A schematic diagram of the on-line preconcentration system
is presented in Fig. 1. The system consists of a Gilson Minipuls
3 peristaltic pump (Anachem Ltd., Luton, UK), a four-way valve
and a mini-column (5 cm × 3 mm, Omnifit, Cambridge, UK), packed
with the Schiff base functionalized Amberlite XAD-4. The peristaltic
pump was connected to the sample uptake capillary and to the
mini-column via the four way rotary valve. This valve also had a
loop fitted that could be filled with the nitric acid eluent. Transport
lines were made using 0.3 mm i.d. PTFE tubing.