Chemistry Reference
In-Depth Information
(a)
(b)
FT
DM
(c)
(d)
Extension of FS
PET
DM
FS
DM
Figure 8.7
Types of microdialysis probes. (a) Concentric cannula probe. (b) Flexible probe. (c) Linear probe.
(d) Flow-through probe. DM, dialysis membrane; FS, fibre skeleton; FT, flexible tubing; PET, polyethylene tubing.
vivo and/or in vitro measurements. Because the sampled system can in fact be a living system or a portion
thereof, a substantial reduction in operational time and sample preparation costs is thus accomplished. After
the probe is inserted in the zone to be sampled, its inner fibre is slowly filled with - or circulated by in
dynamic microdialysis - the sampling solution. As with conventional diffusion, the perfusate solution must
be similar in pH and ionic composition to the sampled system. The membrane, whether porous or non-
porous, allows passage of small molecules of the sample into the acceptor solution for subsequent collection
or transport with a view to on-line separation and/or measurement of the analytes (e.g. by capillary
electrophoresis, which has been successfully coupled to microdialysis for various purposes [34-36]). Figure 8.7
shows the most common types of MD probes. One added advantage of microdialysis is that flow rates can be
up to 10 times lower than in conventional dialysis in FI or SI systems. A special technique known as 'superslow
microdialysis' circumvents some of the typical shortcomings of microdialysis such as the need for time-
consuming calculations in order to offset depletion and incomplete recovery of some analytes.
8.3.2 Liquid-liquid extraction
Conventional liquid-liquid extraction (LLE), which involves mass transfer between two immiscible liquids,
is frequently used in SP procedures to separate an analyte from interfering substances present in the sample
matrix, preconcentrate the analyte for increased sensitivity or improve limits of detection. This highly useful
technique has been widely used in pharmaceutical, environmental, agricultural and industrial analyses. The
main shortcoming of conventional LLE is the need for large volumes of organic, volatile solvents, which
make it a far-from-green technique. In addition, LLE is labour-intensive, time-consuming and usually
implemented in open spaces - which can pose health hazards associated to the high volatility of some organic
solvents. This is especially worrisome in preparing large batches of samples [37]. Similarly to other techniques
