Chromium isotope ratio measurements in ...

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Chromium isotope ratio measurements in environmental matrices by MC-ICP-MS bc c Simon Ponte´r,*a Nicola Pallavicini,bc Emma Engstro ¨ m, Douglas C. Baxter bc and Ilia Rodushkin

Published on 09 May 2016. Downloaded on 09/05/2017 11:54:29.

An analytical procedure consisting of high pressure/temperature acid digestion using an UltraCLAVE system and a one pass, single column matrix separation using DOWEX AG 1X8 anion exchange resin was applied to the determination of Cr concentrations and d53Cr in chromites, soils, and biological matrices (epiphytic lichens and mosses) using a combination of ICP-SFMS and MC-ICP-MS. The overall reproducibility of the method was assessed by replicate preparation and Cr isotope ratio measurements performed by different operators in multiple analytical sessions over a few months and was found to be 0.11& (2s). The accuracy was evaluated using commercially available reference materials for which measured data Received 19th April 2016 Accepted 9th May 2016

were compared with certified values (for Cr concentrations) and previously published results (for isotope data). The results demonstrate a uniform Cr isotope composition in soil depth profiles sampled in different urban environments. A strong negative correlation between d53Cr and Cr concentrations in

DOI: 10.1039/c6ja00145a

lichens and mosses indicates that airborne Cr from local anthropogenic source(s) is depleted in heavy

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isotopes.

1. Introduction Isotopic measurements have emerged as essential tools in environmental sciences, providing valuable information on contamination sources and pathways, quantication of element transport and mixing, directions and rates of environmental processes, etc.1 Continuous improvements in measurement precision and rened separation methods have widened the eld continuously over the last few decades adding heavier, stable isotopes to the isotope ‘toolbox’.1 However, broader application of isotope signatures remains limited by high instrumentation cost and by lengthy, labor-intensive separation schemes. Chromium is widely dispersed in the earth's crust as a trace element in the 200 mg g1 range and around 0.3 mg L1 in the oceans,2 and occurs as four stable isotopes 50Cr, 52Cr, 53Cr, and, 54 Cr with representative relative abundances of 4.345%, 83.789%, 9.501%, and, 2.365%, respectively.3 Earlier studies relying on Cr isotope ratios have been focused on identifying processes during formation of the solar system.4–7 Due to instrument development during the last few decades, Cr isotope measurements in samples with lower concentrations are now possible with recent studies focusing on the minor variations in abundances occurring in environmental matrices.8–10

a

Department of Environmental Science and Analytical Chemistry, Stockholm University, SE-11418 Stockholm, Sweden. E-mail: [email protected]

b c

Division of Geosciences, Lule˚ a University of Technology, S-971 87 Lule˚ a, Sweden

Hexavalent chromium, Cr(VI), being a known carcinogen as well as highly mobile and soluble under oxidized conditions in the form of anions CrO42 and HCrO4 is of particular interest. Trivalent chromium, Cr(III), on the other hand, is innocuous and very inert. Previous studies on reduction mechanisms have shown that Cr isotope fractionation occurs during the reduction of Cr(VI) to Cr(III), with lighter isotopes becoming enriched in the product.1,9,10 Fractionation has also been proposed to take place during the oxidation of Cr(III) to Cr(VI).11 Therefore, there is potential for tracing specic Cr sources and transformations, such as those occurring in chrome-plating processes or in mining and ore renement industries.8,10–13 Since targeted variations in the Cr isotope composition are expected to be minor, the separation of Cr from a sample matrix is crucial for accurate isotope ratio determination. As chromium occurs as either cationic Cr(III) or anionic Cr(VI) species, the behavior of each during ion exchange separation is very different. This dichotomy may be resolved either by converting all Cr to a chosen valence state and adapting the separation accordingly, or by separating Cr(III) and Cr(VI) individually and later merging them prior to making measurements. Both approaches have their advantages and disadvantages. A number of procedures to isolate Cr have been proposed, which are oen very time consuming and elaborate, including several purication steps.3–16 Many published procedures use a double spike technique assuming that isotopically enriched Cr and naturally occurring Cr behave identically (complete sample/spike equilibration), and therefore isotope fractionation caused by incomplete analyte recovery during the matrix separation process as well as

ALS Laboratory Group, ALS Scandinavia AB, Aurorum 10, S-977 75 Lule˚ a, Sweden

1464 | J. Anal. At. Spectrom., 2016, 31, 1464–1471

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instrumental mass bias are corrected for and non-quantitative recovery can be tolerated.17 However, the approach requires isotope ratio measurements using at least three interferencefree Cr isotopes, which can be a challenge, especially for multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). In this study, Cr was separated from a wide range of environmental matrices (Cr-bearing minerals, soils, lichens and mosses) from Northern Sweden using a single column procedure modied from previously published work.15,16 Since double spiking was not used, quantitative recovery of analytes during separation was of paramount importance to prevent articially introduced fractionation. Special care was taken to minimize all sources of spectral interferences and matrix effects. Cr isotope ratios in puried fractions were measured by MC-ICP-MS using a combination of internal standardization and bracketing standards for instrumental mass bias correction.

2.

Experimental

2.1. Chemicals and reagents Nitric acid (HNO3), hydrochloric acid (HCl), hydrogen peroxide (H2O2) (all from Sigma-Aldrich Chemie GmbH, Munich Germany) and hydrogen uoride (HF, 48% Merck, Darmstadt, Germany) were all of analytical grade. MilliQ water (Millipore, Bedford, MA, USA) used in this study was puried by reverse osmosis followed by ion exchange. Anion exchange resin, DOWEX AG 1X8 dry mesh size 200–400 mm (Sigma-Aldrich Chemie GmbH), was suspended in MilliQ water and loaded into 2 mL columns. The oxidizing agent, ammonium peroxodisulfate, (NH4)2S2O8 (Riedel-de Ha¨ en AG, Seelze-Hannover), was weighed and dissolved in MilliQ water to the desired concentration on the day of use. Chromium NBS SRM 979 was used as the d-zero standard. Chromium stock solution (Ultra Scientic Analytical Solutions, Lot. T00606, 10 000 mg L1) was used as the quality control sample (QCS). Nickel stock solution (Ultra Scientic Analytical Solutions, Lot. M00866, 1000 mg L1) was used to prepare the internal standard for MC-ICP-MS measurements. 2.2. Instrumentation Sample digestion was performed by using an UltraCLAVE system (Milestone, Sorisole, Italy) that offers efficient microwave (MW) assisted digestion combining high temperature and high pressure conditions with a relatively high throughput. All element concentrations in sample digests and column fractions were measured by using a single-collector, doublefocusing, inductively coupled plasma sector-eld mass spectrometer (ICP-SFMS, ELEMENT XR, Thermo Fisher Scientic, Bremen, Germany). It was equipped with a demountable quartz torch, a nickel sampler cone, a high sensitivity X-skimmer cone, a PFA spray chamber and a SD2 auto-sampler (ESI, PerkinElmer, Santa Clara, USA) equipped with a six-port valve and 1.5 mL sample loop lled and rinsed utilizing vacuum suction. Details about instrumental parameters and measurement conditions can be found elsewhere.18


The MC-ICP-MS instrument used for Cr isotope ratio measurements was a Neptune PLUS (Thermo Fisher Scientic, Bremen, Germany). The instrument was equipped with a platinum guard electrode and nine Faraday cups (eight movable and one xed center cup) and was operated in high resolution mode (providing a pseudo-resolution of 8500M/DM derived from the peak slope of the rising edge measured at 5% and 95% relative peak height). The instrument can be coupled to three different introduction system congurations: ‘standard’ consisting of a cyclonic/Scott spray chamber, as well as Aridus II (Teledyne CETAC technologies, Omaha, Ne, USA), and Apex (Elemental Scientic, Omaha, Ne, USA) high efficiency desolvating nebulizers. Instrumental operating conditions and measurement parameters are summarized in Table 1. Due to the instrument's hardware limitations of cup positioning limits for a given mass range, 52Cr, 53Cr, and 54Cr as well as 56Fe were measured simultaneously with the main cup conguration, while 60Ni, 61Ni and 62Ni were measured at another magnet setting (Table 1). The use of the so-called dynamic mode certainly is less time effective than true simultaneous measurements, but it allows for on-line correction of dri and matrix-induced changes in mass bias. Aer plasma ignition, the instrument was allowed a minimum of one hour to stabilize while aspirating 0.14 M HNO3 blank solution prior to performing the daily optimization of operational parameters (gas ow, torch position and lens settings) and mass calibration. The best signal stability and isotope ratio in-run precision were obtained by self-aspiration, but a peristaltic pump was used since this decreased sample uptake and wash-out times considerably. All measurement solutions were always diluted to equal concentrations, providing concentration matching between samples and bracketing d-zero standards for each individual run. With a standard introduction system, the Cr concentrations in samples were adjusted to 2 mg L1 in 0.14 M HNO3 (providing 52 Cr intensity of approximately 40 V) and spiked with Ni to 1 mg L1. For samples with low Cr concentrations, an Aridus desolvating nebulizer was tested initially, but severe 40Ar12C+ interference on 52Cr, presumably originating from organics released by the membrane material, made further application impossible. Consequently, the Aridus was replaced by an Apex desolvating nebulizer. For solutions analyzed with this more sensitive introduction system, the Cr concentration was adjusted to 0.4 mg L1 and spiked with Ni to 0.2 mg L1. All solutions were analyzed in duplicate giving a total measurement time per sample of approximately 12 minutes including uptake and washout. Duplicate measurements allow the detection of memory effects in the introduction system, which proved to be non-existent. Outlier elimination was activated, using the 2s criterion in the resident Neptune soware. Signal intensities were transferred to commercially available spreadsheet soware for further off-line calculations. The isobaric interference from 54Fe on 54Cr was corrected mathematically using the monitored 56Fe signal together with tabulated Fe isotope abundances and computed instrumental mass bias for each individual Neptune session. Instrumental mass bias was corrected off-line by using the revised exponential

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Paper MC-ICP-MS parameters for Cr isotope ratio measurements

Rf power (W) Ion lens settings Zoom optic parameters Coolant gas (L min1) Sample gas (L min1) Auxiliary gas (L min1) Sample uptake rate (mL min1) Integration time, s Number of integrations Number of blocks Cycles per block Amplier rotation

1400 Optimized daily for maximum sensitivity and signal stability Adjusted daily for the highest sensitivity and peak shape 16 0.9–1.25 0.8 0.05–0.25 1.262 3 9 5 Le

Cup conguration


Main

L3 52 Cr

L2 53 Cr

Sub-cong.

correction model by Baxter et al.19 using the internal standard (nickel ratio) and the Cr d-values were calculated against bracketing d-zero solution. Three samples were analyzed between two standards, together forming a block (block: standard 1 – sample 1 – sample 2 – sample 3 – standard 2). The mean value of the two consequent measurements of the sample ratio was calculated against ratios for standards in each block. Assuming a linear change in mass bias, ratios for samples 1 and 3 were calculated relative to those for standards 1 and 3, respectively, while sample 2 was calculated against the mean ratio for both standards. Results from the two measurements were used to calculate mean d-values and s for each sample. Chromium d-values were calculated using the following formula " # ðx Cr=y CrÞsample x=y d Cr ¼ x 1 1000 ð Cr=y CrÞNBS 979 where xCr and yCr represent two different isotopes of interest, (xCr/yCr)sample is the measured ratio of sample solution and (xCr/yCr)NBS 979 is the bracketing standard selected as d-zero. The factor 1000 is used to convert ratios to per mil notation. When the d-value refers to a ratio of a heavier isotope composition, a positive d-value corresponds to an enrichment in the heavier isotope compared to the standard.

2.3. Samples and sampling area The study region in northeast Sweden, stretching from the Swedish/Finnish border at the northeast of the sampling transect to the town of Lule˚ a at the southwest, is sparsely populated but heavily industrialized. Major industries include metal foundries processing chromium, iron, copper, lead, and zinc ores as well as paper mills. Of major interest for the present study are the mining of chromium ores from the open pit mine in Kemi on the Finnish side of the border and the processing of the ore in Torne˚ a


L1 54 Cr (54Fe)

H1 56 Fe

H2

H3

60

61

62

Ni

Ni

Ni

steelworks. The area has been studied previously and surveys focusing on heavy metal deposition patterns on the Finnish side of the border have been carried out for more than two decades covering many elements including Cr using Hylocomium splendens and Pleurozium schreberi mosses.20 The distribution of Cr in this area and potential inuence from anthropogenic sources have been mentioned previously in the study devoted to the use of the Os isotope composition for anthropogenic assessment.21 Epiphytic lichen (Usnea spp. and Bryoria spp.) and moss samples (Pleurozium schreberi) were collected from sampling sites at different distances from Torne˚ a steelworks during 2005 and 2006. Exact sampling locations can be seen in the study by Rodushkin et al.21 In order to increase representativeness, pooled samples consisting of >20 individual lichens or mosses were collected from several trees at each sampling location wearing powder-free gloves, dried at 50 C and stored in zip-lock plastic bags marked with the location and sampling date. Six soil proles, approximately 60 cm deep, divided vertically into four sub-samples each, were collected in the city and suburbs of Lule˚ a (65 340 5700 N 22 80 4700 E, Northern Sweden). The area surrounding Lule˚ a is heavily industrialized, with SSAB steelworks as the predominant local industry. The soil consists mainly of clay and silt loam overlying 1.9 Ga granitic bedrock with minor meta-sedimentary constituents.22 During initial stages of method development and testing, three certied reference materials were used, NIST 1547 Peach leaves, NIST 2701 Hexavalent Chromium in Contaminated Soil (both from the National Institute of Standards and Technology, Gaithersburg, MD, USA) and basalt JB-1 (Geological Survey of Japan, Tokyo, Japan), as well as one contaminated soil sample obtained as part of a Swedish environmental monitoring program. The latter was used as an in-house control and was prepared and analyzed repeatedly as part of all sample batches with recoveries ranging from 80%–101% (Table 2).


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Table 2

Cr concentrations and d53Cr in reference and control samplesa

Test sample

Preparation

Cr, mg g1, found s (certied s)

d53Cr (2s), &

QCS T00606 Control soil NIST 1547 NIST SRM 2701 NIST SRM 2701 JB-1

Dilution, n ¼ 28 Digestion/separation, n ¼ 18 Digestion/separation, n ¼ 3 Digestion/separation, n ¼ 2 Digestion/dilution, n ¼ 2 Digestion/separation, n ¼ 3

(10 000) 425 21 0.80 0.04 (1*) 42 300 2700 (42 600 1200)

0.396 (0.072) 0.351 (0.110) NA** 0.091 (0.044) 0.127 (0.066) 0.091 (0.052)

a

*Information values. **Too low Cr content for isotope ratio measurements.

Chromites (black and light-greenish varieties) originating from the open mine pit in Kemi were obtained from the geological collection of Lule˚ a University of Technology. Published on 09 May 2016. Downloaded on 09/05/2017 11:54:29.

415 13 (417)

2.4. Sample digestion 0.05–0.5 g (limited by the recommended maximum sample amount for the digestion vessel used) dried material from each sample location was weighed into a 12 mL Teon vial. Then 5 mL 14 M HNO3 was added and aer one hour of initial oxidation, samples were stirred to ensure complete mixing, before another 1 mL HNO3 and 0.05–0.2 mL HF were added to wash down samples adhering to vial walls. Teon vials were placed into a carousel which was inserted into a Teon-coated UltraCLAVE reaction chamber lled with de-ionized water and H2O2 (10 : 1 v/v). The chamber was pressurized and the preprogrammed digestion cycle (30 min ramp to 230 C followed by 20 min hold at temperature and pressure) was initiated. Processing time, including cooling time and transfer to evaporation vessels, was approximately 2.5 h per digestion batch. Sample digests were transferred into 6 mL Teon screw-cup vials (Savillex, Minnetonka, Minnesota, USA) and dried down on a ceramic-top hot plate. Then 2 mL of 12 M HCl were added to the residue and evaporated again to remove an excess of uorine. The latter procedure was repeated twice followed by residue dissolution in 3 mL 0.2 M HCl for direct Cr separation on anion exchange columns. Small (50 mL) aliquots were taken for preanalysis of sample elemental compositions by ICP-SFMS. 2.5. Matrix separation An anion exchange chromatography procedure was adopted and modied from previously published methods.15,16 Cr(VI) in a weak hydrochloric acid matrix forms negatively charged

Fig. 1

oxyanions where most other elements form cationic or neutral complexes that pass through the column. However, aer sample digestion the majority Cr is present as Cr(III) and an efficient oxidation step is mandatory to quantitatively convert Cr(III) to Cr(VI). To achieve this, 1 mL 0.2 M (NH4)2S2O8 with 0.07 mL 6.7 M NH3 was added to 3 mL 0.2 M HCl sample solution. Teon vessels were tightly sealed and le on a hot plate for a minimum of 2 h at 160 C for sufficient oxidation. Columns loaded with 2 mL DOWEX AG 1X8 anion resin were sequentially cleaned using 2 mL 14 M HNO3 and 10 mL 5 M HNO3 and rinsed with 15 mL MilliQ following by resin preconditioning with 10 mL 0.2 M HCl. Aer cooling to room temperature, sample solutions were loaded onto the anion exchange column. The sample matrix was eluted using 20 mL 0.2 M HCl followed by 10 mL MilliQ water. Aer the matrix wash, 2 mL 5 M HNO3 was added to the column and the ow was arrested by plugging the column to allow reduction of Cr(VI) to Cr(III) which no longer adheres to the resin. Aer at least 2 hours, the column was opened and Cr was eluted by sequentially adding another 28 mL 5 M HNO3. To ensure quantitative elution of Cr, 2 mL 14 M HNO3 followed by 8 mL 5 M HNO3 were added in a nal separate fraction; see Fig. 1 for the full elution scheme. The separation effectiveness was evaluated by multielemental analysis (70 elements measured) of all fractions by ICP-SFMS. This provides (I) direct assessment of Cr recovery, (II) information on separation efficiency from matrix elements, (III) information on Cr concentration, needed for preparation of concentration- and acid strength matched solutions for isotope ratio measurements and (IV) information on the potential presence of spectrally interfering elements and isobars either from the sample matrix or from contamination during sample preparation.

Flow chart of the Cr separation method.



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All puried fractions containing more than 1% of total Cr were pooled in a 40 mL Teon beaker and evaporated to dryness on the hot plate at 160 C, effectively removing residual chloride from the matrix wash stages. An aliquot of 14 M HNO3 was pipetted directly onto the residue, le to react for 10–15 min and then diluted to the desired concentration for analysis on the Neptune. Handling of samples and solutions was performed in clean laboratory facilities (class 10000) by personnel wearing clean room gear, following general precautions to reduce contamination.23 All plastic labware was acid cleaned and rinsed with MilliQ-water before use.

3.

Results and discussion


3.1. Blank contribution The average Cr method blank for the entire procedure, assessed by applying all preparation and separation steps to a set (n ¼ 14) of digestion blanks handled as samples, was 17 5 ng. This corresponds to a Cr contribution of less than 3% for samples containing the minimum analyte content in this study. Re-use of columns gives signicantly higher blanks (59 29 ng, n ¼ 9). As a result of this ‘column memory’, the use of new columns is recommended for samples with low Cr concentrations. 3.2. Separation efficiency Typical elution proles for major matrix elements, Cr, Ni and potentially interfering elements, are shown in Fig. 2. In the majority of samples tested in the present study, Cr recovery

Fig. 2

>93% was achieved in the rst 20 mL of the elution fraction with 2–5% lost during sample loading and matrix wash steps (Fig. 2b). Samples with recoveries below a 90% threshold were re-prepared and re-analyzed or results for such samples were excluded from evaluations. Addition of NH3 proved crucial for quantitative recovery. As evident from initial test separations during method development, Cr recoveries were signicantly lower and resulted in positively biased d53Cr when attempting to oxidize Cr in the absence of NH3. When the negatively charged Cr-oxyanion in the form of HCrO4 attaches to the strong-base anion exchange resin, the majority of major matrix elements (and Ni) pass through the column during sample loading and matrix washing, while 0.1, which would efficiently prevent the use of this isotope in the majority of samples. Moreover, the efficiency of the mathematical correction can be further affected rstly by uncertainty in actual instrumental mass bias deduced from measured and tabulated Ni (or Cr) ratios and secondly by the risk of Fe fractionation (either in the original sample or introduced during column separation). At a S/Cr concentration ratio of 250, d53Cr in the spiked standard was slightly positively biased (0.11& 0.03&). For the rest of the spiked standards, d53Cr was statistically indistinguishable from zero within measurement uncertainty conrming negligible levels of spectral interferences and matrix effects using the proposed method.

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including both QCS solution (no separation) and separations of in-house soil control materials in all measurement sessions providing 2s of 0.07& (n ¼ 28) for the former and 0.11& (n ¼ 16) for the latter. Since measurements of in-house soil replicate digestions, separations and analyses were performed during numerous analytical sessions conducted by various operators, this reproducibility estimate is considered to provide a valid assessment of the entire method. An almost 0.4& difference in d53Cr between our QCS solution and NBS CRM 979 (Table 2) contradicts the presumption of Ellis et al.11 that Cr isotopes are not expected to be fractionated during industrial purication, and thus most or all supplies of industrial Cr should have values close to 0&. Similarly, Schoenberg et al.16 observed an isotopic composition of d53Cr ¼ 0.39& for a Merck Cr(III) standard solution. These ndings serve to conrm the importance of having common d-zero standards for all research groups studying Cr isotope fractionation. Verication of accuracy for Cr concentration measurements by ICP-SFMS was accomplished by analysis of CRMs (Table 2). No matrix reference material with a certied Cr isotopic composition exists, making comparison between the obtained d-values and previously published data the only available option to evaluate accuracy. The d53Cr for JB-1 (0.091& 0.052&) is in acceptable agreement with previously published results by Ellis et al.11 (0.04&, with no stated uncertainty) and Schoenberg et al.16 (0.178& 0.048&) given the aforementioned precision estimate. It should be stressed that these literature values were obtained using very different separation schemes and measurement techniques. Due to the high Cr concentration in NIST 2701 and the chromites, it was possible to measure d53Cr aer simple dilution of digests and to compare data thus obtained with those aer column separation. No statistically

3.4. Throughput Using the UltraCLAVE, up to 40 digestion vessels may be accommodated in the reaction chamber. This limits the batch size to 36 samples along with two method blanks and two reference materials. The entire procedure from sample weighing, digestion, evaporation of digests, separation on anion exchange columns, evaporation of puried fractions, ICP-SFMS concentration determinations, isotope ratio measurements by MC-ICP-MS to data evaluation can be done by one chemist in approximately three days, assuming that evaporations can be done overnight. 3.5. Precision and accuracy In-run instrumental repeatability in d53Cr can be estimated as twice the standard deviation (2s) of results from duplicate consecutive measurements of each sample and as a rule is better than 0.04& or 0.09& for measurements performed with the standard introduction system or Apex, respectively. Reproducibility provides a more realistic assessment of the developed method's ability to detect minor variations in isotope compositions than repeatability.25 Reproducibility was assessed by


Fig. 3 Cr concentrations and d53Cr as a function of soil depth. The arrow represents the typical reproducibility of isotope data.


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signicant differences were found, effectively conrming the absence of column-induced fractionation.


3.6. Cr in soil proles Cr concentrations and d53Cr in 6 soil proles as a function of depth are shown in Fig. 3. Concentrations vary in the (10–35) mg g1 range with a notable trend towards lower levels in topsoil. Similar patterns are also observed for elements such as Al and Ti and are caused by differences in organic matter content, the latter containing less Cr than more predominantly inorganic sections. There is very little variation in the Cr isotopic composition between different locations and depths (Fig. 3b), with the majority of results being within the 0.08–0.14& range. It might appear that topsoils are slightly enriched in heavier Cr isotopes but given a typical method with a reproducibility of 0.11& this tendency is not signicant. No correlation was observed between Cr concentrations and d53Cr (R2 ¼ 0.2). No notable differences were observed for proles taken from the city and from suburbs. It should be noted that the in-house control soil sample used for method development and performance testing has a significantly heavier isotope composition (0.35&). 3.7. Cr in lichens and mosses Concentrations of Cr in lichens increase from approximately 1.5 mg g1 in the vicinity of Lule˚ a to >30 mg g1 near the border between Sweden and Finland. Concentrations in moss samples follow a similar pattern, increasing from approximately 4 mg g1 in Lule˚ a to almost 50 mg g1 when approaching Torne˚ a steelworks. These data reaffirm previously published ndings on the anthropogenic impact of chromite mining and stainless steel production in the area.19 The higher degree of Cr accumulation in mosses than that in lichens also substantiates data acquired using other species in a Nigerian study.26 Interestingly, there is a strong (R2 ¼ 0.7) correlation between 53 d Cr in lichens or mosses and inverse Cr concentrations, as shown in Fig. 4. In lichen samples from Lule˚ a, Cr is signicantly

Fig. 4 d53Cr in chromites and environmental samples as a function of inverse Cr concentration, error bars are 2s.


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heavier (d53Cr approximately +0.4&) than that in local soils indicating different isotope compositions in airborne sources. Recent estimates show that all surface waters are enriched in heavy Cr isotopes27 suggesting that wet deposition may explain the positive d53Cr values in lichens from Lule˚ a. At approximately 30 km distance from the Torne˚ a steelworks, as Cr in both lichen and moss samples increases, d53Cr decreases to approximately 0.2&. Bio-indicators with the highest Cr content, sampled at approximately 2 km from the steelworks, have mainly negative d53Cr values and a high scatter of isotope compositions (Fig. 4). Average d53Cr for Kemi chromites is 0.08&, which is identical to the mean value reported for chromites by Farkaˇs et al.28 Therefore, ne Cr-rich dust from chromite mining and transport activities, unlikely to induce Cr isotope fractionation, can hardly be the major source of local airborne Cr pollution. It seems that smelting and rening processes result in predominant airborne release of lighter Cr isotopes as reected in the Cr isotopic composition found in lichens and mosses. It must also be mentioned that d53Cr values for Torne˚ a lichens and mosses are reminiscent of dust, assumed by Bonnand et al.27 to have the same composition as the continental crust (0.13&).

4. Conclusion Digestion using the UltraCLAVE reaction chamber provides complete oxidation of the organic material as well as digestion of chromite samples, while a one pass column separation allows efficient removal of matrix elements and shows consistently high Cr recoveries. However, Ti, V and Fe are still present at trace levels, hindering accurate measurements of 50Cr and 54Cr isotopes. Incorporation of the present Cr separation procedure into multi-elemental purication schemes shows interesting potential. For example, Cr is quantitatively recovered in the sample loading and matrix wash steps during a column separation designed for B, Cu, Cd, Fe, Pb, Sr, Tl and Zn separation.29 Therefore, using these fractions for Cr separation would signicantly reduce the amount of Fe present and enable the use of 54Cr. Although the Apex desolvation system allows isotope ratio measurements in samples with lower Cr concentrations and reduces oxide formation, in-run precision is inferior while signal stabilization and wash-out times are longer than those with the standard conguration. Therefore, the latter is to be preferred when the Cr concentration is not a limiting factor. The analytical protocol presented has proved to be suitable for precise 53Cr/52Cr isotope ratio measurements in various environmental matrices although the absence of commercially available matrix-matched reference materials with certied isotope ratios for Cr hampers the assessment of true method accuracy and emphasizes the need for such products to be developed and validated. The rst ever Cr isotope data obtained for lichens and mosses indicate the potential of using this approach for tracing and quantifying airborne Cr pollution caused by stainless steel foundries.


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Acknowledgements


We acknowledge ALS Scandinavia AB for technical and nancial support of this project. The Neptune was purchased through a grant from Kempestielsen to the Centre for Isotopic and Trace Element Measurements, Lule˚ a University of Technology. We also wish to thank Katerina Rodiouchkina for help with sampling and sample preparation. The authors also wish to express their gratitude to the Stable Isotopes Laboratory of the Department of Earth Sciences of the University of Oxford for providing NBS 979 reference solution. We also thank Bj¨ orn ¨ Ohlander, Johan Ingri, Dimitry Malinovsky, and Lennart Widenfalk for help in obtaining geological samples.

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