Biomedical Engineering – From Theory to Applications

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Biomedical Engineering – From Theory to Applications
Kaniansky (Kaniansky & Marák, 1990). The analytical benefits of the ITP-CZE combination
have been already well documented (Fanali et al., 2000; Danková et al., 2001; Kvasnička et
al., 2001; Valcárcel et al., 2001; Bexheti et al., 2006; Beckers, 2000b; Křivánková et al., 1991;
Křivánková & Thormann, 1993; Křivánková & Boček, 1997a).
An on-line combination of ITP with CZE appears to be promissing for alleviating some of
the following practical problems (Kaniansky & Marák, 1990):
i. ITP is a separation technique with a well defined concentrating power while the
separands migrate stacked in sharp zones, i.e., it can be considered as an ideal sample
injection technique for CZE,
ii. In some instances the detection and quantitation of trace constituents separated by ITP
in a large excess of matrix constituents may require the use of appropriate spacing
constituents. Such a solution can be very beneficial when a limited number of the
analytes need to be determined in one analysis. It becomes less practical (a search for
suitable spacing constituents) when the number of trace constituents to be determined
in one analysis is high,
iii. In CZE, high-efficiency separations make possible a multi-component analysis of trace
constituents with close physico-chemical properties. However, the separations can be
ruined, e.g., when the sample contains matrix constituents at higher concentrations than
those of the trace analytes.
A characteristic advantage of the ITP-CZE combination is a high selectivity/separability
obtainable due to the CZE as the final analytical step. Hence, the ITP-CZE method can be
easily modified with a great variety of selectors implemented with the highest advantage
into the CZE stage enabling to separate also the most problematic analytes (structural
analogs, isomers, enantiomers). The ITP-CZE methods with chiral as well as achiral CZE
mode have been successfully applied in various real situations (Mikuš et al., 2006a, 2008a,
2008c; Danková et al., 2001; Marák et al., 2007; Kvasnička et al., 2001).
The most frequently used ITP-CZE system works in the hydrodynamically closed separation
mode that is advantageous for the real analyses of multicomponent ionic mixtures because
of the best premises for enhancing sample load capacity (enables using capillaries with very
large I.D.). Such commercial system is applied with just one high-voltage power supply and
three electrodes (one electrode shared by the two dimensions), see Fig. 1. The electric circuit
involving upper and middle electrode (electric field No. 1) is applied in the ITP stage while
upper and lower electrode (electric field No. 2) is applied in the CZE stage. For the
separation ITP-CZE mechanism see chronological schemes in Fig.2. The focused zone in the
first dimension (ITP) is driven to the interface (bifurcation point) by only electric field No. 1.
The cut of the zone of interest in the ITP stage is based on the electronic controlling
(comparation point) of the relative step heigth (Rsh, a position of the analyte between the
leading and terminating ion, it is the qualitative indicator depending on the effective
mobility of the analyte) of the analyte, see Fig. 3. The conductivity sensor (upper D in Fig. 1,
D-ITP in Fig.2) serves for the indication of the analyte zone. This is very advantageous
because such indication is (i) universal and (ii) independent on other comigrating
compounds (sample matrix constituents migrating in the ITP stage) and therefore
independent on sample composition. The electric circuit is switched and electric field No. 2
(upper and lower electrode) is applied in an appropriate time (this time is set electronically
depending on requirements of the composition of the transferred plug) after the indication
of the analyte zone passing through the upper D. From this moment the all ITP zones are
directed to the CZE stage for the final separation and detection. It is possible to carry out