High precision 142 Ce/ 140 Ce stable isotope measurements of puri ﬁ ed materials with a focus on CeO 2 nanoparticles

Engineered CeO 2 nanoparticles (NPs) are becoming increasingly prevalent in consumer products and this has raised concerns about the unknown behaviour and fate of such materials in the environment. Analytical limitations have hampered the detection of CeO 2 NPs in natural systems at environmentally relevant levels. This study presents data on the inherent stable isotope composition of commercially available puri ﬁ ed Ce materials with a particular focus on CeO 2 NPs. The aim of this investigation is to determine whether CeO 2 NPs posses a distinct isotopic signature that may be exploited for their detection in natural systems. To achieve this, suitable stable isotope measurement protocols were developed for the precise determination of the 142 Ce/ 140 Ce isotope ratio by multiple collector ICP-MS using Ba for external normalisation of the instrumental mass bias. The data presented show that precisions of (cid:1) 0.01 & (2se) and (cid:1) 0.04 & (2sd) can be routinely achieved with these techniques. The results also demonstrate that commercially available CeO 2 NPs do not have a distinct Ce isotope composition that may be exploited for the purpose of stable isotope tracing.


Introduction
Engineered CeO 2 nanoparticles (NPs) are amongst the most extensively manufactured nanomaterials (NMs).4][5] In particular, such investigations require methods that can detect small quantities of NPs in complex natural samples against the natural background levels of the elements present.One promising tracing approach is based on mass dependent differences in the stable isotope compositions of elements.
Mass dependent differences, or fractionations, in stable isotope compositions are generated by processes of physical (e.g., diffusion, adsorption), chemical (e.g., precipitation, redox change) or biological nature.Recent advances in mass spectrometry have allowed the resolution of such isotope effects for numerous metals.This includes the elements Ca, Ba, and U, [6][7][8] which resemble Ce due to their refractory, lithophile nature and moderate to high atomic mass number.Such studies have shown that stable isotope fractionations are common in nature and recorded in many natural materials.Furthermore, industrial processes, such as partial evaporation and diffusion during smelting and purication, have been found to create distinct isotopic signatures for some metals, which have been exploited for tracing anthropogenic emissions to the environment. 9n contrast to many other elements, stable isotope variations for Ce have received only little attention to date, with results presented in just two recent studies. 10,11Whilst the ndings of these authors are limited to only a few samples from adsorption and precipitation experiments and 5 natural samples, they do reveal small but clearly resolvable variations in stable Ce isotope compositions.It is therefore conceivable that industrial processing and purication of Ce and the preparation of CeO 2 NPs is also associated with isotopic fractionation.If these effects are large enough they may result in an isotopic signature that is sufficiently distinct from the natural background (e.g., soil, water, biological tissue) such that isotopic tracing is feasible.For example, CeO 2 NPs that differ in their 142 Ce/ 140 Ce isotope ratio from the background value by 10& may be suitable for tracing.In this case, detection and quantication of as little as 0.5 mg kg À1 of NP-derived Ce is already possible in a sample with a natural Ce background of 50 mg kg À1 , if the isotopic measurements achieve a precision of better than AE0.1&.
This study investigates the feasibility of the stable isotope tracing approach for CeO 2 NPs.To this end, high-precision Ce stable isotope data were acquired by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for CeO 2 nano-and bulk-particles and other puried Ce materials.These samples were chosen, to determine whether the purication of Ce and NP synthesis might result in isotopic signatures that are distinct from 'normal' Ce.A sufficiently distinct isotope composition would enable the tracing of CeO 2 NMs in natural systems by means of stable isotope measurements.Such an approach would allow investigations into the behavior and fate of CeO 2 NPs in real world settings and samples, thereby providing the improved understanding of environmental cycling.This knowledge is needed to investigate whether the use and emission of engineered NMs pose a threat to health or the environment and thus develop informed risk assessments.

Samples
Seven commercially available CeO 2 NP samples and three CeO 2 reference materials (of which two are also in NP form), all provided as dry powders, were analysed in this study (Table 1).The reference materials (JRC NM-211, NM-212, NM-213) are available from the European Commission Joint Research Centre (JRC) whilst the commercial CeO 2 NPs were sourced directly from the manufacturers.
Furthermore, three different Ce standard solutions were analysed, whereby the Alfa Aesar Specpure solution was used as in-house Ce isotope reference material, relative to which all other Ce isotope compositions were determined.The Ce solution that was prepared by Willbold 12 at the University of Bristol (termed 'Bristol Ce' hereaer) has also been proposed as a Ce isotope reference material.Additional puried Ce based materials analysed encompass two small chunks of Ce metal that were treated as distinct samples, and the Ce salts ammonium Ce(IV) nitrate (NH 4 ) 2 Ce(IV)(NO 3 ) 6 and Ce(III) nitrate, Ce(III)(NO 3 ) 3 (Table 1).

General procedures and reagents
All sample preparation work was carried out in the ISO class VI clean room facility of the MAGIC Research Centre at the Department of Earth Science and Engineering, Imperial College London.For all blank sensitive procedures ISO class IV laminar ow hoods were utilised.Concentrated 14.5 M HNO 3 and 6 M HCl were puried prior to use by sub-boiling distillation in quartz stills, whilst water was from a Millipore Milli-Q Academic system that produces >18.2 MU cm À1 water.Suprapur quality 30% H 2 O 2 solution (Merck Millipore) and 99.5% purity NaBrO 3 (Alfa Aesar) were used as purchased.

Sample preparation for stable isotope analysis
Working stock solutions with Ce concentrations of 100 mg mL À1 in 0.1 M HNO 3 were prepared from the CeO 2 powders and the puried Ce based materials (Table 1) as follows.For the three Ce standard solutions, suitable aliquots were diluted with 0.1 M HNO 3 .Solutions of the two Ce metal pieces were prepared at room temperature by dissolving about 1 g of metal in 200 mL 2 M HNO 3 .The resulting solutions were diluted with 2 M HNO 3 to obtain concentrated Ce solutions of 1000 mg mL À1 .Aliquots of these were further diluted to obtain the working stock solutions.For the Ce(III)(NO 3 ) 3 and (NH 4 ) 2 Ce(IV)(NO 3 ) 6 salts, about 10-15 mg of material were dissolved in 4 mL 4 M HNO 3 at room temperature.The resulting solutions were evaporated to dryness and taken up in 0.1 M HNO 3 .For the CeO 2 powders, 10-15 mg of material were weighed into 15 mL Teon beakers, 4 mL 14.5 M HNO 3 and 1 mL 30% H 2 O 2 , to reduce Ce(IV) to Ce(III), were added before placing the beakers on a hotplate at 80 C. Dissolution was allowed to proceed overnight, whereupon the temperature was increased to 160 C. The beakers were again le on the hotplate overnight following which the samples were dried down.If complete digestion was not achieved before drying, the above procedure was repeated until CeO 2 powders were no longer visible in the solution.The samples were subsequently dried down three times with 100 mL 14.5 M HNO 3 at 100 C, before taking up in 0.1 M HNO 3 .

Mass spectrometry
The Ce stable isotope measurements were performed with a Nu Instruments Nu Plasma HR MC-ICP-MS at the Imperial College MAGIC Laboratories.The instrument is tted with a large capacity (80 L min À1 ) rotary pump for enhanced evacuation of the expansion chamber to improve sensitivity.A Nu Instruments DSN and a CETAC Aridus I desolvation system were employed for sample introduction, both with glass nebulizers that provided solution uptake rates of about 100 mL min À1 .Normal instrumental operating parameters are summarised in Table 2.
For isotopic analyses, the 100 mg mL À1 stock solutions of the puried Ce samples and the in-house standard (Table 1) were diluted with 0.1 M HNO 3 to prepare measuring solutions with Ce concentrations of between 50 and 125 ng mL À1 .These Ce solutions were doped with a 10 mg mL À1 Ba stock solution (in 0.1 M HNO 3 ) to obtain Ba concentrations of 75 to 350 ng mL À1 and Ce/Ba ratios of between 1.5 and 3.5.
Data collection was carried out by static multiple collection with the Faraday cups of the instrument, which are tted with 10 11 U resistors.Each analysis consisted of 60 integrations of 5 seconds each, in 3 blocks of 20, where the ion beams of the atomic masses 135 (Ba-collector L3), 137 (Ba-L2), 139 (La-Axial), 140 (Ce, Nd-H1), 141 (Pr-H2), 142 (Ce-H3), and 145 (Nd-H5) were monitored.Prior to each block, baseline signals were measured for 15 seconds, whilst the ion beam was deected in the electrostatic analyser.Analyses of mixed Ce-Ba solutions typically yielded sensitivities of about 110 to 160 V mg À1 mL À1 for Ce and 130 to 180 V mg À1 mL À1 for Ba during the course of the study, equivalent to a transmission efficiency of about 0.10-0.15%for both elements (Table 2).Each measurement was followed by a thorough ($2-3 min) washout of the sample introduction system with 0.1 M HNO 3 , and this was generally sufficient to reduce the measured signals to the baseline intensities.
To determine variations in the stable isotope composition of Ce, the analyses focused on the two major Ce isotopes and determination of the 142 Ce/ 140 Ce isotope ratio.The 135 Ba/ 137 Ba isotope ratio of the admixed Ba was monitored for mass bias correction with the exponential law. 13Although only puried Ce materials were analysed, small amounts of Nd were present in the samples and so the 145 Nd ion beam was measured for correction of the isobaric interference of 142 Nd on 142 Ce using a ratio of 142 Nd/ 145 Nd ¼ 3.27711. 14The presence of the light rare earth elements (LREE) La and Pr was also monitored, using 139 La and 141 Pr, even though interference corrections are not necessary.To assess the robustness of the correction for isobaric 142 Nd, a series of Alfa Aesar Ce standard solutions were doped with increasing amounts of Nd.This enabled determination of the maximum Nd/Ce ratios that can be tolerated during analyses.
All sample measurements were bracketed with at least two analyses of the in-house Alfa Aesar Ce isotope reference material.Differences between the corrected 142 Ce/ 140 Ce ratios of the samples and the reference material are reported in the form of d 142 Ce values, which denote the relative deviation of the 142 Ce/ 140 Ce isotope ratio of a sample (R sam ) from the in-house Alfa Aesar standard (R std ) in parts per 1000: 3 Results and discussion

Precision and accuracy of Ce stable isotope data for CeO 2 and puried Ce samples
The isotope data obtained for the CeO 2 powders and puried Ce samples (Table 1) were acquired over a 9 month period that encompassed 11 separate analytical sessions.Each single measurement consumed approximately 50 to 100 ng of Ce and the d 142 Ce data for the samples feature 2sd reproducibilities, based on the results of 5 to 15 individual measurements, of AE0.02& to AE0.07& (Table 1).Based on the multiple analyses, the mean results have 2se uncertainties of only AE0.01& to AE0.02& (Table 1).These performance characteristics are similar to that achieved in MC-ICP-MS stable isotope analyses for elements of similar mass when normalised to a mass difference of 1 amu (comparison of 2sd/amu results).For example, typical 2sd/amu uncertainties of AE0.01& to AE0.03& were reported for MC-ICP-MS analyses of Cd and Mo standard solutions by Abouchami et al. 15 and Goldberg et al., 16 respectively.Our data also compare As the analysed samples were puried Ce materials, it is unlikely that the accuracy of the data is compromised by matrix effects and/or spectral interferences.The absence of the former is conrmed by the observation that none of the samples displayed any unusual mass bias behaviour of Ba during the analyses.In addition, it was observed that the use of two different sample introduction systems and variations in the Ce, Ba concentrations and Ce/Ba ratios between sample and the bracketing standard solutions of up to 460% and 230% respectively, had no discernible effect on the d 142 Ce data at the quoted level of precision.
The purity of the Ce solutions was, furthermore, assessed throughout the isotopic analyses by monitoring the LREEs La, Pr and Nd.These elements are likely to be the most prominent contaminants, as they are difficult to separate from Ce.However, only trace amounts of the LREEs were present, with typical LREE/Ce ratios of less than 10 Â 10 À5 and a maximum Nd/Ce of about 60 Â 10 À5 for a CeO 2 powder and the two metal samples (Table 3).Any Nd contamination is, however, of concern due to the isobaric interference of 142 Nd on 142 Ce, which was corrected during the analyses based on the monitoring of 145 Nd.Table 3 summarises the corrections that were applied to the d 142 Ce data of samples to account for the 142 Nd interference.The corrections were generally observed to be less than 0.2& but with maximum levels of up to 1.0 to 1.4& (Fig. 1).
The controlled doping of the Alfa Aesar Ce standard solution with Nd was used to investigate the maximum Nd/Ce ratios that can be tolerated for reliable Ce isotope measurements.Our data demonstrate that correction of the 142 Nd interference on 142 Ce resulted in 142 Ce/ 140 Ce ratios that were identical, within 2sd uncertainty, to data for non-doped Alfa Aesar Ce solutions at Nd/Ce ratios of up to 0.01 (Fig. 2).As all puried Ce materials analysed here were characterised by Nd/Ce of less than 0.001 (Table 3), the results obtained for these samples can be considered as accurate, as the presence of isobaric 142 Nd did not generate analytical artefacts.This conclusion is underlined by additional observations.First, a plot of d 142 Ce versus magnitude of the 142 Nd correction shows no obvious correlation (Fig. 1).
Second, seven of the nine CeO 2 powders are characterised by a very narrow range of d 142 Ce values, even though they feature variable levels of Nd.It is highly unlikely that the isotopic homogeneity of the CeO 2 powders is a fortuitous result of an inaccurate Nd correction.Third, the two isotopically distinct CeO 2 samples are subject to only relatively small d 142 Ce corrections of 0.03& and 0.05&, which implies that the negative d 142 Ce values are unlikely to be an artefact.In summary, these observations indicate, that the applied corrections on d 142 Ce for the presence of isobaric 142 Nd were accurate even at the highest observed Nd/Ce ratios of about 60 Â 10 À5 (Table 3).

Isotopic variability
The analysed CeO 2 NPs show a total variability in d 142 Ce of 0.23&, with values of between À0.16& and 0.07&.However, 7 of the 9 samples display a very narrow range of d 142 Ce values of 0.03& to 0.07&, whilst two outliers are characterised by d 142 Ce of À0.08& and À0.16& (Table 1, Fig. 3).The CeO 2 bulk particles are essentially indistinguishable from most of the CeO 2 NP powders, with a d 142 Ce value of 0.07 AE 0.01& (2se).The remaining puried Ce samples also show only very limited variability in d 142 Ce.The two Ce salts and the Sigma-Aldrich Ce   1) are plotted against the correction to the d 142 Ce values for the presence of isobaric 142 Nd (Table 3).The error bars associated with the x-axis denote the range of corrections applied to the d 142 Ce values for the presence of isobaric 142 Nd (Table 3), whereas the y-axis error bars denote the 2se uncertainties of the d 142 Ce values based on multiple analyses (n ¼ 5 to 15; Table 1).
standard solution are hereby isotopically identical, within the quoted 2se uncertainties of only 0.01& to 0.02&, to the CeO 2 particles.The two metal samples are slightly heavier with d 142 Ce values that are identical within uncertainty at 0.11& and 0.12& (Table 1, Fig. 3).
No information was provided by the manufacturers on the provenance of the Ce or the methods that were used for element purication and NP synthesis.It is therefore conceivable that the limited isotopic variability observed here for various forms of puried Ce is simply a consequence of similar origin and similar production methods.It is notable, however, that our results appear to be in good accord with the Ce data published by Ohno and Hirata. 10Even though the two investigations cannot be compared directly, as they are expressed relative to different d 142 Ce ¼ 0 reference materials, Ohno and Hirata 10 also found only a limited range of d 142 Ce values of 0.14& for two igneous and three sedimentary rock samples (Fig. 3).This may indicate that, even though some samples display resolvable isotopic differences, the overall variability of Ce stable isotope compositions may be quite limited for most terrestrial materials.
Considering the aims of our study, the observed 0.3& variability in d 142 Ce is too small to be exploitable for routine tracing purposes.This is demonstrated by a best-case scenario, which assumes that CeO 2 NPs have a d 142 Ce value that differs from a constant natural background by 0.3&.In this instance, an increase in the background Ce concentration of 20%, from the addition of CeO 2 NPs, would result in a change of the d 142 Ce value by 0.05&.This example demonstrates that isotopic tracing methods for CeO 2 NPs which are based on inherent differences in d 142 Ce are of no practical value because they do not provide detection capabilities that are clearly superior to those offered by less costly and less time consuming concentration measurements.

Conclusions
We present a robust and novel analytical protocol for the determination of 142 Ce/ 140 Ce isotope ratios by MC-ICP-MS, which offer a reproducibility (2sd) of about AE0.05&.The techniques are, in principle, suitable for analyses of natural materials if used in conjunction with an appropriate procedure for the separation and purication of Ce from the sample matrix.
Our analyses show that commercially available CeO 2 NPs do not have a distinct isotope signature and their detection in natural samples by stable isotope tracing is, therefore, not feasible.However, the stable isotope tracing approach may still be successfully applied if an enriched Ce isotope is used to produce isotopically labelled CeO 2 NPs. 17 1).The grey field denotes the range of d 142 Ce values determined by Ohno and Hirata 10 for 5 rock samples.However, the position of this field in relation to our data may not be accurate, as a different Ce reference material were applied to define d 142 Ce ¼ 0.

Fig. 1
Fig.1The d 142 Ce values of the purified Ce materials (Table1) are plotted against the correction to the d 142 Ce values for the presence of isobaric 142 Nd (Table3).The error bars associated with the x-axis denote the range of corrections applied to the d 142 Ce values for the presence of isobaric 142 Nd (Table3), whereas the y-axis error bars denote the 2se uncertainties of the d 142 Ce values based on multiple analyses (n ¼ 5 to 15; Table1).

Fig. 2
Fig. 2 Plot of measured d 142 Ce values versus the Nd/Ce ratios of the sample solutions.The purple bar and dotted lines denote the 2sd uncertainty determined from multiple analyses of the Alfa Aesar Ce standard solution (AE0.09&, n ¼ 25).The data obtained for Alfa Aeasar Ce standard solutions doped with varying levels of Nd (blue squares with internal 2sd uncertainties) demonstrate that accurate d 142 Ce values can be readily obtained at Nd/Ce ratios of up to 0.01.The orange circles represent the Ce samples that were analysed in this study.

Fig. 3 d
Fig. 3 d 142 Ce data for the Ce samples analysed in this study.Error bars denote the 2se uncertainties that were calculated based on multiple analyses (n ¼ 5 to 15; Table1).The grey field denotes the range of d 142 Ce values determined by Ohno and Hirata 10 for 5 rock samples.However, the position of this field in relation to our data may not be accurate, as a different Ce reference material were applied to define d 142 Ce ¼ 0.

Table 1 Summary
of Ce isotope data for nano-and bulk-sized CeO 2 particles, including several reference materials, and other purified Ce samples Sample type Supplier/comments Particle size a (nm) n b d 142 Ce c (&) AE2sd d (&) AE2se e (&) Ce solutions Alfa Aesar; in-house d-zero reference material AE0 a Particle size as reported by supplier.bNumber of individual analyses.cAll isotopic data are reported relative to the Alfa Aesar Specpure Ce standard solution, which served as in-house d 142 Ce ¼ 0% reference material.dThe quoted 2sd uncertainties are based on the reproducibility of n individual measurements.eThequoted 2se uncertainties (standard error of the mean) were calculated as 2se ¼ 2sd/On.f Bristol Ce solution from Willbold. 12g Supplied directly from Energenics Europe Ltd on request, no product code or particle size data provided.

Table 2
11strumental operating parameters Ce stable isotope results of Nakada et al.11and Ohno and Hirata, 10 who reported d 142 Ce uncertainties (2sd) of AE0.06& and AE0.08& for multiple analyses of Ce solutions.As in this study, these authors also employed MC-ICP-MS but the samples were doped with Sm, and 149 Sm/ 147 Sm was used for external normalisation of the instrumental mass bias.

Table 3
REE/Ce ratios that were determined for the Ce standard and sample solutions and the corresponding correction to the d 142 Ce values Change in d 142 Ce aer correction for the isobaric 142 Nd. a