L-cystine-functionalised Fe₃O₄@SiO₂@CPTES magnetic nanoparticles for the selective solid-phase preconcentration of lead and cadmium from human hair and nails prior to flame atomic absorption spectrometry.

Occupational exposure to lead and cadmium in the petroleum industry is recorded in keratinous tissue. In hair and nails, however, the concentrations are low and the digest is rich in alkali and alkaline-earth cations, which together place these analytes below the practical reach of flame atomic absorption spectrometry (FAAS) unless a selective preconcentration step is applied. We therefore developed a silica-coated magnetite adsorbent bearing covalently grafted L-cystine (Fe₃O₄@SiO₂@CPTES@Cystine) for the magnetic solid-phase extraction (MSPE) of Pb(II), Cd(II), Cu(II), Zn(II) and Fe(III) prior to FAAS. L-Cystine was chosen over cysteine because its sulfur is already in the oxidised, dimeric state; it is not consumed by the oxidative pathway that shortens the operational life of thiol-bearing adsorbents, and so trades peak affinity for stability on repeated regeneration. Grafting was confirmed by disulfide stretches at 500 and 540 cm⁻¹ , present only in the final material, and by a fall in surface chlorine below 0.1 at% (X-ray photoelectron spectroscopy, XPS). The mesoporous product (127.3 m²/g) retained a saturation magnetisation of 22.0 emu/g, allowing magnetic capture within 1 min. Under optimised conditions (pH 8.0, 30 mg adsorbent, 100 mL sample, elution with 4 mL of 1.0 M NaOH), recoveries were 90-98% with enrichment factors of 22.5-24.5, maximum Langmuir capacities of 58-105 mg/g, linearity over 0.5-10.0 mg/L (R² = 0.9978-0.9998), and method detection limits of 1.4-2.1 µg/L; trueness was verified against NIST SRM 1643 f and a certified human-hair reference material (NCS ZC81002). All five analytes reached maximum uptake between pH 8.0 and 8.5, but Pb, Cd and Cu were recovered 16-27 %age points more efficiently than Zn and Fe, showing that the selectivity arose from a difference in affinity rather than in optimum pH. Pb and Cd retained above 90% recovery over seven regeneration cycles. Applied to petroleum-facility workers and matched controls, the method revealed six-fold higher Pb and ten-fold higher Cd in workers' hair, whereas the homeostatically regulated elements Zn and Fe differed by no more than 16%. This contrast provides an internal check that the difference reflects accumulated exposure rather than analytical bias, and shows that the method offers a selective, low-cost route to occupational biomonitoring on widely accessible flame instrumentation.