On-line solid phase extraction of Ni and Pb using carbon nanotubes and modified carbon nanotubes coupled to ETAAS

A study about the capabilities of three kinds of nanomaterials namely, carbon nanotubes (CNT), oxidized carbon nanotubes (ox-CNT) and l-alanine immobilized on carbon nanotubes (ala-CNT) to serve as sorbents for preconcentrating Ni and Pb using an on-line system coupled to electrothermal atomic absorption spectroscopy (ETAAS) technique, was accomplished. The solid phase extraction was performed in a conical minicolumn used as sorbent holder. After loading a fixed volume of the analytes, they were eluted with a discrete volume (50 μL) of nitric acid, placed directly into the platform of a L’Vov tube. After that, each analyte was determined individually. Ni and Pb retention was strongly influenced by pH but exhibiting different behaviors. The study demonstrated that the sorbent based on ox-CNTs was the one that exhibited the highest capacity and linearity for both analytes when compared with CNT or ala-CNT. The analytical performance was evaluated for the three sorbents to establish the best conditions regarding sensitivity, reproducibility and accuracy. The precision expressed as relative standard deviations (n = 6) were 3.9 and 0.5% for Ni²⁺ and Pb²⁺, respectively The limit of detection (LOD), calculated as the concentration required to yield a net peak equal to three times the standard deviation of the background signal (3σ) was 30 and 10 ng L⁻¹ for Ni²⁺ and Pb²⁺ respectively. Alternatively, the limit of quantification (10σ) was calculated and resulted to be 0.79 and 0.07 μg L⁻¹ for Ni²⁺ and Pb²⁺ respectively. After optimization, the method that involved the use of ox-CNT associated to an on-line preconcentration was tested in samples of relevant environmental importance. Accuracy was evaluated analyzing a certified reference material namely, Municipal Sludge (QC MUNICIPAL SLUDGE A) and a reference sample of Lake Sediment (TRAP-LRM from IJS).