Ana M. Afonso

The pillared-layer Zn-triazolate metal-organic framework (CIM-81) was synthesized, characterized, and used for the first time as a sorbent in a dispersive micro-solid phase extraction method. The method involves the determination of a variety of personal care products in wastewaters, including four preservatives, four UV-filters, and one disinfectant, in combination with ultra-high performance liquid chromatography and UV detection. The CIM-81 MOF, constructed with an interesting mixed-ligand synthetic strategy, demonstrated a better extraction performance than other widely used MOFs in D-µSPE such as UiO-66, HKUST-1, and MIL-53(Al). The optimization of the method included a screening design followed by a Doehlert design. Optimum conditions required 10 mg of CIM-81 MOF in 10 mL of the aqueous sample at a pH of 5, 1 min of agitation by vortex and 3 min of centrifugation in the extraction step; and 1.2 mL of methanol and 4 min of vortex in the desorption step, followed by filtration, evaporation and reconstitution with 100 µL of the initial chromatographic mobile phase. The entire D-µSPE-UHPLC-UV method presented limits of detection down to 0.5 ng·mL⁻¹; intra-day and inter-day precision values for the lowest concentration level (15 ng·mL⁻¹)-as a relative standard deviation (in %)-lower than 8.7 and 13%, respectively; average relative recovery values of 115%; and enrichment factors ranging from ~3.6 to ~34. The reuse of the CIM-81 material was assessed not only in terms of maintaining the analytical performance but also in terms of its crystalline stability.

Four metal-organic frameworks (MOFs), specifically UiO-66, UiO-66-NH₂, UiO-66-NO₂, and MIL-53(Al), were synthesized, characterized, and used as sorbents in a dispersive micro-solid phase extraction (D-µSPE) method for the determination of nine pollutants of different nature, including drugs, phenols, polycyclic aromatic hydrocarbons, and personal care products in environmental waters. The D-µSPE method, using these MOFs as sorbents and in combination with high-performance liquid chromatography (HPLC) and diode-array detection (DAD), was optimized. The optimization study pointed out to UiO-66-NO₂ as the best MOF to use in the multi-component determination. Furthermore, the utilization of isoreticular MOFs based on UiO-66 with the same topology but different functional groups, and MIL-53(Al) to compare with, allowed us for the first time to evaluate the influence of such functionalization of the ligand with regards to the efficiency of the D-µSPE-HPLC-DAD method. Optimum conditions included: 20 mg of UiO-66-NO₂ MOF in 20 mL of the aqueous sample, 3 min of agitation by vortex and 5 min of centrifugation, followed by the use of only 500 µL of acetonitrile as desorption solvent (once the MOF containing analytes was separated), 5 min of vortex and 5 min of centrifugation. The validation of the D-µSPE-HPLC-DAD method showed limits of detection down to 1.5 ng·L⁻¹, average relative recoveries of 107% for a spiked level of 1.50 µg·L⁻¹, and inter-day precision values with relative standard deviations lower than 14%, for the group of pollutants considered.

An in situ dispersive liquid–liquid microextraction (DLLME) method together with flame atomic absorption spectroscopy (FAAS) is developed for the extraction and determination of copper and cadmium from aqueous samples using as an extraction solvent a low cytotoxic ionic liquid (IL)-based surfactant, specifically decylguanidinium chloride (C₁₀Gu-Cl). The IL-based surfactant 1-hexadecyl-3-butylimidazolium bromide (C₁₆C₄Im-Br) is also used for comparative purposes. In the method, N,N′-bis(salicylidene)thiocarbohydrazide is the complexing agent, and the metathesis reaction in the in situ DLLME is accomplished using lithium bis[(trifluoromethyl)sulfonyl]imide. The main parameters affecting the extraction efficiency of the method, i.e. pH of the sample, NaCl content, and vortex time, are properly optimized with a Doehlert experimental design. The method using C₁₀Gu-Cl exhibited a limit of detection (LOD) four times lower and with slightly better reproducibility than that of the imidazolium IL-based surfactant. The validation of the in situ DLLME-FAAS method with C₁₀Gu-Cl is accomplished for the simultaneous extraction and quantification of copper and cadmium from water. The optimum method requires 10 mL of aqueous sample and ∼25 μL of the IL-based surfactant to obtain a microdroplet of ∼21 μL, which is diluted up to 120 μL with acetonitrile to perform the analytical determination of both metals. LODs down to 0.3 and 0.5 μg L⁻¹ were reached with the entire in situ IL-DLLME-FAAS for copper and cadmium, respectively. The inter-day relative standard deviation values were lower than 17% for both metal ions; with relative recoveries of 97.8% and 102%, and enrichment factors of 85 and 94, for copper and cadmium, respectively, at a low spiked level (4 and 6 μg L⁻¹, respectively). Furthermore, the developed method shows adequate precision and accuracy evaluated with a certified reference material, the absence of matrix effects, and high tolerance limits towards common cations (between 80 and 600) for the analysis of tap water, while accomplishing the requirements of green analytical chemistry.