Chemistry Reference
In-Depth Information
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Figure 8.10
Comparison of extraction and subsequent hydrolysis in an alkaline medium in the presence and
absence of ultrasound.
phase; ultrasound-accelerated mass transfer between the two immiscible phases. A more complex manifold
(Figure 8.9(b)) was also developed for ultrasound-assisted extraction, albeit from an organic phase to an
aqueous phase, with monitoring of the two mass-transfer zones (one in the presence of ultrasound and the
other in its absence). Avoiding passage of the organic phase through the peristaltic pump dispensed with the
need for special, expensive tubing and its frequent replacement [50].
Overall, ultrasound energy increases separation efficiency, thereby shortening extraction times and
reducing reagent consumption. This is clearly apparent from Figure 8.10, which compares the results obtained
in the extraction of paracetamol with and without ultrasound. USALLE experimental set-ups require the
ultrasound application variables (particularly probe position, radiation amplitude and pulse duration) to be
optimized since extraction times are typically short and, with low reagent and samples volumes, it is crucial
to produce as little environmentally hazardous waste as possible.
Attempts at reducing solvent and sample volumes have led to the development of LLE in microfluidic
chips consisting of two microfabricated glass plates with a microporous membrane sandwiched in between.
Gravity was used to drive the aqueous and organic flows through channels separated by the membrane. The
native fluorescence of the analyte (butyl rhodamine B) extracted into the organic phase was monitored in situ
with a laser-induced fluorescence detector. The operational complexity of this system makes it impractical
for routine use, however [51].
8.3.3 Single-drop microextraction
By virtue of its simplicity and good analytical results, single-drop microextraction (SDME) has become a
very popular non-chromatographic separation technique since the mid-1990s, when Dasgupta used a very
small amount of solvent holding from a tip of a syringe to separate analytes from their matrix.
There are seven different modes of solvent microextraction falling under the SDME category; all use two
or three phases (see Figure 8.11) [52]. The most common modes are probably head-space SDME (41
%
of all
reported procedures) and direct immersion SDME (38
). Both are quite simple and use inexpensive
equipment. Also, both use a very small drop, in the microlitre range, of an appropriate organic solvent held
by the tip of a syringe to effect extraction. This dramatically reduces the consumption of organic phase
relative to alternative liquid-liquid extraction approaches.
Because they use similar volumes of organic solvent and sample, direct immersion and head-space SDME
are also similar in environmental friendliness. However, the two differ operationally; thus, unlike other
%
