Chemistry Reference
In-Depth Information
(a)
(b)
(c)
Sample
waste
Sample
Buffer
Stacking
buffer
Stacking
waste
Buffer
waste
Figure 17.18
The three process steps for full-column stacking in a coupled column configuration: (a) filling
the stacking channel, (b) sweeping anions back to the separation channel, and (c) separation of stacked sample
plug. Adapted from [136] with permission from Wiley-VCH Verlag GmbH & Co. KGaA © 2001.
In MCE, the first application of the ITP technique was reported by Walker
et al
. [140]. The ITP
preconcentration was carried out on a microchip with Raman spectroscopic detection system. As the samples
only line along the separation channel in the order of their electrophoretic mobilities, a baseline separation
cannot be obtained in the ITP condition. Also, in MCE, the first report of the application of tITP to MCE-MS
has been given by Zhang
et al
. [141]. Improved resolution and 10-fold signal enhancement for peptides are
obtained by the tITP preconcentration. For further improvement of the preconcentration efficiency in tITP-
MCE, Williams and co-workers have developed a 'floating injection' method [142-145]. As a result, a 13-fold
enhanced sensitivity is obtained for the tITP-MCZE analysis of tyrosine kinase compared to the conventional
injection scheme.
When a highly diluted sample solution is introduced into a microchannel filled with a highly concentrated
LE solution by the floating injection in tITP, a million-fold increase in the SEF is obtained on the conventional
cross channel chip [146]. In this work, a simple and robust isotachophoresis (ITP) method was integrated on
a CE microchip-base to achieve million-fold sample stacking. Figure 17.19 shows the strategy proposed: (a)
the north and the south reservoirs are filled with LE, and the west reservoir is filled with a mixture of TE
sample. TE/LE boundary is formed by applying vacuum at the south reservoir (while arrows show the
direction of pressure-driven flows). (b) ITP stacking is initiated by applying high voltage and ground at the
east and west reservoirs, respectively. The black arrows denote the direction of electric filed. Sample anions
electromigrate toward the anode as EOF is suppressed. The early stage of ITP stacking results in a partial
separation (i.e. moving boundary electrophoresis). (c) The field is switched toward the north reservoir to
inject LE ions behind the sample and initiates C.
