Chemistry Reference
In-Depth Information
fluid solely by ultrasonic forces and by agglomerating with other particles [124]. Although the use of
ultrasound avoids the production of disposable materials (waste), it should be minimized by using the
optimum US frequency, intensity, power and propagation direction in each case. Downscaling ultrasound-
assisted filterless filtration avoids the need to handle the separation cell and allows separation to be
integrated in microfluidic systems. Ultrasound-assisted filtration has proved an effective continuous
filtration approach [125].
Filtration can be preceded by a precipitation step. In this case, precipitation and filtration can be both
implemented in a continuous flow system to exploit the analytical and environmental advantages of continuous
approaches. Continuous precipitation-filtration can be followed by dissolution if needed. This introduces an
additional reagent (solvent) in the process. The precipitation-filtration-dissolution sequence has been used in
immunochemical methods involving the formation of high-molecular weight immunocomplexes such as that
for the determination of anti-canine immunoglobulin G [126]. Again, filtration can rely on a filterless device
(a knotted reactor, preferably). In fact, a knotted reactor was recently used to immobilize the ligand on its
inner walls for the simultaneous separation-preconcentration of traces of various metals before detection by
ICP-TOF/MS [127]. The typical purposes of using precipitation-filtration without precipitate dissolution
include interference removal and indirect determinations of analytes based on the signal decrease caused by
analyte precipitation, which will be proportional to the analyte concentration in the sample [40]. Inorganic
anions such as halogenides [128] and sulfate [129] have been determined in this way, using Ag(I) and Pb(II),
respectively, as precipitants, and so have organic compounds such as sulfonamides [130] and local anaesthetics
[131] with Cu(II) and Co(II), respectively. The main operational problem of this methodology is the need for
frequent cleaning of the filter, which can be overcome by placing it in the loop of an injection valve and
cleaning in the opposite direction to filtration after each precipitation step.
8.5
A Green future for sample preparation
As shown in this chapter, non-chromatographic separation techniques provide a wide range of choices for
sample preparation. Sustained research in this field and today's growing environmental awareness are bound
to boost the development of increasingly greener analytical methods. Analytical chemists can play a prominent
role in the search for environmentally benign non-chromatographic separation methods, and so can other
professionals involved in the design of analytical equipment and procedures.
Acknowledgement
The Spanish Ministerio de Ciencia e Innovación (MICINN) and FEDER program are thanked for financial
support through project CTQ2009-07430.
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