José Ángel Rodríguez-Castrillón

Isotope pattern deconvolution for internal mass bias correction in the characterisation of isotopically enriched spikes

Enriched spikes of 53Cr, 57Fe, 67Zn, 95Mo, 111Cd, 143Nd, 150Sm, 199Hg and 207Pb were characterised on a multi-collector ICP-MS instrument both in terms of isotopic composition and concentration. The isotopic composition of the spikes was determined using either a natural abundance or certified standard for mass bias correction, except for lead where thallium internal correction was used. It was observed that the use of weighted regression lines for mass bias correction provided similar or, in most cases, lower uncertainties in the isotopic composition of the spikes in comparison with unweighted regression lines or single ratio measurements. For the establishment of the spike concentration by reverse isotope dilution analysis, isotope pattern deconvolution was evaluated and mass bias could be corrected internally in each blend by minimising the variance of the multiple linear regression model. It was also observed that isotope pattern deconvolution with internal mass bias correction provided lower uncertainties in the concentration of the spikes in comparison with the usual procedure based on external mass bias correction.

Enriched stable isotopes and isotope pattern deconvolution for quantitative speciation of endogenous and exogenous selenium in rat urine by HPLC-ICP-MS

Interest in selenium metabolism and its role in health and disease continue to stimulate a lot of bioanalytical research. The use of enriched stable isotopes of Se and isotope pattern deconvolution (IPD) with an ICP-MS in lactating rats is approached here for total selenium distribution studies. A mathematical tool has been developed to calculate supplemented and endogenous total Se contents in urine and faeces by ICP-MS. Also, quantification of endogenous (natural) and exogenous seleno-species by HPLC-ICP-MS has been worked out. The proposed IPD methodology, for total determinations and for quantitative speciation, was applied to reference materials to validate total selenium quantification in faeces and quantitative selenium speciation in urine samples. Selenium apparent absorption and retention and natural and exogenous selenium distribution in urine samples were calculated. Interesting quantitative information about selenium bio-transformations and final catabolism can be obtained from careful examination of the results observed, using such analytical tools, of the urinary endogenous and exogenous selenium metabolites in urine of rats and their kinetics throughout the two weeks of selenium supplementation.

Internal correction of spectral interferences and mass bias in ICP-MS using isotope pattern deconvolution: Application to the determination of selenium in biological samples by isotope dilution analysis

The internal and simultaneous correction of mass bias and spectral interferences (BrH+ and SeH+) in the determination of total selenium in biological samples by isotope dilution analysis is evaluated here using isotope pattern deconvolution (IPD). The procedure takes the measured intensities at masses 76 to 82 in the spiked samples (using 77Se as spike) and calculates the isotopic composition of selenium. This isotopic composition would be a linear function of the isotopic composition of natural abundance selenium and that of enriched 77-selenium if no mass bias or spectral interferences were present or if they were adequately corrected. So, the molar fractions of natural abundance selenium and isotopically enriched selenium are calculated using IPD along with the variance of the multiple linear regression model. The variance of the regression is first calculated when no correction is applied (either mass bias or spectral interferences) and then this value is minimised by applying different factors for mass bias and the ratios SeH/Se and BrH/Br. It is demonstrated that, using the SOLVER application of Excel, the regression variance can be minimized and the correction factors converge to those factor which could have been calculated by external correction. Hence, this procedure eliminates the need for any external correction of mass bias or isobaric interferences in isotope dilution analysis provided that enough isotopes of the element are measured. To illustrate the applicability of this method, raw data obtained by Hinojosa Reyes et al. in a previous paper (J. Anal. At. Spectrom., 2003, 18, 11) were recalculated using the proposed methodology with satisfactory results.

Internal correction of hafnium oxide spectral interferences and mass bias in the determination of platinum in environmental samples using isotope dilution analysis.

AbstractA method has been developed for the accurate determination of platinum by isotope dilution analysis, using enriched 194Pt, in environmental samples containing comparatively high levels of hafnium without any chemical separation. The method is based on the computation of the contribution of hafnium oxide as an independent factor in the observed isotope pattern of platinum in the spiked sample. Under these conditions, the ratio of molar fractions between natural abundance and isotopically enriched platinum was independent of the amount of hafnium present in the sample. Additionally, mass bias was corrected by an internal procedure in which the regression variance was minimised. This was possible as the mass bias factor for hafnium oxide was very close to that of platinum. The final procedure required the measurement of three platinum isotope ratios (192/194, 195/194 and 196/194) to calculate the concentration of platinum in the sample. The methodology has been validated using the reference material “BCR-723 road dust” and has been applied to different environmental matrices (road dust, air particles, bulk wet deposition and epiphytic lichens) collected in the Aspe Valley (Pyrenees Mountains). A full uncertainty budget, using Kragten’s spreadsheet method, showed that the total uncertainty was limited only by the uncertainty in the measured isotope ratios and not by the uncertainties of the isotopic composition of platinum and hafnium. FigureSimultaneous correction of hafnium oxide spectral interferences and mass bias in the determination of platinum in environmental samples using isotope dilution analysis

Development of a Dual-Isotope Procedure for the Tagging and Identification of Manufactured Products: Application to Explosives

A novel chemical tagging approach, based on a dual-isotope procedure, is presented. The method has been applied to explosives tagging. The method is based on the addition to the explosive of two enriched isotopes of the same element, which may be already present within it, at a given molar ratio. This dual-isotope approach will give a unique fingerprint to the tagged explosive. Further, the authentication of the tagged explosive or its residues will be obtained by comparison of the ratio of molar fractions experimentally measured by inductively coupled plasma mass spectrometry (ICP-MS) with the molar fraction ratio of the tagging mixture. The novelty of this tagging method relies on working with isotope abundances and molar fraction ratios instead of the classical isotope ratios, and this fact constitutes the strong point of the described approach since the molar ratio is not affected by physical, chemical, or biochemical processes, and it is also not disturbed by environmental contamination with the natural abundance element. Furthermore, the use of molar fraction ratios overcomes the nonhomogeneous distribution of the tagging element within the explosive. As the tagging element can be present at trace or ultratrace levels, a very small amount of enriched isotopes needs to be added, denoting a low cost solution. Also, the use of enriched stable isotopes of nontoxic elements will have negligible health effects or affect the environment.

Use of the stable isotope 57Fe to track the efficacy of the foliar application of lignosulfonate/Fe3+ complexes to correct Fe deficiencies in cucumber plants

BACKGROUND During the last decade, environmental concerns regarding the use of recalcitrant synthetic chelates to overcome iron chlorosis has increased and new ligands such as lignosulfonates (LS) have been evaluated. However, the efficacy of these products is variable. In this work a hardwood (eucalyptus) and softwood (spruce) LS were compared to try to relate their physico-chemical characteristics and their efficacy. Also two more products derived from the eucalyptus lignosulfonate were tested. RESULTS All the LS tested presented a good ability to complex Fe, but only the spruce LS was capable to maintain significant amounts of soluble Fe above pH 8. According to the FTIR data, structural changes related to the Fe source (Fe(2+) or Fe(3+) ) used to form the complex occurred in the LS molecule and might influence their efficacy. Cucumber (Cucumis sativus L. cv Ashley) chlorotic plants were used to test lignosulfonate efficacy when applied through foliar sprays in comparison with FeSO(4) and EDTA/(57) Fe(3+) . The (57) Fe content of plants sprayed with LS was very low in respect to the EDTA treatment, but this was not reflected in the biomass and re-greening rates. Eucalyptus LS modifications improve its efficacy for iron chlorosis recovery to levels similar to those found for the spruce LS. Two applications of the LS are recommended. CONCLUSIONS Lignosulfonates did not require surfactants for their application; they did not burn the leaves, and had a stimulating effect on the vegetative growth of the plants. So these by-products could be a good alternative when applied through foliar sprays for cucumber plants.

Evaluation of alternative procedures for the provision of consensus and assigned values to sixteen trace elements in natural water used for an ICP-MS proficiency testing exercise

The University of Oviedo, in collaboration with the company ISC-Science, has conducted a proficiency testing exercise for Spanish ICP-MS users focused on trace elements determination in drinking waters. A total of 47 laboratories participated in this exercise where 16 elements (Al, Sb, As, B, Cd, Cu, Cr, Fe, Mn, Ni, Pb, Se, Sr, Mo, U, and Zn) were determined in a natural water sample. The analytical procedure used at the University of Oviedo was validated by analysis of the certified reference material SLRS-4 (riverine water) from the National Research Council of Canada. Assigned values were provided for 12 multi-isotopic elements by isotope dilution analysis. For this purpose, isotopically enriched spikes (10B, 53Cr, 57Fe, 61Ni, 65Cu, 67Zn, 77Se, 86Sr, 95Mo, 111Cd, 123Sb and 207Pb) were characterised by reverse isotope dilution analysis both by multicollector ICP-MS and collision cell ICP-MS (10B and 77Se). Finally, the collision cell instrument was adopted for the provision of the assigned values taking into account the results from the homogeneity and stability studies on the natural water sample. Considering the differences in expertise between the different laboratories, consensus values for all sixteen measured elements were calculated using alternative statistical tests for outlier removal. It was observed that the combination of the Cochran and Hampel tests provided the best agreement between the consensus and the assigned values. Finally, the analytical results from the different laboratories were evaluated by different scoring procedures taking into account the requirements of European Directive 98/83/EC for drinking water testing. As these types of exercises will be used by many laboratories to seek or maintain accreditation, in intercomparison exercises where consensus values are usually applied, the evaluated procedures for outlier elimination and scoring are critically compared.