2. Experimental
2.1. Reagents and materials
Analytical reagent-grade chemicals were used as well as deionised water from a Milli-Q-system. Standard solutions of manganese were prepared by the appropriate dilution of 1000 mg l−1 of a stock solution from Merck. 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) from Fluka was used without further purification.
The macroporous anion-exchange resin Amberlite IRA-904 (50–100 mesh) from Bio Rad Laboratories was applied to prepare the microcolumns.
Buffer components were of analytical grade and used without further purification. The phosphate buffer (0.05 M) was used for pH 6–8 and borate buffer (0.05 M) for pH 8–10.
2.2. Apparatus
A Perkin-Elmer 3110 atomic-absorption spectrometer equipped with a deuterium background corrector utilised the Mn 279.5 nm line. An air–acetylene flame was adjusted according to the manufacturer's recommendation. Time-resolved absorbance signals of the elution peaks were displayed on the monitor and printed together with the peak height values (evaluation of the spectrometer was operated with a time contact of 1 s).
2.3. Procedures
The FI systems used, shown schematically in Fig. 1, consisted of an Ismatec MS/4 Reglo peristaltic pump, two Rheodyne RH 5020 low-pressure injection valves and a microcolumn manufactured from a pipette tip (the ends of which were fitted with cotton to retain the sorbent in the tube). The microcolumn (packed with 200 mg of sorbent) was integrated into the system between the eluent injection valve and the nebulizer of the instrument. It was initially washed with 2 M HNO3 and deionised water before use. PTFE tubes (0.8 mm i.d.) were employed for the coils. The flow system was operated in the time-based mode and deionised water served as the carrier stream.