Técnicas de microextracción con nanomateriales aplicadas a la determinación de especies inorgánicas con espectrometría de absorción atómica

Resum

This Memory deals with dispersive solid-phase microextraction (DSPME) processes coupled to electrothermal atomic absorption spectrometry (ETAAS) for the purpose of the analytical determination and speciation of several metallic elements and arsenic in waters, beverages and some foods. Three materials with a low particle size (nanomeric scale) are studied, namely, graphene oxide (GO), microcrystalline cellulose (MCC) and ferrite. As a consequence, six new analytical procedures that use relatively simple protocols are proposed for the determination and speciation of five elements at very low concentration levels. For the case of using ferrite as the adsorbent, the separation of the dispersed solid-phase is carried out by means of a neodymium magnet. For the procedures involving GO or MCC, the solid material is collected with the help of micelles obtained when a tensioactive (Triton X-45 or similar) is added to the dispersed suspensions and the mixture is heated at a temperature above the critical temperature of the surfactant (cloud point extraction, CPE). A conventional centrifugation step then allows the final separation of the coacervate (the surfactant-rich phase). The reliability of the procedures is, in all cases, validated using certified reference materials with matrices similar to that of the sample studied. The treatment of GO with a reducing agent (sodium tetrahydroborate or sodium citrate) in the presence of silver nitrate results in the formation of a nanocomposite (Ag@rGO) containing partially reduced GO useful for the CPE of trace amounts of cadmium and lead which are concentrated in the surfactant-rich phase. Using the optimized conditions, a linear relationship between the analytical signal (peak area obtained during the atomization stage in the ETAAS heating program) and the concentration of cadmium and lead in the original sample was found in the 0.006 - 0.1 and 0.1 - 2 µg L-1 with detection limits of 0.002 and 0.03 µg L-1 respectively for a typical 10 mL sample volume. When a CPE process is carried out in the presence of GO, very small amounts of vanadium are concentrated in the coacervate obtained being thus the basis for a new and sensitive procedure for its determination. Calibration graphs are linear in the 0.06-3.3 µg L-1range with a detection limit of 0.02 µg L-1 vanadium. The feasibility of vanadium speciation is also proved. On the other hand, a new analytical procedure for chromium (III) and chromium (VI) determination based in the fast adsorption of their ions by MCC is optimized. In this case, referring again to a typical 10-mL sample volume, calibration graphs are linear in the 0.02-0.5 µg L-1 chromium range. The procedure permits an easy speciation of chromium at low concentration levels. The use of freshly prepared ferrite nanoparticles allows metallic silver and monovalent silver to be speciated in a variety of samples (waters, lixiviates obtained from materials treated with silver nanoparticles, lixiviates obtained from the washing of edible vegetables¿.). The same nanomaterial, when used at pH close to neutrality, also permits a selective retention of chromium (III) and chromium (VI) which is the basis of another new, sensitive speciation procedure for determining this metal. Of a special relevance is the use of the freshly prepared ferrite particles for the speciation of several arsenic compounds, namely trivalent and pentavalent arsenic and monomethyl arsonic acid (MMA). In these procedures, once the rapid retention step is finished, the nanoparticles are separated by means of a magnet and then resuspended in a low volume (0.1 mL) before the final ETAAS measurement. As with the other procedures here optimized, the typical sample volume recommended is 10 mL so that enrichment factors close to 100 are obtained and excellent sensitivities are achieved. Thus, for the procedures for silver, chromium and arsenic species, the linear calibration graphs cover the 0.01-0.3, 0.03-0.4 and 0.05 y 2 µg L-1 ranges, respectively. The research here summarized corroborates that the combination of electrothermal atomic absorption spectrometry, a technique available in all laboratories, together with relatively simple reagents to carry out microextraction processes allows extreme sensitivities in the determination of metals to be achieved without the need of using more expensive instrumentation such as inductively coupled plasma mass spectrometry.