Chemistry Reference
In-Depth Information
(a)
(b)
Figure 17.11
Images of the chamber (a) at an initial stage of electrokinetic packing and (b) after it is
completely filled with beads. Reprinted from [119] © 2000 American Chemical Society.
In this work, electrokinetic packing of conventional capillaries has been demonstrated, and adapted for the
microchip format. The packing procedure involved applying a suitable voltage program. The applied voltage
induced EOF to flow down the bead channel, carrying the beads into the cavity. An organic solvent was
required to suspend the chromatographic beads and prevent them from aggregating and plugging the relatively
narrow bead channel. At the early stages of packing, the beads entering the cavity contacted the weir structures
on either side of the cavity. The beads were unable to traverse the weir because the distance from the top of
the weir to the bottom of the cover plate (1.0
μ
m) was less than the diameter of the individual particles of the
packing material (1.5-4.0
m). Figure 17.11 shows illustratively an image of the bead cavity midway through
the packing procedure (a), and the cavity 10 s later (b) [119].
Similar in-stream SPE microdevices have been proposed for sample clean-up and enrichment of protein
and peptide samples prior to MALDI-TOF MS analysis [121]. Silicon/glass devices incorporating a weir
structure facilitate the packing of reverse-phase chromatographic beads. These beads are then used to
successfully purify and enrich a 10 nM peptide mixture containing 2 M urea in 0.1 M phosphate-buffered
saline prior to MS analysis. Subsequent modelling of the fluid dynamics in this system has allowed an
improved grid-SPE device to be fabricated and tested for on-line proteomic sample preparation [122].
Other approaches based on pillar-like frits [123, 124] and photopolymerized frit-structure [125] have
been fabricated for packing the SPE beads. Regarding the first ones, in a very interesting work [123], the
channel pattern of a CEC microchip employed is shown in Figure 17.12(a), where the expanded view
shows the bead chamber with frit structures (Figure 17.12b). The structures of a microchip include an
injector and a bead chamber with integrated frits, where the particles of the stationary phase were
completely retained. Dimensions of the frit structures are 25 × 20
μ
μ
m, and the space between the structures
is 3
m. PDMS channels were coated with polyelectrolyte multilayers to avoid adsorption of hydrophobic
analytes to PDMS and chromatographic beads are packed in a bead chamber with integrated frits. Silica
beads (5
μ
in acetonitrile) were loaded and packed into the bead chamber by applying
different pressures from the reservoirs. Neutral compounds and coumarins, have been successfully
preconcentrated and separated.
Besides, in a photopolymerized frit-structure approach [125], a porous plug of polymethacrylate polymer
(200
μ
m diameter, 2
%
m in length) was fabricated by ultraviolet irradiation in microchannels. Microcolumns of hydrophobic
beads packed against the polymethacrylate plugs were utilized for the quantitative extraction of rhodamine B
using a microchip layout shown in Figure 17.13(a). This work demonstrates the first utilization of
μ
