Flow injection on-line sorbent extraction pre-concentration for graphite furnace atomic absorption spectrometry
Flow injection (FI) techniques were used to develop an efficient on-line sorbent extraction pre-concentration system for graphite furnace atomic absorption spectrometry with lead as a model trace element. Bonded silica with octadecyl functional groups (C18) was used as sorbent in a 15-µl conically shaped micro-column. The lead diethyldithiocarbamate chelate was formed on-line, loaded on to the column for 60 s, washed with de-ionised water and eluted with ethanol into a collector with a volume of 75 µl, made from 0.35 mm i.d. polytetrafluoroethylene tubing. The collected eluate was introduced into the graphite tube by an air flow on initiation of the FI system for pre-concentration of the next sample. The reproducibility of FI peak gradients was exploited through precise timing of the collection of the eluate fraction so that end sections of the eluate bolus containing lower analyte concentrations were discarded. A 26-fold enhancement in peak area compared with 50 µl direct introduction was obtained with 60-s pre-concentration while achieving a precision of 1.9% relative standard deviation for 0.1 µg l–1 of Pb (n= 11), and a detection limit of 0.003 µg l–1(3σ). The complete cycle of operation was 150 s which conveniently fitted into the time interval between successive graphite furnace firings. Determination of Pb in sea water samples showed that the interfering matrix had been almost completely removed during the pre-concentration, and determinations were made without using chemical modifiers. Results obtained for National Research Council of Canada Standard Reference Materials CASS 1, CASS 2 [Seawater (Coastal)] and NASS 1 [Seawater (Open Ocean)] using matrix-matched standards and for the SLRS 1 Riverine Water using simple aqueous standards agreed well with certified values.