Page 375 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

478 Organic Chemistry, Compound Detection

covered with a reagent. After a very short time, (mostly a It should be pointed out that typical measurements in a
few seconds or minutes) the plate is developed in a suitable laboratory take several hours to perform, but many of the
solvent system whereupon the reaction products are sepa- desired applications of on-chip analytical systems require
rated and identified by color or by spectroscopic methods. data in seconds, or at most a few minutes. Fast analysis is
A great deal of information can be gained from the in therefore an important aspect of miniaturization.
situ reaction technique with the expenditure of very small
amounts of material. The following in situ reactions on TL IX. FIELD-FLOW FRACTIONATION (FFF)
plates have been carried out so far: acetylation, dehydra-
tion, formation of derivatives: (acetates, dinitrobenzoates,
FFF is a chromatographic-like separation technique which
DNPS, methyl esters, phenylisocyanates, and semicar-
is designed for fractionation of macromolecules, colloids,
bazones), diazotization, esterification, halogenation, cat-
and particles, primarily on an analytical scale. While the
alytic hydrogenation, acid and alkaline hydrolysis, iso-
chromatographic separation is achieved by differential
merization, nitration, oxidation, photochemical reactions,
distribution of species between one stationary phase and
reduction, and Diels–Alder reactions.
one moving phase. FFF uses only one moving phase.
However, there are velocity difference within the moving
F. Lab on-a-Chip phase, and separation is achieved by differential distribu-
tion of species between the regions of different velocity.
A Lab on-a-chip is a planar device on which or in which
FFF has broad application.
a number of chemical processes are being performed in
Sedimentation FFF has been used for the fraction-
order to go from reactants to products or from a sample
ation of polystyrene, latex beads, emulsions, artificial
to analysis. There is a distinction between microfluidic
blood, viruses, and aqueous colloids, liposomes, albumin
devices and microarray devices and DNA analysis.
spheres, and DNA. Thermal FFF has been applied to dif-
DNA microchip arrays contain from several hundred to
ferent types of synthetic polymers. A recent development
several thousand immobilized DNA reagents. They pro-
in FFF has been the increase of separation speed to allow
vide a systematic way to survey DNA and RNA variation
fractionation on a minute scale instead of hours.
and may well provide a standard tool for molecular bi-
ology research and clinical diagnostics. The peripherals
such as MS and fluorescence detectors sit off-chip and are X. ION CHROMATOGRAPHY (IC)
typically benchtop instrument sized.
At present, the typical commercially produced mi- lon chromatography (IC) usually contains an ion ex-
crochip used for miniaturized chemical systems consists changer or ‘pseudo ion exchanger’. The ion exchangers
of a 2- or 3-cm-square sliver of silicon, glass, quartz, or are derived from cross-linked organic polymers by affix-
plastic that is 50 µm. The chip is covered with a plate ing to the polymer ionogenic groups that are the source or
to contain the samples and reagents. At present it is pos- the vital ion exchange processes. The IC is used for the
sible to inject volumes as low as 1 pL onto chips with determination for both inorganic or organic compounds
miniaturized chemical systems. and even species as ‘non-ionic’ as carbohydrates.
For its operation, a microfluidic chemical device re- The determination of ions in environments as diverse
quires a number of microcomponents, such as microfilter, as brine, milk, diesel exhaust, plating baths, urine and
microreactors, and microseparation columns, depending foodstuffs is a measure of its versatility. Improvements in
on the application. It also needs some means of driv- resins and other stationary phases and in the sensitivity
ing the fluids around the chip. Recently, a microtech- and automation of IC techniques should help to sustain
nology for producing “bioprocessors-on-a-chip” was de- the growth in this new area of analytical technology.
veloped. Such chips can be described as microfactories
that can within minutes diagnose infections or diseases
by separating bacteria, fatal cells, or cancer cells from XI. CAPILLARY
blood. This technology relies on dielectrophoresis for sep- ELECTROCHROMATOGRAPHY (CEC)
arating bioparticles, such as cells, viruses, bacteria, pro-
teins, and DNA with a high degree of selectivity and CEC may be regarded as a combination of high-per-
sensitivity. formance capillary electrophoresis (HPCE) and HPLC—a
With improvement in layout and packaging of mi- hybrid technique combining the best of both worlds.
crochips, it might even be feasible to integrate the de- CEC is a derivative of HPCE, wherein a packed bed
tection and computer circuiting on the chip. capillary is used. The essential difference between HPCE