Chemistry Reference
In-Depth Information
(a)
(b)
Accumulation point
Cover
plate
S
SW
Edge of cover plate
V
7 parallel channels
C
T
B1
B2
AW
Sample channel
(c)
Cover plate
1
2
3
4
5
6
7
Substrate
Figure 17.14
(a) Schematic of the microchip for sample filtration, concentration and separation. The labels are
(B1) buffer with 20
acetonitrile, (S) sample, (SW) sample waste, (AW)
analysis waste, (T) T-intersection, and (V) valve. The circles represent fluid reservoirs. (b) An expanded view
and (c) cross section of the filter element. The filer channels are numbered 1-7. Reprinted from [127]
%
acetonitrile, (B2) buffer with 60
%
© 2003
American Chemical Society.
Because of packing columns are often complex and ill-defined, an alternative is to replace conventional
stationary phase materials with a continuous, porous bed of support (a porous monolith) formed by in situ
polymerization of organic monomers [129]. The process of bed formation is facile, since a low-viscosity
monomer solution can be introduced by vacuum or pressure into the microfluidic channel prior to initiation.
Afterwards, the continuous polymer bed is attached to the channel walls, making a retaining frit or weir
redundant. To this end, the preparation of microfluidic devices for SPE using porous monoliths with
hydrophobic and ionisable surface chemistries has been described [130, 131]. In this work reported by Yu
et al
., monolithic porous polymers have been prepared by photoinitiated polymerization within the channels
of a microfluidic device and used for on-chip solid phase extraction and preconcentration. The preparation of
the monolithic material with hydrophobic and ionizable surface chemistries was easily achieved by
copolymerization of butyl methacrylate with ethylene dimethacrylate. Monoliths were prepared with two
different surface chemistries at the desired location in the channel of the microchip shown in Figure 17.15(a).
The large pore size of the monolith materials minimizes the flow resistance enabling use of very high flow
rates of up to 10
μ
l min
−1
without causing mechanical breakage of the monolith (Figure 17.15b) [130].
17.3.4
Preconcentration approaches using electrokinetics
As we have already mentioned in previous sections, in the miniaturization scene electrokinetic flow-driven
systems have a relevant role because of their inherent and easy miniaturization, simplicity of fabrication and
high-controlled fluidic manipulation and motivation. In addition, the analytical potency of electrokinetics has
been enhanced since this principle has also been demonstrated to be very useful to analyte pre-concentration
on microchips because their high versatility and control for fluidic handling offering a wide group of strategies
with this purposes. Indeed, on-line sample preconcentration techniques can easily enhance the sensitivity
compared to the development of the sensitive detection methods or the application of off-line preconcentration
techniques. Employing on-line sample preconcentration, analytes can be rapidly and easily enriched and
