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We present here the comparison of three mass spectrometric techniques coupled to gas chromatography for the speciation of Hg in complex matrices in order to evaluate the limiting factors for providing accurate and precise quantification results.

The measurement of isotope ratios (within individual species), for a range of organolead compounds, was carried out by coupling a capillary gas chromatograph (HP 6890) to an inductively coupled plasma mass spectrometer (HP 4500). Optimum conditions for the separation and detection of the derivatised lead organometallic compounds were studied focusing on the precise and accurate measurement of isotope ratios on each chromatographic peak. Three lead isotopes were monitored, 206, 207 and 208 using 66 ms integration time per isotope. Under optimum conditions adequate chromatographic peak profiles could be measured. Based on peak area ratios, isotope ratios were determined for five butylated lead compounds (trimethyllead, dimethyllead, triethyllead, diethyllead and inorganic lead) and mixed methyl-ethyl tetraalkyllead compounds from leaded gasoline. Mass bias was corrected using both ethylated and butylated NIST 981 isotopic standard (Common Lead) and a certified enriched 204Pb solution. No isotope fractionation effects were observed during derivatisation of the different organometallic compounds. Applications of this methodology include the differentiation between Pb sources of the different standards used, the measurement of Pb isotope ratios in the organolead compounds present in airborne particulate matter of Oviedo to ascertain their origin and the comparison with isotope ratios in the BCR CRM 605 urban dust reference material. Significant differences between lead isotope ratios in the different species of organolead compounds and in total lead (after acid digestion of the samples) were observed. These differences can be used to trace back sources of lead contamination, by resorting to the speciation data, in cases where the isotope ratios of total lead in the samples are inconclusive in identifying Pb contamination sources.

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.

Following the concepts and applications included in our tutorial review published in 2005, this review presents and critically discusses new concepts, methodologies and trends that have appeared during the last five years of scientific development in the field of isotope dilution analysis for elemental speciation. Although general concepts have been detailed previously in several high quality reviews, we will highlight selected applications that, in our opinion, have significantly contributed to the improvement of the state of the art in this field. Methodological advances, such as the irruption of the Isotope Pattern Deconvolution theory, are commented in detail. Important metrological achievements, such as the organisation of international comparisons by the Comite consultatif pour la quantite de matiere (CCQM) or the certification of new reference materials using Isotope Dilution Analysis are also included. Particular attention has been paid to the developments in the field of bioanalytical chemistry. The application of enriched stable isotopes in metallomics, clinical chemistry and quantitative proteomics are covered in this review. Also, metabolic applications of enriched isotopes are included when speciation work is carried out. Finally, future trends are identified together with the extension of concepts and applications typical of elemental speciation to other scientific fields.

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.

The combined use of single and multiple spike solutions for the determination of butyltin compounds in sediments by species-specific isotope dilution mass spectrometry has been evaluated in this work in order to validate an analytical procedure proposed for routine determinations. For this purpose, a test sediment sample was analysed during the course of an intercomparison exercise using both a single isotope spike solution (a mixture of mono-, di- and tributyltin enriched in 119Sn) and a triple spike solution containing each butyltin species enriched with a different tin isotope. The triple spike methodology (able to correct for interconversion reactions) was employed for the optimisation of the microwave assisted extraction procedure that was subsequently applied for the analysis of the test sediment. The determination of butyltin compounds in the test sediment was performed using both spike solutions under the same extraction conditions. An excellent agreement between the results obtained using both approaches and those provided by all laboratories participating in the intercomparison exercise was obtained, validating in this way the methodologies employed in this work. Uncertainty budgets were calculated for both spiking methodologies. The complexity of the triple spike methodology, in which the simultaneous measurement of nine tin isotope ratios must be carried out, provided additional sources of error in obtaining degradation-corrected concentrations of the three butyltin species. Additionally, uncertainty budgets were obtained also for the calculation of degradation factors F1 (degradation of TBT to DBT) and F2 (degradation of DBT to MBT). The results demonstrated that their uncertainty was independent of the uncertainty of the measurement of the concentration of the butyltin species, both in the spike solution and in the sample, when using a multiple spiking approach, and depended only on the measured isotope ratios and on the uncertainty in the isotopic composition of the spike.