. The collapse
of these bubbles generates extremely high pressures and temperatures at the interface between the bubble and
another phase. Moreover, the extremely high effective temperatures and pressures at the interface between an
aqueous solution and a solid matrix, combined with the oxidative energy of hydroxyl radicals and hydrogen
peroxide created during the sonolysis of water, enhances chemical reactivity. Acoustic cavitation thus
provides a unique interaction between energy and matter and more extractive power is obtained from the
solvent.
15.3.2.1
Accelerated extraction and leaching
Ultrasonic radiation is another alternative for accelerating some steps in solid sample pre-treatment, such as
dissolution, fusion and leaching. Cavitation from acoustic vibrations facilitates the removal of analytes from
the matrix surface and creates microenvironments with high temperatures and pressures. Sonication-assisted
extraction (SAE) is faster (5-30 min per sample) than the Soxhlet mode and allows large amounts of sample
to be extracted with lower energy consumption. The same range of solvents is suitable for SAE as for Soxhlet
extraction. SAE is usually conducted statically in discontinuous batch mode; however, SAE can also be
dynamic (DSAE). DSAE has several advantages over conventional batch mode, such as continuous exposure
of the sample to fresh solvent, which improves extraction kinetics. Furthermore, post-extraction filtration and
rinsing are eliminated and solvent consumption and the risk of loss and/or contamination of the extracted
species during manipulation is reduced. Ultrasonic leaching of metals from sediments, although not yet
sufficiently exploited, could be an attractive alternative to conventional, acid bomb and microwave digestion;
except for the time required for digestion, unloading reactors is faster in the absence of high temperature and
pressure. The type and concentration of acid in the liquid extractant is the most critical parameter for
ultrasound leaching. Unfortunately, SAE uses as much solvent as Soxhlet extraction and filtration is required
after extraction. Moreover, it is labour-intensive because apart from the polarity of the solvent, the efficiency
of the extraction is dependent upon the nature and homogeneity of the sample matrix, the ultrasound frequency
and the sonication time. The EPA has approved sonication-assisted extraction as Method 3550C [14].
Ultrasound has been used to overcome the effects of electrode passivation in electrochemical systems,
allowing sensitive electroanalysis to be carried out in hostile media such as blood, effluents, foodstuffs and
fuels [15]. Mass transport is greatly increased via acoustic streaming and micro-jetting that results from
cavitational collapse on the surface of the electrode. In addition, surface activation via cavitational erosion
can be used to activate otherwise passivated electrodes. Therefore, the coupling of power-ultrasound with
electroanalytical techniques dramatically improves analysis times, detection limits and electrode lifespan.
Sono-electroanalysis methods have been developed using readily available and relatively inexpensive
equipment that can be tailored to specific analytical situations [16].
15.3.2.2
Assisted matrix solid-phase dispersion
Matrix solid-phase dispersion (MSPD) is a form of microextraction for both liquids and solids. For liquid
samples, the sorbent is dispersed in the sample to absorb the analyte. The sorbent is then separated from the
liquid and transferred to a column and then the analyte is desorbed thermally for further analysis. For solid or
semi-solid samples, a suitable solid-phase material (e.g. derivatized silica, silica gel, sand or Florisil) is
manually blended with the sample. The solid support and sample are then transferred to a column and eluted
with an appropriate solvent. In MSPD, sample disruption and dispersal onto the particles of the support
