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was developed in the mid-seventies by the proposal of the concept of Solid
Phase Spectroscopy. The principle of this new type of sensors is that the native
absorbance of the analytes is measured after absorption of the latter on a
suitable solid support immobilized in the cell of the detector. Such
configurations have been automated extensively by flow injection techniques.
The resulting flow optosensors combine the advantages of enhanced selectivity
and sensitivity with the high precision and sampling rate of flow injection
techniques. Additionally, based on the different retention properties of the
analytes of interest, there are potentials for multi-analyte determinations.
Typical solid phase supports range from hydrophobic reversed phase and ion-
exchange beads to tailor-made molecularly imprinter polymers.
The simultaneous determination of pairs of xanthines, caffeine-theophylline
and caffeine-theobromine, using a flow optosensor, has been proposed by
Llorent-Mart´nez and co-workers (Llorent-Mart´nez et al 2005). Reversed
phase silica gel beads served as the sensing zone in the flow cell, while all
analytes were monitored at 272 nm. In order to achieve separation of the
above-mentioned xanthine-pairs prior to entering the sensing zone, the authors
placed a mini-column with the same material between the injection valve and
the detector. Using an aqueous carrier stream, caffeine was retained in the
mini-column, while theophylline (or theobromine) was detected in the flow-
cell. Elution of caffeine was carried out by replacing the carrier with a solution
containing 25% MeOH. The detection limits were at the 0.1 mg L
21
level and
the precision at 3%. The analytical features were sufficient for the simultaneous
determination
d
n
0
t
2
n
g
|
3
of
caffeine-theophylline
in
pharmaceuticals
and
caffeine-
theobromine in soft drinks and food samples.
The same research group adopted a similar approach for the separation-
determination of mixtures of caffeine and accompanying pharmaceutical
compounds: caffeine, salicylamide, propyphenazone (Gilbert-L ´pez et al
2007); caffeine, paracetamol, propyphenazone (Vidal et al 2003); caffeine,
paracetamol (Ortega-Barrales et al 2002). In all cases, polar analytes e.g.
paracetamol and salicylamide were not retained on the reversed phase pre-
column, while more hydrophobic compounds were eluted by suitable organic-
water mixtures. Critical analytical figures of merit included: (i) detection limits
in the range of 0.21-0.61 mg L
21
(Gilbert-L ´ pez et al 2007), 0.65-12 mg L
21
(Vidal et al 2003) and 0.5-0.75 mg L
21
(Ortega-Barrales et al 2002); (ii) relative
standard deviations between 2.97-4.86% (Gilbert-L ´ pez et al 2007), 2.7-3.8%
(Vidal et al 2003) and 0.5-3.1% (Ortega-Barrales et al 2002); (iii) sampling
throughput between 11-20 h
21
(Vidal et al 2003) and 15-20 h
21
(Ortega-
Barrales et al 2002).
11.5 Determination of Caffeine and Related Compounds
by On-line IR Detection
The coupling of IR detectors to continuous flow systems has proven to be an
advantageous alternative to conventional detection systems that are typical in
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