Cristina Sariego Muñiz

Multi-elemental trace analysis of human serum by double-focusing ICP-MS

A method is described for the determination of a selected range of trace and ultratrace elements in human serum, using a double-focusing ICP-MS instrument (ELEMENT, Finnigan-MAT). Serum samples were diluted 1+4 with ultrapure water and addition of Sc, Ga, Y and Tl as internal standards, to a final concentration of 10 µg L –1 , was used for matrix interference correction. Low and medium resolution scans were performed on the instrument for the analyses, the first scan, at m/Δm=300, for the determination of Rb, Sr, Mo, Cd, Pb and U and the second scan, at m/Δm=3000, for the determination of those elements subject to interference at low resolution: Al, Ca, Cr, Mn, Fe, Co, Cu, Zn and Se. A SRM, 1598 Inorganic Constituents in Bovine Serum (NIST), was analysed for validation of the multi-element analysis method. Good agreement was achieved between the found levels and the certified values. The method was also applied to the multi-analysis of trace elements in serum samples from healthy subjects and from uraemic patients. Preliminary experiments carried out with both quadrupole and double-focusing ICP-MS instruments proved to be invaluable for the study of spectral and non-spectral interferences in ICP-MS detection.

Accurate determination of iron, copper and zinc in human serum by isotope dilution analysis using double focusing ICP-MS

The development of an ICP-MS method for the determination of three essential trace elements, Fe, Cu and Zn, in human serum by isotope dilution analysis is described. A double focusing ICP mass spectrometer (ELEMENT) working at a nominal resolution of 3000 allowed the complete resolution of polyatomic ions overlapping the main isotopes of these elements. The isotope ratios measured for the determination of Fe, Cu and Zn were 57 Fe/ 56 Fe, 65 Cu/ 63 Cu and 67 Zn/ 64 Zn, respectively. Instrumental parameters of the mass spectrometer were optimised for optimum precision, while keeping the total time of analysis under 6 min (2 min per element). The RSD values obtained for the isotope ratios of Fe (57/56), Cu (65/63) and Zn (67/64) were 0.7, 0.2 and 0.8%, respectively, for a natural standard aqueous solution of 50 µg l –1 . The measured isotope ratios were corrected for dead time on the detector and mass discrimination errors. The sample to spike ratio was optimised to minimise the relative error on the isotope ratio measurement. Serum samples were diluted 1+19 with ultrapure water after the addition of the 57 Fe, 65 Cu and 67 Zn spikes. The proposed method was validated by the analysis of NIST SRM 1598 Inorganic Constituents in Bovine Serum, the levels found being in agreement with the certified values. Finally, the method was applied to the analysis of 21 serum samples from healthy blood donors.

Reference values for trace and ultratrace elements in human serum determined by double-focusing ICP-MS

Reference values for trace and ultratrace elements concentrations in healthy human serum, measured by double-focusing inductively coupled plasma-mass spectrometry (ICP-MS), are presented. Blood donors from Asturias (Spain) were selected as the reference population (n=59). Blood samples were collected, after donation, taking the necessary precautions to avoid contamination. All subjects analyzed had normal renal function and nutritional status, as shown from their creatinine and albumin levels. A total number of 14 elements (Al, Ca, Cr, Mn, Fe, Co, Cu, Zn, Rb, Sr, Mo, Cd, Pb, and U) were monitored almost simultaneously. Serum samples were diluted 1+4 with ultrapure water and matrix interferences were corrected using Sc, Ga, Y, and Tl as internal standards. Fe, Cu, and Zn were also determined by isotope dilution analysis (IDA).Reference trace element concentrations intervals observed containing 95% of the reference distribution after excluding outliers are presented. Fourteen serum samples from hemodialysis patients were also analyzed for comparison. High levels of Al, Cr, Sr, Mo, Mn, Pb, U, Co, and Cu and low levels of Fe, Zn, and Rb were found in the serum samples from hemodialysis patients compared to the corresponding reference values observed in this work.

Metal distribution patterns in the mussel Mytilus edulis cytosols using size-exclusion chromatography and double focusing ICP-MS detection

In order to estimate metal distribution patterns in biomolecules of different sizes and their possible relationship with environmental heavy metal contamination, multi-elemental distributions in different fractions of the cytosols of mussels were studied. To do so, samples were collected from natural populations of two coastal regions in Spain: a wild (uncontaminated) coast and an industrialised (contaminated) area in Asturias. Moreover, some commercial mussels from the Galicia coast were also investigated for comparison. Aliquots of the mussel cytosol extracts from each sample were applied to a calibrated Sephadex G-75 column (100 x 1 cm) and forty 3 ml fractions were obtained. After suitable dilution, 18 trace metals were determined by double focusing inductively coupled plasma mass spectrometry (DF-ICP-MS). The use of DF-ICP-MS detection allowed the resolution of several spectral interferences that cannot be resolved by quadrupole ICP-MS. Accurate results for ultratrace elements at basal levels are possible even after sample dilution to prevent matrix effects. After biomolecule-metal association pattern has been established, quantitative analysis of mussel cytosols from the three coastal areas was carried out, using external aqueous calibration plus standard additions to correct for possible matrix effects. Results showed that total metal contents increased following the expected order: wild coast < Galicia coast < industrial area coast. Speciation of Cu, Zn, Ca, U, Ni, Mo, Mn, Cr, V, Cd, Al and Sb showed a similar distribution pattern among cytosolic ligands for all the studied samples. Conversely, Fe, Pb, Sn, Co, Hg and Ag were found to exhibit different speciation patterns when samples from industrialised (contaminated) and non-industrialised areas were compared.

Comparison of electrothermal atomic absorption spectrometry, quadrupole inductively coupled plasma mass spectrometry and double-focusing sector field inductively coupled plasma mass spectrometry for the determination of aluminium in human serum

A detailed examination of the performance of ICP-MS, using both a quadrupole-based and a double-focusing sector field instrument, for the determination of Al in human serum was carried out and the results were compared with those obtained by ETAAS as the reference technique for this type of analysis. Both ICP-MS techniques were compared in terms of observed matrix effects, spectral interferences and overall analytical performance. The results showed that ICP-MS was superior to ETAAS in terms of detection power [detection limits of 0.35 µg l–1 using sector field ICP-MS and 0.85 µg l–1 with quadrupole ICP-MS as compared with 2 µg l–1 for ETAAS determinations of Al in serum (3σ)]. Negligible matrix effects were observed using the quadrupole instrument for serum diluted 1+4 with water. However, the double-focusing instrument showed matrix effects even for a 1+9 dilution of the serum. In any case, such matrix effects could be eliminated by the use of Be and Sc as internal standards. Spectral interferences at m/z=27 in the serum matrix were observed by using the sector field instrument at a resolution of 3000. The analytical methods developed were applied to the determination of Al in reference serum samples with satisfactory results. However, Al serum determination in healthy subjects (n=20) showed that typical basal Al levels in serum required high resolution and, as a rule, were below the detection limit of sector field ICP-MS (i.e., less than 0.35 µg l–1).

Unobserved heterogeneity in work absence

Labour absenteeism may be detrimental to firms and society because of the economic costs, organizational problems and production cuts that it involves. Although involuntary absenteeism due to accident or illness that prevents workers from performing their work is unavoidable, avoidable voluntary absenteeism may also emerge due to asymmetric information given that neither employers nor doctors have perfect information about workers’ health status. Assuming that there is heterogeneity in individual’s behaviour and thus some workers are more likely to take sick leave than others due to differences in observable and unobservable characteristics, we specify a Finite Mixture Model to analyse sick leave days per year using a sample of employees from the 2014 European Health Survey in Spain. This specification accounts for unobserved heterogeneity in a discrete way assuming that there are two types of workers even though the data do not allow us to identify which group any individual belongs to. Our results reveal that, although health indicators have the greatest impact on the proportional change in days of absenteeism, there is heterogeneity in sick leave decisions and individual and job characteristics have different effect on the absenteeism of each group.

Effects of scarcity, aesthetics and ecology on wildlife auction prices of large African mammals

For successful integration of biological conservation into economic markets, economic processes need to capture ecological values. South African wildlife ranching is a tourist-based activity that generates unique information on the economic value of wildlife species. We used public data from South African wildlife auctions to evaluate if annual prices 1991–2012 related to species characteristics associated with scarcity, aesthetics and ecology of South African carnivores and ungulates. While none of the species characteristics influenced carnivore prices, ungulate prices were related to characteristics associated with novelty and aesthetics, which relative importance had increased over time. We raise both ecological and economic concerns for this apparent focus. Our results also suggest a potential importance of non-species-related factors, such as market and buyer characteristics. We encourage further evaluation of the relative influences of species characteristics versus factors that are intrinsically linked to economic processes on price variations in South African wildlife.