Christophe R. Quétel

Evaluation of detector dead time calculation models for ICP-MS

Four methods of pulse counting detector dead time calculation were evaluated on two quadrupole ICP-MS instruments, using the criteria of ease of application and accuracy (i.e., reliable measurement uncertainty). Three of the methods were based on measurements of the 204Pb/208Pb isotope ratio in the NIST Standard Reference Material NBS-982, while the fourth method was based on the comparison of pulse counting signals versus the corresponding analog signals across a range of signal intensities. The uncertainty in the dead time result derived from each method was evaluated and found to vary considerably, with some approaches underestimating this parameter, while others yielded large uncertainties. Overall, a method based on studies of the 204Pb/208Pb isotope ratio measured for different Pb concentration solutions, recalculated using different dead time settings (herein referred to as Methodxa02), was found to be the most suitable in terms of the specified criteria, particularly with respect to its uncertainty evaluation.

Anthropogenic vs. lithogenic origins of trace elements (As, Cd, Pb, Rb, Sb, Sc, Sn, Zn) in water column particles: northwestern Mediterranean Sea

Abstract The distribution of heavy metal was analyzed in water column particles collected in autumn (October 1985) and spring (March 1986) by two series of sediment traps from a mooring located in the northeastern Mediterranean Sea continental slope. Four traps were set, at 50, 100, 300, 600 m depths on the mooring in 645 m deep water in the Lacaze-Duthier canyon. The total metal concentrations were determined by ICP-MS. Results show that Rb and Sc contents display typical shale values. As, Cd, Pb, Sb, Zn (normalized to Sc) display high enrichment factors (up to 50) over shale compositions. Distinctive temporal variability as well as the respective contributions of local (Tet, Aude) and remote (Rhone) rivers and Sahara-derived aerosols have been identified. Fluxes of most elements observed in the upper 100 m can be attributed to atmospheric fluxes. In the deepest traps (300 and 600 m) these fluxes are, however, mostly dominated by riverine particles advected from the continental shelf. Most of the trace-element enrichments are more likely to be related to the anthropogenic input rather than to biological cycling. Isotopic composition of lead determined by ICP-MS enabled to evaluate that the proportion of anthropogenic lead derived from European gasoline consumption ranged between 50 and 100%.

Flow injection analysis as a tool for enhancing oceanographic nutrient measurements—A review

Macronutrient elements (C, N and P) and micronutrient elements (Fe, Co, Cu, Zn and Mn) are widely measured in their various physico-chemical forms in open ocean, shelf sea, coastal and estuarine waters. These measurements help to elucidate the biogeochemical cycling of these elements in marine waters and highlight the ecological and socio-economic importance of the oceans. Due to the dynamic nature of marine waters in terms of chemical, biological and physical processes, it is advantageous to make these measurements in situ and in this regard flow injection analysis (FIA) provides a suitable shipboard platform. This review, therefore, discusses the role of FIA in the determination of macro- and micro-nutrient elements, with an emphasis on manifold design and detection strategies for the reliable shipboard determination of specific nutrient species. The application of various FIA manifolds to oceanographic nutrient determinations is discussed, with an emphasis on sensitivity, selectivity, high throughput analysis and suitability for underway analysis and depth profiles. Strategies for enhancing sensitivity and minimizing matrix effects, e.g. refractive index (schlieren) effects and the important role of uncertainty budgets in underpinning method validation and data quality are discussed in some detail.

Factorial optimization of data acquisition factors for lead isotope ratio determination by inductively coupled plasma mass spectrometry

Abstract A factorial optimization of data acquisition factors was performed to minimize non-random instrumental noise on an inductively coupled plasma mass spectrometer. Results have identified instrumental and analytical operating conditions under which lead isotope ratios can be measured precisely and accurately in natural samples. Precision was checked periodically over weeks and ranged 0.10–0.30, 0.15–0.35, 0.15–0.35, 0.10–0.30 and 0.10–0.20 (%) for 206/204, 207/204, 208/204, 206/207 and 208/206 ratios respectively. Mass bias was minimized and the correction factors were typically of 0.6% for 206/204, 1.5% for 207/204, 1.8% for 208/204, 0.8% for 206/207 and 1.2% for 208/206. These results were performed with a dwell time of 20 ms, 40 sweeps per replicate, 8 replicates by analysis, a time factor of 15 applied on mass 204 and three points per peak in the low resolution mode. The total duration time of an analysis was 10:50 min and it was found that optimum concentration of lead in solution was 50 ng ml −1 on our instrument.

SI-Traceable Certification of Methylmercury Amount Content in a Tuna Material.

Speciation analysis has an ever-increasing relevance in environmental studies, for risk assessment and, eventually, for legislation. There is a concomitant need for reliability and comparability of speciation measurements made at different places and at different times, to assist with which the concepts of traceability (to a stated system of reference) and combined uncertainty budgetting can be applied. Traceability can be visualised as a chain, or a combination of chains, between a measurement result and a reference where each link in the chain(s) is a mathematical expression representing our best understanding of a measurement process, within a declared level of uncertainty. The combined uncertainty of the measurement result is calculated by combination of the uncertainties associated with each parameter of the traceability chain. This also helps method optimisation and in the diagnosis of possible errors in results by highlighting components that contribute significant uncertainty, provided that an appropriate uncertainty was attached to each parameter affecting the result. This work shows how a methylmercury, MeHg, measurement was made, and how understanding of the measurement process could make the result reliable and comparable to results between laboratories. The measurement process was validated according ISO 17025 through the use of reference materials for calibration, comparison of results with different methods, a systematic assessment of factors influencing the result, by calculating the combined measurement uncertainty and by participation in an inter-laboratory comparison. A tuna material was distributed for the laboratory inter-comparison studies for the Comite Consultatif pour la Quantite de Matiere (CCQM) pilot study 39 and International Measurement Evaluation Programme (IMEP) comparison 20. MeHg was measured using species-specific isotope dilution, SSID, by blending samples with an isotopic certified reference material, ERM®-AE670, which contains CH3HgCl isotopically enriched in 202Hg, followed by extraction and derivatisation of Hg species and detection by gas chromatography-inductively coupled plasma mass spectrometry. SSID was applied to provide mass fractions traceable to the SI through the comparison of isotope ratios measured in blends prepared by metrological weighing, with reference materials carrying values traceable to the SI. Uncertainties associated with each of the experimental steps were investigated and combined according to ISO guidelines. The mass fraction of Hg in the form of MeHg in the tuna sample carried an expanded combined uncertainty estimated at 5.6% n (k n = 2). The greatest uncertainty contributions stemmed from the degree to which sample + spike equilibration could be proved, the certified MeHg content of the spike material and (for the IMEP-20 value) sample homogeneity. Uncertainty contributions from the known natural Hg isotopic composition, isotope ratio measurement and instrumental correction factors were minor in comparison. The level of equivalence was demonstrated by CCQM-P39 in that the standard deviation of mass fractions reported by 9 participating laboratories that applied SSID was 2.6%, and the IRMM value lay 0.24% from the mean.

Certification of the Cu and Cd amount contents in artificial food digest using isotope dilution inductively coupled plasma mass spectrometry for Pilot Study 13 of the Comité Consultatif pour la Quantité de Matière

Abstract The Comite Consultatif pour la Quantite de Matiere (CCQM) launched the Pilot Study 13, an interlaboratory comparison between the metrological organizations worldwide on the determination of Ca, Cu and Cd in artificial food digests. These samples (available in 7% HNO3 and with a salinity evaluated around 370 mg kg−1, including approx. 30 mg Na kg−1) were prepared by gravimetrical mixing, and thus reference values traceable to the Kg for the three elements were available eventually. This paper describes the contribution of IRMM for the certification of the Cu and the Cd amount contents. The analytical protocol developed was based on isotope dilution associated to inductively coupled plasma mass spectrometry (ID-ICP-MS). The Cu measurements required 125-fold dilution of the initial sample solution. An interference of 23Na40Ar+ on 63Cu+ was identified but, since the ratio between both species was over 1000, it was successfully overcome by the calculation of a correction factor for its effect on the Cu amount content directly. Dilution of the sample was not possible for Cd only present at the low ng g−1 level. Up to 1% difference was observed on Cd isotope ratio results between measurements performed directly or after matrix separation. This is rarely shown. As similar results could be obtained either way after the necessary corrections, the direct measurements approach associated to a correction for mass discrimination effects using the CCQM-P13 sample itself (and the IUPAC table values as reference for the natural Cd isotopic composition) was preferred as it was the easiest. SI traceable values were obtained for Cu and Cd with less than 1 and 1.5% combined uncertainty, respectively (6 995±55 (k=2) nmol kg−1 and 45.53±0.64 (k=2) nmol kg−1). The excellent agreement between these results and the reference values (less than 0.6 and 0.08% difference) further validated the analytical protocols developed.

First results on Fe solid-phase extraction from coastal seawater using anatase TiO2 nano-particles

This paper describes the application of TiO2 nano-particles (anatase form) for the solid-phase extraction of iron from coastal seawater samples. We investigated the adsorption processes by infra-red spectroscopy. We compared in batch and on-(mini)column extraction approaches (0.1 and 0.05xa0g TiO2 per sample, respectively), combined to external calibration and detection by inductively coupled plasma mass spectrometry at medium mass resolution. Globally, this titania phase was slightly more efficient with seawater than with ultra-pure water, although between pHxa02 and pHxa07, the Fe retention efficiency progressed more in ultra-pure water than in seawater (6.9 versus 4.8 times improvement). Different reaction schemes are proposed between Fe(III) species and the two main categories of titania sites at pHxa02 (adsorption of [FeLx](3u2004−u2004x)+ via possibly the mediation of chlorides) and at pHxa07 (adsorption of [Fe(OH)2]+ and precipitation of [Fe(OH)3]0). Under optimised conditions, the inlet system was pre-cleaned by pumping 6% HCl for ∼2xa0h, and the column was conditioned by aspirating ultra-pure water (1.7xa0g min−1) and 0.05% ammonia (0.6xa0g min−1) for 1xa0min. Then 3xa0g seawater sample was loaded at the same flow rate while being mixed on-line with 0.05% ammonia at 0.6xa0g min−1 to adjust the pHxa0to 7. The iron retained on the oxide powder was then eluted with 3xa0g 6% HCl (<0.002% residual salinity in the separated samples). The overall procedural blank was 220u2009±u200946 (2xa0s, nu2009=u200916) ng Fe kg−1 (the titania was renewed in the column every 20 samples, with 2-min rinsing in between samples with 6% HCl at 1.5xa0g min−1). The recovery estimated from the Canadian certified reference material CASS-2 was 69.5u2009±u20097.6% (2xa0s, nu2009=u20094). Typically, the relative combined uncertainty (ku2009=u20092) estimated for the measurement of ∼1xa0µg Fe kg−1 (0.45xa0µm filtered and acidified to pHxa01.5) of seawater was ∼12%. We applied our method to a similar sample, from the coastal region of the North Sea. The agreement well within stated uncertainties of our result with the value obtained independently by isotope dilution mass spectrometry further validated our method.

Investigation on sources of contamination, for an accurate analytical blank estimation, during measurements of the Fe content in seawater by isotope dilution inductively coupled plasma mass spectrometry

It is well known that direct measurement of the Fe content in seawater by means of inductively coupled plasma mass spectrometry (ICPMS) is hardly possible. For open ocean water samples, typically, it requires a separation from the matrix associated to significant concentration factors. Furthermore, in the low ng kg−1 range, a correct assessment of the analytical blank is of crucial importance. Despite years of experiments and publications, blank estimation remains a fundamental analytical challenge for the realisation of reliable profiles of dissolved Fe data. Not only must this blank be low and reproducible, which is particularly difficult considering the ubiquity of Fe and the complexity of the seawater matrix, but also realistic. Thus, the complete analytical sequence must be applied to real seawater with zero concentration of Fe to produce a “field blank” (FB), according to the International Union of Pure and Applied Chemistry terminology. This paper investigates the sources of contamination and describes two ways of establishing reliable field blank values for the isotope dilution (ID) ICPMS procedure described by Wu and Boyle (J. Wu and E. A. Boyle, Anal. Chim. Acta, 1998, 367, 183), based on a multiple steps protocol, including a co-precipitation with magnesium hydroxide after ammonia loading, and consecutive dissolution with hydrochloric acid. The analytical protocol is optimised to achieve reproducible separation–pre-concentration of ∼100% Fe under stable pH conditions. Typically, at ∼30 ng Fe kg−1 level a concentration factor of up to 15 can be achieved, leading to samples containing residual salinity ∼0.06%. The first approach proposed for field blank determination applies mostly to Fe mass fractions >500 ng kg−1 and results from the comparison of two IDMS-based Fe content values of an identical sample, where the first is produced by Mg(OH)2 co-precipitation and the second after a simple dilution. For the second approach, adapted to lower Fe content samples (down to 3 ng Fe kg−1), the absolute field blank is the intercept of a linear regression between sample masses and corresponding absolute Fe contents for a given set of sample replicates. The estimated field blanks for both approaches were 16 ± 12 ng kg−1 and 6 ± 2 ng kg−1, respectively. We found that manipulations (i.e. sample handling and the environment) are by far the largest source of contamination as they contribute ∼75% of the total. The 2% nitric acid used to dissolve the precipitate and the instrumental background come next, with, respectively, 3–10% and 5–9% contributions. The measurement procedure was validated through the use of reference materials, a systematic assessment of factors influencing the result, by calculating the combined measurement uncertainty, and from the results obtained on a test material of a recent inter-laboratory comparison.

Solution to data integration problems during isotope ratio measurements by magnetic sector inductively coupled plasma mass spectrometer at medium mass resolution: application to the certification of an enriched 53Cr material by isotope dilution

Abstract The suitability of a single-detector magnetic sector inductively coupled plasma mass spectrometer for low uncertainty Cr isotope ratio measurements was evaluated. Operation at medium mass resolution ( m /Δ m ⩾4000) was required to eliminate the interferences from polyatomic ions commonly observed on Cr isotope masses. However, the repeatability of the ratios appeared to be far worse than expected and extremely unstable. The mass calibration was found to drift by up to 0.0016 amu on peak center (i.e. ∼12.5% of the peak width) for the duration of a measurement (i.e. 675 s). Moreover, for individual peak signals (0.12–0.36 s duration depending on isotopes) the instabilities observed, particularly for low abundant isotopes, lead to multiple maxima that could potentially complicate the data integration step. However, the major problem turned out to be the instrument software, failing to integrate the data in a reproducible and predictable manner. An ‘off line’ method of data integration was developed to overcome these problems that led to a nearly tenfold improvement in the repeatability of natural n ( 52 Cr)/ n ( 50 Cr) isotope ratio measurements. The stability of the repeatability over 45 min improved by a factor of 2.6, the reproducibility of the ratios improved by more than a factor of 4 and the average ratio changed by ∼0.75% (and by up to 1.5% in the worst case). Under these stabilized conditions, direct isotope dilution could be applied as a primary method of measurement for the certification of the Cr amount content in a 53 Cr enriched material. The isotope ratio measurements, whose repeatability varied from 0.1 to 0.7% depending on the value of the ratio, were calibrated (corrected for mass discrimination effects) using the IRMM-625 certified isotopic reference material. Combined uncertainties were estimated for all results following the ISO guide to the expression of uncertainty in measurements. A combined uncertainty (expanded, with k= 2) on the Cr amount content of less than 0.6% relative was achieved, where the repeatability of the isotope ratio measurements accounted for less than 1% of this value.

Cadmium determination in natural waters at the limit imposed by European legislation by isotope dilution and TiO2 solid-phase extraction

AbstractThe cadmium content in surface water is regulated by the last European Water Framework Directive to a maximum between 0.08 and 0.25xa0μg L−1 depending on the water type and hardness. Direct measurement of cadmium at this low level is not straightforward in real samples, and we hereby propose a validated method capable of addressing cadmium content below μgxa0L−1 level in natural water. It is based on solid-phase extraction using TiO2 nanoparticles as solid sorbent (0.05xa0g packed in mini-columns) to allow the separation and preconcentration of cadmium from the sample, combined to direct isotope dilution and detection by inductively coupled plasma mass spectrometry (ID-ICP-MS). The extraction setup is miniaturised and semi-automated to reduce risks of sample contamination and improve reproducibility. Procedural blanks for the whole measurement process were 5.3u2009±u20092.8xa0ng kg−1 (1xa0s) for 50xa0g of ultrapure water preconcentrated ten times. Experimental conditions influencing the separation (including loading pH, sample flow rates, and acid concentration in the eluent) were evaluated. With isotope dilution the Cd recovery rate does not have to be evaluated carefully. Moreover, the mathematical model associated to IDMS is known, and provides transparency for the uncertainty propagation. Our validation protocol was in agreement with guidelines of the ISO/IEC 17025 standard (chapter 5.4.5). Firstly, we assessed the experimental factors influencing the final result. Secondly, we compared the isotope ratios measured after our separation procedure to the reference values obtained with a different protocol for the digested test material IMEP-111 (mineral feed). Thirdly, we analysed the certified reference material BCR-609 (groundwater). Finally, combined uncertainties associated to our results were estimated according to ISO-GUM guidelines (typically, 3–4% ku2009=u20092 for a cadmium content of around 100xa0ng kg−1). We applied the developed method to the groundwater and wastewater samples ERM-CA615 and BCR-713, respectively, and results agreed with certificate values within uncertainty statements.n Figurexa0