Chemistry Reference
In-Depth Information
efficiency of this approach has conveniently been proved through the analysis of model buffered solutions, to
the best of our knowledge, real-life applications are still needed.
SDME can also be accomplished by direct exposure of the drop to the headspace of the investigated
sample. In this case, the technique is named headspace single-drop microextraction (HS-SDME) and can
be applied to gaseous, aqueous and solid samples. The technique performs very efficiently for the
preconcentration of volatile non-polar analytes, and has the advantage over direct-immersion SDME of
providing cleaner extracts in shorter analytical times due to the possibility of using higher stirring rates.
For the rest, experimental parameters affecting the efficiency of the process are essentially the same as for
the immersion mode.
All previously described SDME-based techniques are static and consequently, the main factor determining
both the extraction efficiency and the extraction time is the diffusion of the extracted analytes from the drop
surface to its inner part. Although the use of less viscous solvents and higher stirring rates and temperatures
can contribute to increase this diffusion rate, the constant renovation of the solvent surface by using a dynamic
approach is probably a more effective approach. Two type of dynamic SDME are possible: in-syringe and
in-needle SME. In the former approach, the aqueous sample or headspace is withdraw into the syringe needle
or lumen and ejected repeatedly to perform the desired solvent enrichment [12]. In the in-needle dynamic
approach [13, 14], around 90
of the extraction drop is withdrawn into the syringe needle and then pushed
out again repeatedly for sample exposure. For obvious reasons, the in-syringe approach is more effective
when dealing with relatively pristine samples. Meanwhile, the in-needle one may be more useful for the
analysis of relatively 'dirty' samples, that is, samples containing relatively high amount of matrix components
that could affect the subsequent instrumental analysis.
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17.2.1.3
Hollow fibre-protected two/three-phase solvent microextraction
Hollow fiber-protected two-phase solvent microextraction (HF(2)ME) was introduced by He and Lee in 1997
[6] with the name of liquid-phase microextraction. In its simplest version, the technique involves a small-
diameter microporous polypropylene tube (the hollow fibre), usually sealed at one end, to contain the organic
extracting solvent. The open end of the hollow fibre is attached to a syringe needle used to fill the fibre with
the organic solvent. Once filled, the fibre is immersed in the vial containing the investigated aqueous sample
to allow analytes migration through its walls. After a preselected extraction time, the solvent is withdrawal
with the syringe and transferred to the instrument selected for analytes determination, typically GC. HF(2)
ME can consequently be considered a liquid-liquid membrane extraction [4] and so it is more appropriate
than SDME for the analysis of 'dirty' aqueous samples. The use of larger extractant volumes (typically in the
4-20
l range) and the possibility of using higher stirring rates are other advantages of HFME over SDME.
On the other hand, HF(2)ME usually involves longer extraction times than SDME (20-60 min versus 5-15 min
with SDME), and, at least LVI was used, only a fraction of the organic extractant is transferred to the
instrument selected for final determination. In addition, and although it can be adapted for use with an
autosampler [15], probably its main limitation is that each individual hollow fibre should carefully be sized
and prepared before use [4].
The three phases involved in HF(3)ME are the aqueous sample investigated, the water-immiscible organic
solvent that fills the pores of the hollow fibre polymer before this is attached to the syringe needle, and an
aqueous acceptor phase that is placed in the lumen of the fibre with the help of the syringe [16]. HF(3)ME
is operated in a way similar to HF(2)ME but, since the final acceptor solution is aqueous, the technique is
used to extract water-soluble analytes from aqueous matrices, and LC and CE are usually preferred for final
instrumental determination of the tested analytes. Similarly to that explained for three-phases SDME, the
pH of the aqueous sample and the acceptor phase are key parameters controlling the efficiently of the HF(3)
ME process.
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