María R. Flórez

Direct determination of sulfur in solid samples by means of high-resolution continuum source graphite furnace molecular absorption spectrometry using palladium nanoparticles as chemical modifier

This work investigates the potential benefits of using Pd nanoparticles, in combination with Ru as permanent modifier, for sulfur monitoring as CS by means of high-resolution continuum source graphite furnace molecular absorption spectrometry. Upon heating, these small (approx. 20 nm diameter) Pd nanoparticles are evenly distributed over the platform surface, offering a larger surface for interaction with the analyte during the drying and pyrolysis steps. In this way, a more efficient stabilization of sulfur species can be achieved. Furthermore, a similar analytical response is obtained regardless of the chemical form in which sulfur species are originally found, thus making it easier to develop quantitative analytical methods for this analyte. When using these modifiers, and under optimized working conditions, it is possible to use this technique for direct analysis of different types of solid samples (biological, petroleum coke, polyethylene and steel CRMs), thus circumventing the traditional drawbacks associated with sample digestion when sulfur determination is aimed at. Accurate results are obtained using only the central CS line found at 257.958 nm, with precision values in the 5–10% RSD range (14 ng characteristic mass; 9 ng limit of detection). Moreover, the combined use of the main six CS lines available in the spectral area simultaneously monitored by the detector permits to further improve precision to 3–5% RSD, while decreasing the limit of detection down to 3 ng (the characteristic mass is also 3 ng), which represents a relative limit of detection of approx. 1 μg g−1, as calculated for the sample with the lowest sulfur content. In all cases, straightforward calibration with aqueous standards was proved to be feasible, and the sample throughput is of 3–4 samples per hour (5 replicates per sample).

Progress in the determination of metalloids and non-metals by means of high-resolution continuum source atomic or molecular absorption spectrometry. A critical review

AbstractThis work examines the new possibilities introduced with the arrival of commercially available high-resolution continuum source atomic absorption spectrometers for the determination of metalloids (B, Si, Ge, As, Se, Sb and Te) and non-metals (P, S, F, Cl, Br, I and N-based species), such as the improved potential to detect and correct for spectral overlaps and the strategies available to correct for matrix effects. In particular, and considering the increasing number of papers reporting on the use of molecular absorption spectrometry using graphite furnaces and flames as vaporizers, the work discusses in detail the advantages and limitations derived from the monitoring of molecular spectra from a practical point of view, in an attempt to guide future users of the technique. FigureHeating of a graphite furnace with platform

Simultaneous determination of Co, Fe, Ni and Pb in carbon nanotubes by means of solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry

This work explores the possibilities of solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry for the direct analysis of carbon nanotubes. In particular, the simultaneous determination of Co, Fe, Ni and Pb is intended, as these elements are typically found in these samples as impurities. The results demonstrate that it is possible to find spectral (monitoring of the region between 283.168 and 283.481 nm), furnace (2500 °C for atomization; use of 100 ng Pd as a chemical modifier) and detector (use of side pixels to expand the linear range) conditions that permit the development of a fast and straightforward method for the simultaneous determination of the target elements at the levels at which they are typically found (mg g−1 for Co, Fe and Ni; μg g−1 for Pb) in carbon nanotubes. Limits of detection of 23 pg (Pb), 6 ng (Fe), 65 ng (Ni) and 86 ng (Co) were obtained, which are suitable for this type of sample. In this way, it was feasible to carry out the analysis of the samples investigated (one candidate reference material and three commercially available samples) and achieve accurate results when constructing the calibration curve with aqueous standards. Precision values for 5 solid sample replicates varied between 7 and 15% RSD in most cases. Overall, the proposed method shows important benefits for the cost-effective analysis of such complex samples in routine labs.

Al determination in whole blood samples as AlF via high-resolution continuum source graphite furnace molecular absorption spectrometry: potential application to forensic diagnosis of drowning

In this work, a new methodology for direct determination of Al in whole blood samples by means of high-resolution continuum source graphite furnace molecular absorption spectrometry has been developed, based on the formation of the AlF diatomic molecule in the graphite furnace and the subsequent monitoring of its molecular absorption. The proposed methodology provides an alternative method to conventional atomic absorption, solving most of the problems related to the latter technique, particularly matrix effects, providing a straightforward alternative for blood analysis. The addition of NH4F·HF, which is required for promotion of the AlF molecule, was found to improve sample matrix removal for whole blood samples, whether they contain EDTA or heparin as anticoagulant agents. Besides minimizing residues in the graphite platform, this circumstance enabled the use of aqueous standards to build a calibration curve, avoiding the need for the cumbersome method of standard additions, while not affecting significantly detection capabilities (1.8 μg L−1 LOD). The method developed was also used for exploring the possibilities of Al as a chemical marker assisting forensic diagnosis of death-by-drowning. For this purpose, a set of samples (water and blood) obtained from 8 drowning suspects and two controls were analysed for their Al levels. Although additional studies with a large number of samples would be needed in order to draw definitive conclusions from a forensic point of view, a positive correlation between Al concentration in the drowning water and Al concentration in the blood of drowning suspects was found, supporting the validity of Al as a marker for drowning diagnosis.

Isotope ratio mapping by means of laser ablation-single collector-ICP-mass spectrometry : Zn tracer studies in thin sections of Daphnia magna

In this paper, the possibilities of LA-single collector-ICP-mass spectrometry for obtaining isotope ratio images of thin sections of Daphnia magna specimens exposed to isotopically enriched Zn tracers were evaluated. Zn was selected considering its importance in ecotoxicological studies. All aspects of the analytical methodology deployed were carefully studied and optimized for obtaining the best isotope ratio precision and accuracy. The development of this methodological approach consisted of: (i) evaluation of the performance of two different medium mass resolution exit slits, a conventional one providing a mass resolution of m/Δm ≈ 4000 and triangular shaped peaks, and another (wider) one, offering a lower mass resolution (m/Δm ≈ 2000), but providing enhanced ion transmission and flat-topped peaks; (ii) the use of a wet plasma for improving plasma robustness; (iii) thorough optimization of the ablation conditions (such as laser spot size, repetition rate, scan speed and laser fluence) and data acquisition parameters (such as scan mode, the number of nuclides monitored, mass window, the number of samples per peak and dwell times); and (iv) adequate data treatment including the use of the moving average of 5 individual values. With the methodology developed, isotope ratio images with 30 micrometer spatial resolution scale were obtained for exposed and unexposed Daphnia magna individuals. The typical overall uncertainty of the measurements was approximately 5% RSD for the 66Zn/64Zn and 68Zn/64Zn ratios, which can be considered as satisfactory taking into account that the amount of Zn present per 30 μm × 30 μm pixel only reached a few picograms in the most favourable cases (giving rise to 10 000–20 000 counts per s for 64Zn). These results open the possibility for tracer studies with stable isotopes to be carried out using single collector instrumentation, a much more widespread analytical technique than multi-collector ICP-MS.

Liposomes as an alternative delivery system for investigating dietary metal toxicity to Daphnia magna

Dietary metal toxicity studies with invertebrates such as Daphnia magna are often performed using metal-contaminated algae as a food source. A drawback of this approach is that it is difficult to distinguish between the direct toxicity of the metal and indirect effects caused by a reduced essential nutrient content in the metal-contaminated diet, due to prior exposure of the algae to the metal. This hampers the study of the mechanisms of dietary metal toxicity in filter-feeding freshwater invertebrates. The aim of the present study was to develop a technique for producing metal-contaminated liposomes as an alternative delivery system of dietary metals. These liposomes are not vulnerable to metal-induced shifts in nutrient quality. Liposomes were prepared by the hydration of phosphatidylcholine in media containing either 0 (control) or 50mg Ni/L. The liposomes had average diameters of 19.31 (control) and 10.48 μm (Ni-laden), i.e., a size appropriate for ingestion by D. magna. The liposome particles were then mixed with uncontaminated green algae in a 1/10 ratio (on a dry wt. basis) to make up two diets that differed in Ni content (i.e., 2.0 μg Ni/g dry wt. in the control and 144.2 μg Ni/g dry wt. in the Ni-contaminated diet, respectively). This diet was then fed to D. magna during a 21-day chronic bioassay. The experiment showed that the Ni content and the size distribution of the liposomes were stable for at least 7 days. Also the use of phosphatidylcholine as a liposome component did not affect the reproduction of the daphnids. Exposure to increased level of dietary Ni resulted in 100% mortality after 14 days of exposure and in an increased whole-body Ni concentration in D. magna of 14.9 and 20.4 μg Ni/g dry wt. after 7 and 14 days of exposure, respectively. The Ni-exposed daphnids also exhibited a reduced size (i.e., 30% smaller than the control) after 7 days and a completely halted growth between day 7 and day 14. In terms of reproduction, the size of the first brood (number of juveniles) of the Ni-exposed daphnids was significantly reduced (by 85%) compared to the control. None of the Ni-exposed individuals were able to produce a second brood before dying. The algal ingestion rate - after correction for the indirect effect of a reduced size - was increased (by 68%) by dietary Ni after 6 days of exposure compared to the control, but was severely reduced (by 80% compared to the control) after 13 days. These data suggest that an inhibition of the ingestion process may have contributed to the observed effects of dietary Ni on growth and reproduction beyond 6 days of exposure, although the involvement of other mechanisms cannot be excluded. The mechanism(s) which led to the reduced growth during the first week of exposure remain unclear, although inhibition of the ingestion process can likely be excluded here as an explanation. Overall, this paper demonstrates, using this new method of delivering dietary Ni via liposome carriers and thus excluding potential diet quality shifts, that dietary Ni can indeed induce toxic effects in D. magna. This method may therefore be a promising tool to help further elucidate the mechanisms of dietary metal toxicity to filter-feeding invertebrates.

Natural Fe isotope fractionation in an intestinal Caco-2 cell line model

In this work, Fe isotopic analysis of samples obtained from an in vitro intestinal model was performed via multi-collector ICP-mass spectrometry (MC-ICP-MS) to evaluate the isotope fractionation accompanying Fe uptake and transport mechanisms at a cellular level. The Caco-2 cell line has been used, after cell differentiation, as an enterocyte model and a bi-cameral experimental setup has been developed and optimized for stimulating intracellular Fe fluxes. An Fe : ascorbic acid mixture with a molar ratio of 1 : 5 was used as a source of non-heme bioavailable Fe. Good experimental repeatability and reproducibility were attained with low blank contribution levels, allowing precise and reliable Fe isotope ratio results. Both Fe absorption and transport processes were accompanied by Fe isotope fractionation in favor of the lighter isotopes. After 3 hours of exposure, the isotopic composition of the apical solution and the cells did not significantly differ from that of the original solution added to the cells. After 24 hours of exposure, the trend observed was towards a light Fe isotopic composition in the cells, whereas the apical solutions were enriched in the heavier isotopes. These results were in good agreement with previous in vivo and ex vivo findings. An overall increase in delta Fe values of the cell layers exposed to Fe treatment relative to the corresponding values for the untreated cells also seems to support the assumption of a preferential accumulation of heavy isotopes in enterocyte ferritin. The consistency of the results obtained supports the usefulness of in vitro cell culture models as an interesting complementary tool for studying Fe metabolic pathways at the intestinal level.