.5. Determination of caffeine by the proposed on-line SPE–FI method
In the assembly of the flow manifold, attention was emphazised
to reduce as much as possible the distance between the selection
valves and the column, as well as between the selection valve
SV2 and the detector, minimizing, as much as possible the signal
broadening. The flow system is schematically depicted in Fig. 1.
200 lL of aqueous extract treated with Carrez reagents (Solution
A) were injected into carrier (distilled water) and passed through
the C18 minicolumn for 90 s in the retention position shown in
Fig. 1. The target analyte was retained in and the matrix was sent
to waste. Then SV1 was changed to position 2 and a 25:75 methanol:water
mixture (eluent) was aspirated toward the holding coil
(HC) for 30 s at 0.8 mL/min (400 lL) changing the way of the peristaltic
pump (Fig. 1A, elution step). Later, the flow was changed
and the SV1 and SV2 valves were switched as shown in the elution
step of Fig. 1B (SV1 position 1 and SV2 position 3) then the mixture
of methanol:water was passed through the C18 minicolumn. Eluted
caffeine was driven to the flow cell where it was measured at
276 nm. The minicolumn was regenerated by passing the carrier