Sorbent extraction ETAAS

The dual-column and single-column ion-exchange precoilcentration flame AAS systems as well as the DDC-Cis sorbent extraction flame AAS and ETAAS systems used for the determination of cadmium were also used for the determination of copper in water samples. The detection limit for the dual-column system was 0.07-0.09 /ig l at 60 samples h [7] for the sorbent extraction preconcentration flame AAS system, 0.2 /xg r at 120 samples h [9] for the sorbent extraction ETAAS system, 0.02 /xg 1 at approximately 20 samples h [10]. The sensitivity of sorption column preconcentration methods with flame AAS detection should be sufficient for the determination of copper in most natural water samples. 

Fig.4.11 a-c FI manifold and operation sequence of sorbent extraction preconcentration for ETAAS. P], P2, peristaltic pumps C, micro-conical column (15 pi packed with Ci8 sorbent) V, multifunctional valve L, eluate collector (75 /xl, 0.35 mm i.d.) W, waste and GF, graphite furnace [10]. 

Ibble 4.4 Sequence of operation of FI on-line sorbent extraction preconcentration for ETAAS... 

Despite the improved sensitivity of the flame AAS methods with on-line column preconcentration, the detection limits achieved are still insufficient for most unpolluted water samples. Cadmium in such samples (CASS-1, CASS-2, NASS-2, SLRS-1, etc.) have been successfully determined using an ETAAS system with on-line sorbent extraction preconcentration similar to the one described in Sec. 8.8.3. A detection limit of 0.0008 /ig 1 Cd was achieved with almost complete removal of the interfering matrix of sea water samples [10]. 

The sorbent extraction preconcentration system was miniaturized to be adapted to an ETAAS system achieving an enrichment factor of 26, and a detection limit of 0.003 /xg l without lowering the original sampling frequency of the ETAAS procedure [16]. The interfering matrix of sea water samples was removed in the separation, so that no chemical modifiers were necessary in the ETAAS determination. Results obtained for standard reference sea water CASS-1, CASS-2, NASS-1 and riverine water SLRS-1 agreed well with certified values. The procedure is described in detail in Sec. 8.8.3. 

Fig Recordings of ETAAS atomization signals for lead in water samples using on-line sorbent extraction separation and preconcentration, (a) Blank (b) 0.100 fig P Pb standard (c)... 

The majority of publications on preconcentration for FAAS and ETAAS using FI techniques are based on solid sorbent extraction [47,51]. A typical FI manifold for this type of application is shown schematically in Fig. 8. A microcolumn with 50-100 jiL of sorbent for FAAS or 15 p.L for ETAAS is mounted on the FI valve instead of the sample loop. Sample and complexing agent are mixed online and the analyte complex collected for typically 30-60 sec. The analyte is then eluted directly into the flame or a graphite tube atomizer, usually reversing the direction of flow compared to sample loading. 

Most applications of online solid sorbent extraction for FAAS and ETAAS were for water and seawater samples. That is not surprising because this matrix is somewhat ideal to demonstrate the improvement brought about by the extraction procedure. Seawater causes a lot of problems, particularly in ETAAS, because of its high sodium chloride concentration. Sodium chloride is not retained at all at the packed columns used for sorbent extraction, so that only the trace elements are introduced into the atomizer. However, some biological materials contain fairly high concentrations of iron and copper, elements which may also be retained at the colunm and hence compete with the analyte element for active sites. Copper forms particularly stable complexes over a wide pH range and is retained very easily on a variety of solid sorbents. For this reason, values obtained for cadmium were low in samples with high copper content [52]. 

Nevertheless, some of the data published in the literature on FI online sorbent extraction preconcentration look very promising and should stimulate more research in this field. One of the advantages appears to be that the combined effects of separation of the analyte from concomitants, preconcentration, and sensitivity enhancement by the organic solvent permit several elements to be determined by FAAS which were previously accessible only by ETAAS. 

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