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In this work, a fast and low-cost method for the highly selective determination of ultratrace Hg(II) ions by total-reflection X-ray fluorescence spectrometry (TXRF) is presented. The procedure is based on dispersive micro-solid phase extraction (DMSPE) using thiosemicarbazide-functionalized carbon nanotubes (CNT-TSC). The experiment shows that Hg(II) ions are affectively adsorbed on CNT-TSC at pH 2 within 10 min using an extremely low adsorbent dosage of 1 μg mL−1. Moreover, CNT-TSC is characterized by excellent selectivity toward Hg(II) ions, which can be quantitatively adsorbed from the solution of very high ionic strength (5 mol L−1) and in the presence of the coexisting ions at extremely high concentrations (100 000-fold excess). After the preconcentration/separation step, the suspension of CNT-TSC with adsorbed Hg(II) ions is pipetted onto a quartz reflector and dried. This article shows that the procedure allows overcoming the problems with the volatilization losses of mercury and eliminating the matrix effects in high salinity water. The quantitative DMSPE/TXRF analysis can be performed using the classical calibration curve and simplified approach using only an internal standard. The method is characterized by a very low detection limit of 2.6 pg mL−1 and uses minimal amounts of chemical reagents and a low-power TXRF instrument. The method was applied to analyze water samples, including seawater, and biological samples: lobster hepatopancreas, herring, cormorant, and cod tissues. The excellent selectivity and high adsorption capacity of CNT-TSC toward Hg(II) ions (188 mg g−1) also open the path for the effective removal of Hg(II) ions from aqueous solutions. [ABSTRACT FROM AUTHOR]

An ultrasound-assisted methodology for the determination of dissolved silicate in water has been developed by combining the miniaturized ion-associated based preconcentration method with energy dispersive X-ray fluorescence spectrometry (EDXRF). The method has been devised for the determination of silicate through molybdenum in the ion-associated complex between 12-molybdosilicate and crystal violet. Because silicate is determined indirectly via molybdenum fluorescent radiation the difficulties resulting from low fluorescence yield and low energy of silicon radiation were successfully overcome. A good ratio of silicon to molybdenum (1 to 41) and a sensitive Kα line of molybdenum make possible the determination of low concentration of silicon in the form of dissolved silicate. Under optimized conditions, good linearity, up to 5 μg of silicon in the form of silicate (r = 0.9990) and a detection limit of 9 ng mL−1 were achieved. The total RSD for the EDXRF determination of silicate, followed by precipitation of the ion-associated complex and its dissolution in a microdrop of isoamyl alcohol, was 6.5%. The enrichment factor was equal to 142. The developed method was used to determine the dissolved silica content in surface waters. The accuracy and repeatability of the proposed procedure were checked by the standard addition method and compared to the results obtained using the ICP-OES technique. The recovery (in the 93–97% range) was satisfactory and indicated the usefulness of the developed procedure. [ABSTRACT FROM AUTHOR]

In this paper the combination of dispersive micro solid-phase extraction (DMSPE), using multiwalled carbon nanotubes (MWCNTs) as solid sorbents, with total-reflection X-ray fluorescence spectrometry (TXRF) is proposed for preconcentration and determination of lead and cadmium ions in water samples. The proposed sample preparation is quite simple and economic. After the sorption processes of the metals on the MWCNTs, the aqueous sample is separated by centrifugation and the metal loaded MWCNTs are suspended using a small volume of an internal standard solution and analyzed directly by TXRF. Parameters affecting the extraction process (complexing agent, pH of the aqueous sample, amount of MWCNTs) and TXRF analysis (volume of the deposited suspension on the reflector, drying mode, and instrumental parameters) have been carefully evaluated to test the real capability of the developed methodology for the determination of Cd and Pb at trace levels. For both elements the linear range is observed up to 50 ng mL−1. Under optimized conditions detection limits are 1.0 ng mL−1 and 2.1 ng mL−1 for Cd(ii) and Pb(ii) ions, respectively. Both of the examined elements can be determined with quantitative recoveries (ca. 100%) and with an adequate precision (RSD = 6.0% and 10.5% for Cd(ii) and Pb(ii), respectively). Our results give insight into the possibilities of the combination of DMSPE and TXRF for trace metal determination in different types of environmental waters (sea, river and waste water). [ABSTRACT FROM AUTHOR]

The rapid development in nanomaterials and nanotechnologies has provided many new opportunities in the area related to analytical chemistry. In this paper the unique properties of these materials were used in order to preconcentrate heavy metals prior to detection by spectroscopic techniques. A novel, simple and highly reliable method for the preconcentration of trace amounts of chromium, manganese, cobalt, nickel, copper and zinc using electrochemically assisted sorption on oxidized multiwalled carbon nanotubes (MWCNTs) is developed. The proposed method is based on the application of an electric field to support the sorption process on oxidized MWCNTs. The proposed method was used to preconcentrate trace elements from water of pH = 4. After the preconcentration process, oxidized MWCNTs were directly measured using an energy dispersive X-ray fluorescence (EDXRF) spectrometer that reduces the number of chemicals required and minimizes sample handling. Besides EDXRF, any spectroscopy technique allowing the measurement of solid samples can be applied. The various parameters including pH of the solution, amounts of oxidized MWCNTs, sample volume and the influence of voltage as well as the time of assisted electric field on the sorption process were investigated for the optimization of the analytical procedure. Under optimized conditions the proposed preconcentration method offers a high recovery of 99% (Cr), 97% (Mn), 94% (Co), 96% (Ni), 92% (Cu) and 93% (Zn) and good precision of sample preparation (RSD) within 3.5–7%. If measurements are performed using the EDXRF spectrometer, the detection limits are 1 ng mL−1, 5 ng mL−1, 5 ng mL−1, 7 ng mL−1, 1 ng mL−1, 8 ng mL−1 for Cr(iii), Mn(ii), Co(ii), Ni(ii), Cu(ii) and Zn(ii), respectively. [ABSTRACT FROM AUTHOR]

Ultrasound-assisted solid-phase extraction (SPE) with multiwalled carbon nanotubes (MWCNTs) as a solid sorbent and 1-(2-pyridylazo)-2-naphthol (PAN) as a chelating agent was developed for the determination of trace and ultratrace amounts of Cd(ii) ions in aqueous samples. The solution containing Cd(ii) complexed with PAN was passed through the syringe filter with a PVDF membrane loaded with 25 mg of MWCNTs. The elution process was performed with 2.0 mL of 2.0 mol L−1 HNO3 with assistance of sonication. In order to obtain the high recovery of the Cd(ii) ions, the proposed procedure was optimized for various analytical parameters such as pH, amounts of PAN, sample volume, and elution conditions such as volume and concentration of eluent and influence of sonication and temperature. The investigation shows that sonication considerably improves the recovery, whereas temperature does not influence the elution process. Under optimized conditions Cd(ii) ions can be determined with very good recovery (99 ± 3%), precision (RSD = 3.2%) and detection limit (0.3 ng mL−1) for an enrichment factor of 50. The effect of common coexisting ions was also investigated. Cd(ii) can be determined in the presence of heavy metal ions and alkali metals. The proposed ultrasound-assisted SPE was applied for the determination of Cd(ii) in water samples and biological samples (Lobster Hepatopancreas). [ABSTRACT FROM AUTHOR]

A dispersive solid-phase microextraction (DSPME) with multiwalled carbon nanotubes (MWCNTs) as solid sorbent and ammonium pyrrolidinedithiocarbamate (APDC) as chelating agent was developed for determination of selenium. In the proposed procedure, the Se(iv)–APDC complex is adsorbed on MWCNTs dispersed in aqueous samples. After the adsorption process, the aqueous samples are filtered and MWCNTs with selenium chelate are collected onto a filter. The loaded filters are directly measured using X-ray fluorescence (XRF) spectrometry. In order to obtain high recovery of the Se ions on MWCNTs, the proposed procedure was optimized for various analytical parameters such as pH, amounts of MWCNTs and APDC, sample volume and time of the sorption process. Under optimized conditions Se ions can be determined with very good recovery (97 ± 3%), precision (RSD = 3.2%) and detection limits (from 0.06 to 0.2 ng mL−1, depending on counting time and XRF equipment). The effect of common coexisting ions was also investigated. Se(iv) can be determined in the presence of heavy metal ions and alkali metals. The chemical interferences observed for high concentrations of Cu(ii), Fe(iii), and Zn(ii) can be completely eliminated using precipitation with NaOH. The proposed method was applied for the determination of Se in mineral water and biological samples (Lobster Hepatopancreas). The proposed method can also be applied for selenium speciation. The concentration of selenate can be obtained as the difference between the concentration of selenite after and before prereduction of selenate to selenite. [ABSTRACT FROM AUTHOR]

Directly suspended droplet microextraction (DSDME) for determination of phosphate by energy-dispersive X-ray fluorescence spectrometry (EDXRF) is proposed. The method is based on the determination of phosphate, through molybdenum in the phosphomolybdenum blue complex. Because phosphorus is determined indirectly via molybdenum fluorescent radiation the difficulties of low fluorescent yield and low energy of phosphorus radiation are successfully overcome. A good ratio of phosphorus to molybdenum (1 to 37) and a sensitive Kα line of molybdenum give the possibility to determine a low concentration of phosphorus. Under the optimized conditions, good linearity, up to 20 μg of phosphorus (r = 0.9995), and the detection limit (2.8 ng mL−1) for the sample of 1.5 mL were achieved. The total RSD for the EDXRF determination of phosphorus followed by DSDME was 2.6% and taking into account all steps preceding the determination and the uncertainty of EDXRF measurements, the proposed method can be recognized as highly precise. The enrichment factor was equal to 56.8. The developed method was used to determine the total phosphorus content in surface waters. The accuracy and repeatability of the proposed procedure were checked by the standard addition method. The recovery in the range 93.5–97% was satisfactory and indicated the usefulness of the developed procedure. [ABSTRACT FROM AUTHOR]

The adsorptive properties of graphene oxide (GO) towards divalent metal ions (copper, zinc, cadmium and lead) were investigated. GO prepared through the oxidation of graphite using potassium dichromate was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR). The results of batch experiments and measurements by flame atomic absorption spectrometry (F-AAS) indicate that maximum adsorption can be achieved in broad pH ranges: 3-7 for Cu(II), 5-8 for Zn(II), 4-8 for Cd(II), 3-7 for Pb(II). The maximum adsorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) on GO at pH = 5 are 294, 345, 530, 1119 mg g(-1), respectively. The competitive adsorption experiments showed the affinity in the order of Pb(II) > Cu(II) ≫ Cd(II) > Zn(II). Adsorption isotherms and kinetic studies suggest that sorption of metal ions on GO nanosheets is monolayer coverage and adsorption is controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO. Chemisorption was confirmed by XPS (binding energy and shape of O1s and C1s peaks) of GO with adsorbed metal ions. The adsorption experiments show that the dispersibility of GO in water changes remarkably after complexation of metal ions. After adsorption, the tendency to agglomerate and precipitate is observed. Excellent dispersibility of GO and strong tendency of GO-Me(II) to precipitate open the path to removal of heavy metals from water solution. Potential application of GO in analytical chemistry as a solid sorbent for preconcentration of trace elements and in heavy metal ion pollution cleanup results from its maximum adsorption capacities that are much higher than those of any of the currently reported sorbents.