Chemistry Reference
In-Depth Information
(a)
(b)
Direction of flow
Laser detection point
Monolithic concentrator
Figure 17.15
(a) Microfluidic chip layout. (b) SEM image of the monolithic ion-exchange concentrator.
Adapted from [130] © 2001 American Chemical Society.
separated with a simple procedure by the variation in the migration velocity between the sample and running
solution zones, which can be adjusted by several factors such as the conductivity, buffer components, pH of
the solutions and so on. Although these methods have been developed in conventional scale (CE), several
researchers have transferred these techniques from conventional format to the microchip one.
In the following sections, selected strategies and progress of integrated sample preconcentration
electrokinetic techniques on microchip platforms will be studied. Firstly, preconcentration strategies based
on the velocity change of the analytes between the sample and separation solution zones will be presented. In
a second stage, 'focusing' techniques where the analytes are focused at the points where the migrating
velocities become zero will be given toward selected examples. Some excellent general literature is proposed
for further reading [132, 133].
17.3.4.1
Preconcentration based on the velocity change of the analytes between the sample and
separation solution zones
Figure 17.16 shows conceptually the preconcentration mechanisms most common used on microchips based
on the velocity change of the analytes between the sample and separation solution zones: field amplified
stacking (FAS) and (t-) isotachophoresis (ITP). These approaches and the termed sweeping will be studied in
this section.
Firstly, in the field amplified stacking (FAS), a background solution (BGS) with a high conductivity and a
sample solution (S) with a low conductivity are prepared. The S is introduced into the capillary filled with the
BGS, and then an appropriate voltage is applied to both ends of a capillary. The local electric field in the
sample zone is higher than that in the BGS as the electric current in the capillary is constant. Therefore, the
electrophoretic velocity of the analytes in the sample zone is faster than that in the BGS. This difference in
the electrophoretic velocities between the S and BGS zones generates the 'stacking' effect at the S/BGS
boundary, so that the analytes are concentrated around the boundary.
In MCE, the first applications of FAS have been reported by Jacobson and Ramsay [134, 135].
Preconcentration of the sample was performed using gated injection and compared with the pinched injection
protocol [134]. Both principles are shown in Figure 17.17. Chip operation consisted of two modes: sample
loading and separation. In the gated injection protocol, the sample migrates through the injection cross toward
