Silk fibroin as a sorbent for on-line extraction and preconcentration of copper with detection by electrothermal atomic absorption spectrometry

Silk fibroin is a kind of polypeptide with functional amino acids in its structure. The electric charges in its molecular chains originating from the dissociation of acidic groups, i.e., hydroxyl, phenol and carboxyl, provide vast potentials for the retention of metal species of interest. In this study, the selective retention of Cu²⁺ with silk fibroin at pH 6.0 was investigated and a novel on-line procedure for separation/preconcentration of Cu²⁺ from complex sample matrices was thus developed by using a sequential injection system with an electrothermal atomic absorption spectrometry. A novel concept of enrichment index (EI), i.e., defined as enrichment factor (EF) obtained by consuming unity of sample volume (ml), was proposed for evaluating the enrichment efficiency of a flow-based preconcentration procedure. With a sampling volume of 900 μl, an EI of 30.3 (EF = 27.3) was achieved, which was much improved as compared to that of reported procedures. A detection limit of 8.0 ng l⁻¹ was achieved within a linear range of 0.025-1.5 μg l⁻¹ along with a precision of 2.2% R.S.D. at 0.5 μg l⁻¹. The practical applicability of this procedure was validated by analyzing a certified reference material of riverine water (GBW08608) and a certified reference material of seawater (NASS-5) achieving satisfactory agreements between the certified and the obtained values. A spiking recovery was also performed by using a cave water sample.