Mariella Moldovan

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.

Measurement of strontium isotope ratios by MC-ICP-MS after on-line Rb–Sr ion chromatography separation

Large volume injection ion chromatography has been evaluated for the precise and accurate measurement of strontium isotope ratios by multicollector ICP-MS. On-line coupling of the ion chromatograph to the MC-ICP-MS instrument allowed the complete separation of isobaric rubidium and other matrix elements (Ca, Mg, Na and K) and the measurement of strontium isotope ratios on flat-topped chromatographic peaks of ca. 3 minutes duration. No drift in the strontium isotope ratios was observed during strontium elution and mass bias could be corrected using either the 86Sr/88Sr isotope ratio (internal correction) or the observed 87Rb/85Rb isotope ratio (external correction) in the real samples. Typical internal precisions obtained with the selected measurement and correction conditions were about 2 times lower than the precisions obtained with continuous sample introduction for a solution 50 ng g–1 of Sr NIST SRM 987 in HNO3 2% (typical standard deviations of 0.00022–0.00036 for the corrected 87Sr/86Sr ratio in comparison to 0.00014–0.00020 for continuous sample introduction). The method was applied to the measurement of strontium isotope ratios in cider samples and the results agreed with previous data obtained after an off-line Rb–Sr separation procedure using the Sr-Spec chelating material. The precision obtained for the real samples was poorer than previous data due to the low concentration of strontium in the measured samples. The method was also applied to other sample types (soil leachates, apple leaves and apples) to study the transference of strontium isotope ratios in the soil–tree–cider system.

Multiple linear regression and on-line ion exchange chromatography for alternative Rb–Sr and Nd–Sm MC-ICP-MS isotopic measurements

A mathematical procedure of multiple linear regression in combination with on-line liquid chromatography was applied for the measurement of Sr and Nd isotope ratios by multicollector ICP-MS in the presence of isobaric isotopes. The separation of Rb and Sr and the separation of Nd and Sm were accomplished by ion exchange chromatography with large volume injection. This mode of injection allowed wide flat-topped chromatographic peaks to be obtained while maintaining chromatographic resolution and avoiding sample dilution. The application of the multiple linear regression procedure to the raw chromatographic data allowed the determination of the isotope ratios of Sr or Nd without interferences from isobaric isotopes. For Sr measurements, the raw 87Sr/86Sr isotope ratio precision (0.006 to 0.010%) was similar to that of continuous sample introduction (ca. 0.003%) at the same concentration level (50 ng g−1). The mathematical procedure was first evaluated for the cation exchange separation of Rb and Sr. Strontium isotope ratios, obtained using NIST SRM 987, were in agreement with the certified values (bias ca. 0.01%). For the case of Nd and Sm, their chromatographic separation was accomplished by anion exchange using EDTA as mobile phase. Neodymium eluted at 12.8 minutes while samarium eluted at 17.5 minutes, showing flat-topped chromatographic peaks and maintaining baseline neodymium–samarium separation. Precise neodymium and samarium isotope ratios suitable for Nd–Sm dating could be measured by MC-ICP-MS using the proposed multiple linear regression procedure using only one cup configuration.

Individual-specific transgenerational marking of fish populations based on a barium dual-isotope procedure.

The present study focuses on the development and evaluation of an individual-specific transgenerational marking procedure using two enriched barium isotopes, (135)Ba and (137)Ba, mixed at a given and selectable molar ratio. The method is based on the deconvolution of the isotope patterns found in the sample into four molar contribution factors: natural xenon (Xe nat), natural barium (Ba nat), Ba135, and Ba137. The ratio of molar contributions between Ba137 and Ba135 is constant and independent of the contribution of natural barium in the sample. This procedure was tested in brown trout ( Salmo trutta ) kept in captivity. Trout were injected with three different Ba137/Ba135 isotopic signatures ca. 7 months and 7 days before spawning to compare the efficiency of the marking procedure at long and short term, respectively. The barium isotopic profiles were measured in the offspring by means of inductively coupled plasma mass spectrometry. Each of the three different isotopic signatures was unequivocally identified in the offspring in both whole eggs and larvae. For 9 month old offspring, the characteristic barium isotope signatures could also be detected in the otoliths even in the presence of a high and variable amount of barium of natural isotope abundance. In conclusion, it can be stated that the proposed dual-isotope marking is inheritable and can be detected after both long-term and short-term marking. Furthermore, the dual-isotope marking can be made individual-specific, so that it allows identification of offspring from a single individual or a group of individuals within a given fish group.

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

Comparison of gas chromatography-combustion-mass spectrometry and gas chromatography-flame ionization detector for the determination of fatty acid methyl esters in biodiesel without specific standards.

GC-FID has been effectively used as a universal quantification technique for volatile organic compounds for a long time. In most cases, the use of the ECN allows for quantification by GC-FID without external calibration using only the response of a single internal standard. In this paper we compare the performance characteristics of GC-FID with those of post-column (13)C Isotope Dilution GC-Combustion-MS for the absolute quantification of organic compounds without the need for individual standards. For this comparison we have selected the quantification of FAMEs in biodiesel. The selection of the right internal standard was critical for GC-FID even when ECN were considered. On the other hand, the nature of the internal standard was not relevant when GC-Combustion-MS was employed. The proposed method was validated with the analysis of the certified reference material SRM 2772 and comparative data was obtained on real biodiesel samples. The analysis of the SRM 2772 biodiesel provided recoveries in the range 100.6-103.5% and 96.4-103.6% for GC-combustion-MS and GC-FID, respectively. The detection limit for GC-combustion-MS was found to be 4.2ng compound/g of injected sample. In conclusion, the quantitative performance of GC-Combustion-MS compared satisfactorily with that of GC-FID constituting a viable alternative for the quantification of organic compounds without the need for individual standards.

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.

Instrumental Setup for Simultaneous Total and Speciation Analysis of Volatile Arsenic Compounds in Gas and Liquefied Gas Samples

Although analysis of metals and metalloids, such as arsenic, is widely spread in many different fields, their analysis in gas and liquefied gas samples is still a challenge. A new GC-ICP-MS set up has been developed for their simultaneous total and speciation analysis in gas and liquefied gas samples without the need of a preconcentration step. An arsine in nitrogen standard was used for optimization and evaluation of the system. Good linearity and detection limits in the very low ppt level for both total and speciation analyses were found. Liquefied butane pressurized under nitrogen and doped with arsine and a propylene real sample from a cracker plant were analyzed using both external calibration and standard additions methods. The good match between both quantifying approaches demonstrated almost negligible matrix effects, even for the total analysis. Application of the approach to check repartition of volatile elements or species between gas and liquid phases was performed in the real propylene sample. Finally, its potential applicability for the simultaneous total and speciation analysis of other elements, such as Hg, was also proved.