Schiff Base-Modified Magnetic Nanosorbents for Selective Preconcentration of Trace Metal Ions: A Review of Donor-Atom Design, Selectivity Mechanisms and Analytical Performance

Trace metal ions in water, foods and biological samples often are below the detection limits of atomic spectrometric detectors. They come with a hefty matrix. For a ligand specific to the sorbent surface, magnetic solid-phase extraction provides a straightforward response. Schiff bases are formed in a single condensation step and can coordinate the azomethine nitrogen with O, N or S donors, which makes them interesting for this purpose. This review discusses Schiff base modified magnetic nanosorbents from the standpoint of donor atoms. We detail the assembly of the magnetic core, the protective shell and the anchoring imine, and we classify the ligands into N,O, N,N, N,S, N2O2 and N,N,S families. Their behaviour is connected to hard-soft acid-base principles, chelate ring size, stiffness and substituent effects. Selectivity is then discussed in terms of non-specific background uptake, the pH window, ion imprinting and competition from matrix ions. Twenty verified studies are compared in a table of analytical performance. Reported detection limits run from below 1 ng L−1 with ICP-MS, through 0.05 to 0.24 µg L−1 with flame AAS, to 10 µg L−1 with spectrophotometry, and enrichment or preconcentration factors from 1 to 1000. The comparison also shows recurring weak points: hydrolysis of the imine in acidic eluents, loosely held ligands, mixed use of enrichment and preconcentration factors, limited multi-element selectivity tests and infrequent use of certified reference materials. A minimum reporting set is proposed so that future methods can be compared on equal terms.