Determination of Caffeine in Various Coffee Types by Capillary Electrophoresis Through the Anionic Complex with 3,4-Dimethoxycinnamate - Caffeine: Chemistry, Analysis, Function and Effects - page 181

region, the UV detection in CE is possible. However, another growing
approach for detection in CE does not give the same good compatibility:
capacitively coupled contactless conductivity detection (C 4 D). Although
previously used as a detection scheme for MEKC, we are not able to reach
the desirable limit of detection of caffeine in different matrices by using C 4 D.
The foundations for this kind of detection will be discussed later, but in a few
words the point is the occurrence of a noisy baseline caused by the high
conductivity BGE and the small difference caused by caffeine in the mobility of
the micelles.
The seeking of another way to promote the electrophoretic mobility of
caffeine resulted in the p-complex formation with chlorogenic acids or their
analogues. Such a complex occurs naturally in plants that synthesise this
complex as a way to store caffeine. Along the next sections, the CE-C 4 Dis
discussed as well as the experimental conditions and results obtained in this
new approach to separate neutral species in electrophoresis.

d n 0 t 2 n g | 9

10.2 Capillary Electrophoresis with Capacitively
Coupled Contactless Conductivity Detection
There are many good text books and review articles dealing with capillary
electrophoresis (Altria 1996; Camilleri 1997) with different detection systems,
including C 4 D (Brito-Neto et al 2005a; 2005b; Kuban and Hauser 2004a;
2004b). Of course, the purpose of this chapter is not to cover in detail all the
aspects of this technique, but to show to the reader the most relevant features
to improve the understanding about how it is possible to determine the
concentration of caffeine, a neutral species, by capillary electrophoresis, a well-
known technique for ions (Table 10.1).

Key features of CE-C 4 D. This table lists some fundamental
aspects

Table 10.1

about capillary electrophoresis and particularly how

detection occurs.

1. Separation by electrophoresis is not only for separation of naturally charged
species. There are plenty of chemical reactions that can be used to change neutral
species into ions previously or during the electrophoretic run.
2. While migrating under the application of an electric field, a species changes the
chemical composition of that region and, consequently, the electric field there.
Thus, an analyte changes the local concentration of the co-ion and counter ion.
3. The perturbation of the concentration profile of all ionic species is the base for
conductivity detection as well as UV indirect detection. In this last case, the co-ion
or the counterion is an absorbing species, and negative peaks are formed when
analytes reach the detection point.
4. Contactless conductivity detection is an advantageous approach, because it
prevents corrosion of the electrodes, eases the electrode positioning, and precludes
the contact of the detector electronics with the inner side of the capillary, which can
be at as high as tens of kilovolts.