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Platinum Group Elements determination in seawater by ICP-SFMS: initial results Clara Turetta1*, Giulio Cozzi2, Anita Varga2,3, Carlo Barbante1,2, Gabriele Capodaglio1,2, Paolo Cescon1,2 1
Institute for the Dynamics of Environmental Processes - CNR, Dorsoduro 2137, 30123 Venezia (I); Dept. of Environmental Sciences - University Ca' Foscari Venice, Dorsoduro 2137, 30123 Venezia (I); 3 Research Group of Environmental and Macromolecular Chemistry, MTA-ELTE Budapest (H). 2
*
[email protected]
Abstract. Studies performed on platinum group elements (PGE) in several environmental matrices have already highlighted their diffusion at a planetary level and their potential risk for human health. These elements, especially Rh, Pd and Pt, have been released in the environment over the previous decades, principally by cars equipped with catalytic converters. In order to evaluate their distribution in the environment and to understand their geochemical behaviour, it is important to study these elements also in non-conservative matrices, such as seawater. PGEs are present in seawater at very low levels (usually less then 1 pg/g); only very sensitive instrumentation like ICP-SFMS, permits the determination of these elements in diluted seawater without any other manipulation of samples, reducing the risk of contamination associated with preconcentration techniques.
1. INTRODUCTION Platinum group elements represent a potential risk for human health; their diffusion at a planetary level has already highlighted, by previous researches, especially in conservative environmental matrices. In order to understand geochemical behaviour of PGEs and their distribution in the environment we have studied the characteristics of these elements in seawater, a non-conservative matrix. The evaluation of PGEs in seawater requests the use of very sensitive instrumentation, like HR-ICP-SFMS, because they are at very low concentrations and the use of a methodology which prevents the risk of contamination. 1.1 Instrumentation The ICP-SFMS instrument used was the Element (Finnigan-MAT, Bremen, Germany). The device can be operated in low-resolution mode (LMR, m/∆m=300), medium resolution mode (MRM, m/∆m=3000) and high-resolution mode (HRM, m/∆m=7500). Intensity optimisation was carried out daily using as tuning solution MilliQ (Millipore) water containing 1ng/g of In; the optimisation is then repeated using a diluted (10 folds) sea-water sample containing a bi-elemental standard solution (In and W - 500 pg/g). Before to commencing the analyses an accurate mass calibration was performed using a solution containing elements with m/z values covering the whole mass range of interest. The chosen parameters lead to a total acquisition time per sample of about 11 minutes excluding wash times. Two systems for direct sample introduction were utilised: a µ-Flow Nebulizer (Elemental Scientific) coupled with a teflon spray chamber and a µ-Flow Nebulizer coupled with a desolvation unit (Cetac Technologies, fig. 1), in which the sample gas is heated to 95°C to prevent droplet accumulation in the spray chamber and then swept into a permeable membrane to significantly reduce oxide formation.
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Figure 1. Micronebulization/desolvating introduction system (Aridus, Cetac Technologies).
1.2 Methodology We present here the optimisation of our analytical methodology using ICP-SFMS to determine PGE in seawater samples. The main problems were to prevent contamination of samples, usually linked to sample pretreatment, and the saline-matrix of the samples and the spectral interferences which causes great problem in PGE quantification. In this respect all preparations were carried out in a class 100 clean room under a laminar flow bench. Pre-treatments were restricted to dilution, acidification and internal standard addition. Five aliquots of a sample were spiked with a multi-standard solution in order to obtain a PGE concentration of 2.0, 5.0, 10.0, 20.0, 50.0 pg/g respectively. These solutions were used for the quantification of PGEs as a matched calibration curve. For acidification UPA grade HNO3 was used. All material used for sampling, treatment and storage of samples and solutions were acid-cleaned as reported in previous works [1, 2, 3]. To minimise matrix effects we have done some dilution/acidification tests; we have carried out experiments with 5, 10, 15 and 20 fold dilutions and 5, 10, 15% acidification. Even though a 5 minute wash time was used after each analysis (wash solution was MilliQ water with 10% HNO3 v/v), a 5-fold dilution lead to clogging of the nebulizer after few sample analyses even with a 15% acidification. Also the test with 10-fold dilution and 5% acidification lead to clogging of the µ-flow nebulizer. 10-fold dilution with 10 and 15% acidification respectively did not show appreciable differences between themselves and give a good repeatability of signal and no clogging of the nebulizer after more than five hours of continuous analyses. 15 and 20 fold dilutions greatly reduced matrix effects but the very low level at which PGEs were present did not permit a good repeatability of measurements. All these tests lead us to choose 10-fold dilution with 10% acidification to reduce the additions and sample pre-treatment required. In and W were used as internal standards at a final concentration of 500 pg/g. The ways to reduce spectral interferences with instrumentation like HR-ICP-SFMS, which has a high-resolution capability, are to use specialised sample introduction systems and mathematical correction. Main spectral interferences on selected isotopes are listed in table 1. The use of HR-ICP-SFMS allows increase in resolution (from low resolution to high resolution m/∆m varies from 300 up to 7500) which result in the resolution of some interferents listed on table 1, but it also leads to a decrease in sensitivity which prevent the determination of the very low level of PGEs in diluted seawater.
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Table 1. Main spectral interferences on Rh, Pd and Pt.
Analyte Isotope Abundance (%) 103
Pd
195
Pt
Resolution (m/∆m)
100
40
Ar63Cu+ Sr16O+ 87 Rb16O+ 206 Pb2+
68.89 7.00 27.78 25.15
7609 60603 73241 1260
22.33
40
Ar65Cu+ 89 16 + Y O 87 18 + Sr O 87 Rb18O+
30.71 99.76 0.03 0.06
7042 24373 328855 32102
33.80
179
13.76
8108
Rh
105
Interferent Species Abundance (%) 87
Hf16O+
The use of a desolvation introduction system instead of teflon spray chamber has several advantages such as reducing oxide and argon interferences and improving sensitivity due to an improved analyte transport efficiency. These characteristics of the introduction sample system mean that minor mathematical correction is necessary for Rh and Pd. Thanks to the use of the desolvation unit we can directly measure Pt levels without any correction: the HfO contribution is less then 0.03% of the total intensity instead of more then 30% of HfO contribution in measurements performed with a spray chamber and a µ-flow nebulizer. In Rh quantification the importance of interferents are strongly reduced by the use of the desolvating system but are still significant. Without the desolvation system it was impossible to evaluate the Pd data because the very low level of this element in comparison to the interferent species; the use of this introduction system lead to a significative reduction in the importance of interferents but do not eliminate the need for mathematical corrections for some of these interferents. In the determination of PGE content, mathematical corrections were used, as reported in various papers [4, 5, 6], and listed below are: IRh = IRh,s - (ICu,s . RArCu,Cu(63) + ISr,s . RSrO,Sr(87) + IRb,s . RRbO,Rb(87) + IPb,s . RPb(2+),Pb(206)) IPd = IPd,s - (ICu,s . RArCu,Cu(65) +IY,s . RYO,Y(89) + ISr,s . RSrO(18),Sr(87) + IRb,s . RRbO(18),Rb(87)) IPt = IPt,s - (IHf,s . RHfO,Hf(179)) As reported above, using the desolvation unit, mathematical correction is no longer necessary for Pt. On the other hand, even with this introduction system, mathematical corrections are still necessary for Rh and Pd. Instrumental operating conditions are reported in table 2. Table 2. Plasma and desolvation unit operating conditions.
Plasma conditions Cool gas Auxiliary gas Sample gas Power
12.50 l/min 1.30 l/min 0.950 l/min 1140 W
Desolvation system conditions Sweep gas flow 3.51 l/min Nitrogen flow 0.16 ml/min Spray Chamber t° 95 °C Desolvator t° 175 °C
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2. RESULTS Table 3 reports some preliminary data for the Pt content in the dissolved fraction of seawater samples from the lagoon of Venice (surface microlayer and subsurface water filtrate from a 0.4 µm pore size filter), Antarctic seawater (Ross sea), Coastal Atlantic seawater (CRM-CASS4) and Atlantic seawater (CRM-NASS5). Table 3: Pt content in dissolved fraction of seawater. Values are in fg/g.
Antarctic seawater
Venice Lagoon samples Microlayer Subsurface water
CASS4 NASS5 Ross Sea LV01 2-1 LV01 4-1 LV02 1-1 LV01 2-2 LV01 4-2 LV02 1-2 90.7
67.9
51.5
775.6
529.4
393.0
223.4
300.3
191.2
Values reported for Oceanic and Antarctic seawater are in agreement with expected values [7, 8]. For Venice lagoon samples we can observe that the Pt values in the microlayer are higher than in subsurface water. The microlayer represents a particular surface able to accumulate substances of natural and anthropic origin at higher levels than in subsurface water. The sampling site is near the bridge between Venice and the mainland and is near an industrial area; by taking into account values of Pt in aerosols from inland and industrial areas, we can assume a strong aerosol input for these samples caused by dry and wet deposition on the microlayer. For Rh more tests are necessary because the values found are higher than expected (we found a concentration of Rh at the pg/g level). In our opinion Sr and Rb oxide and Cu argide play an important role as interferents in Rh evaluation in seawater samples, also if desolvation system are used. Pd in seawater is at a very low level; the 10-fold dilution used for these measurements do not permit us to detect accurately this element and to separate its peak from that of its interferents even when mathematical corrections are applied. References [1] Capodaglio G., Toscano G., Cescon P., Scarponi G., Muntau H., Ann. Chim., 84 (1994) 329-345. [2] Scarponi G., Capodaglio G., Barbante C., Cescon P. (1996), in: Element speciation in bioinorganic chemistry. (Caroli S. Editor), London, Chem. Anal. Series, pp. 363-418. [3] Field M.P., Cullen J.T., Sherrell R.M, J. Anal. Atomic Spectroscopy. 11 (1996) 1425-1431. [4] Parent M., Vanhoe H., Moens L., Dams R., Talanta, 44 (1997) 221-230. [5] Lustig S., Zang S., Michalke B., Schramel P., Beck W., Fresenius J. Anal. Chem., 357 (1997) 11571163. [6] Moldovan M., Gomez M.M., Palacios M.A., J. Anal. Atom. Spectrom., 14 (1999) 1163-1169. [7] Hodge V, Stallard M., Koide M., Goldberg E.D., Anal. Chem., 58 (1986) 616-620. [8] Barefoot R.R., Env. Sc. Technol., 31 (1997) 309-314.
