- Email: [email protected]
Speciation of inorganic platinum–chloride complexes in spiked environmental samples by SPE and ICP–AES
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
Speciation of inorganic platinum–chloride complexes in spiked environmental samples by SPE and ICP–AES Valentina Ljubomirova ∗ , Rumyana Djingova Faculty of Chemistry, University of Sofia, 1 J. Bouchier Boulevard, 1164 Sofia, Bulgaria
a r t i c l e
i n f o
a b s t r a c t
Article history:
A method for the separation of tetrachloroplatinate PtCl4 2− , and hexachloroplatinate PtCl6 2− ,
Received 6 November 2007
by solid-phase extraction, using a Dowex 1 × 10 anion exchange resin is proposed. The
Received in revised form
sequential elution and separation of PtCl4 2− , and PtCl6 2− is achieved using selective com-
18 March 2008
plexing agents. The eluates, containing Pt(II) and Pt(IV) were analyzed by inductively coupled
Accepted 19 March 2008
plasma–atomic emission spectroscopy (ICP–AES). Recoveries of 102% for PtCl4 2− and 94% for
Published on line 27 March 2008
PtCl6 2− and detection limit of 15 ng g−1 were achieved. Using this method determination of Pt(II) and Pt(IV) in soil samples, spiked with different platinum species was performed. The
Keywords:
comparison with GFAAS determination showed a very good agreement.
Speciation
© 2008 Elsevier B.V. All rights reserved.
Tetrachloroplatinate Hexachloroplatinate Anoin-exchange ICP–AES GFAAS
1.
Introduction
The determination of total Pt contents of environmental and biomedical matrices has gained a considerable interest within the last decade, which is due to two main applications [1–8]. On the one hand, Pt compounds like cisplatin and carboplatin are used as anti-cancer drugs [9]. On the other hand, Pt containing vehicle exhaust catalyst (VECs) are introduced in the middle 1970s (USA-1975, Germany-1984, EU-1993). Platinum has been used as a catalytic converter material for the refinement of automobile exhaust gases [10–18]. The clinical application is characterized by relatively high concentrations of Pt species (up to the low mg L−1 range in whole blood of cancer patients) with a defined and locally limited distribution [9]. The emissions from VECs show exactly the opposite behavior. They occur in trace concentrations (in the low g kg−1 range in road dust) and are distributed nearly ubiquitously in the
∗
urbanized areas [3]. Moreover, Pt from VECs is emitted in the form of fine suspended particulate matter and therefore might be inhaled and accumulated by living organisms [19–21]. The high biochemical activity of some Pt species, e.g. the cytotoxicity and mutagenicity of cisplatin or the sensitizing effect of hexachloroplatinate bears the risk of significant negative effects on human health after chronic exposure even to trace amounts of Pt species. A detailed review on the health effects of Platinum Group Elements (PGE) was recently published by Ravindra et al. [10]. However, the (eco)toxicity and bioavailability of the emitted Pt are strongly dependent on oxidation state and the binding forms of Pt in the environmental matrix, i.e. the Pt speciation [22–24]. For example, elemental Pt and platinum dioxide are rather insoluble under environmental conditions while chloro–Pt complexes and further halogenated Pt compounds are well soluble in water and toxic due to their high allergenic
Corresponding author. Tel.: +359 88 5930 751; fax: +359 2 9625 438. E-mail addresses: valentina [email protected] (V. Ljubomirova), [email protected]fia.bg (R. Djingova). 0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2008.03.037
120
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
potential which is related to the halogen ligands and to the negative charge of these complexes [10]. A recent study concluded that platinum uptake mechanism depends on the form of platinum present in the environment: Pt(IV) being more likely to be taken up at a higher rate than Pt(II) by freshwater isopod Asellus aquaticus [25]. Another study with freshwater periphyton communities showed that the uptake did not depend on the oxidation state, Pt(II) exhibited higher toxicity to them compared with Pt(IV), perhaps because of the higher capacity of Pt(II) to bind to amino-acids and proteins [26]. The order of relative toxicity, determined by visual appraisal and in terms of oxidation states for different plant species, was Pt2+ > Pd2+ > Ru3+ ≈ Ru2+ ≈ Ir3+ > Pt4+ ≈ Os4+ Rh3+ . It was concluded that the toxic effects of Pt substantially depend on the type of plant species treated [10]. Studies of human tissue have shown that only Pt(II) binds to proteins in human blood, but Pt(IV) can be reduced to Pt(II) before binding [22,27,28]. All the above topics need further investigation to reach a better understanding of the behavior of Pt in the environment. Although the knowledge of Pt cycle in the environment is of great importance, studies on Pt speciation are rarely reported in the literature. A procedure for the analysis of several platinum species such as the anionic platinum–chloro complexes and some of their hydrolysis products based on ion chromatography was presented [22]. A modification of the chromatographic conditions was proposed by Nischwitz et al. to realize an on-line HPLC–ICP–MS hyphenation for the speciation of Pt(II) and Pt(IV) [23]. The method was validated with aqueous standard solutions as well as spiked road dust extracts with respect to an application for the characterization of Pt species in road dusts. The stability of the model species was investigated in mixtures with several extractants under variation of the pH as well as the concentration of the complexing agents and buffers respectively. Additionally the effect of energy supply by refluxing, sonication and microwave exposure was studied. The optimum conditions were applied for the extraction of platinum species from spiked and unspiked road dust samples [24]. In spiked samples PtCl6 2− was not recovered while in unspiked samples less than 5% from total platinum was extracted and the detection limit was not low enough to detect any soluble platinum complexes. Michalke and Schramel developed an on-line CE–ICP–MS method [29]. Messerschmidt et al. [30] and Alt et al. [31] applied gel permeation chromatography and preparative isotachophoresis combined with adsorptive voltametric measurements to platinum speciation in grass. All the described methods use hyphenated techniques for separation and quantification of Pt species. The aim of the present paper is to propose a method for ICP–AES determination of water-soluble species of Pt (PtCl4 2− and PtCl6 2− ) in environmental samples using anion-exchange procedure.
2.
Experimental
2.1.
Reagents and materials
(NH4 )2 PtCl4 (99%, Aldrich) and K2 PtCl6 (99%, Merck) were used for the preparation of stock solutions of PtCl4 2− and PtCl6 2−
(1000 g mL−1 ) in 1 M HCl. Diluted platinum metal solutions were daily freshly prepared in 1 M HCl. HCl (37%, p.a., Merck) and HNO3 (65%, p.a., Merck) were purified by sub-boiling distillation and stored in quartz vessel. The eluent solutions 2,2 -bipyridyl (p.a., Fluka), sodium-diethyldithiocarbamate (NaDEDTC) (p.a., Fluka) and thiourea (p.a., Fluka) were freshly made by dissolving 2.34 g 2,2 -bipyridyl, 3.38 g NaDEDTC, and 0.39 g of thiourea in 0.1 M HCl in a volume of 50 mL. The reducing solution of SnCl2 (99%, Fluka) was prepared by dissolving 2.84 g in 50 mL 3 M HCl. The elution of Pt-DEDTC complexes was performed with methylisobuthylketon (MIBK) (99%, Merck). Dowex 1 × 10 resin (Fluka) 100–200 mesh (ionic form: Cl− ) was used in the experiment. The resin was packed in a 1 cm × 3 cm glass column containing a glass frit at the bottom and a socket at the top. The resin was prepared by soaking in 1 M HCl overnight. For each ion-exchange experiment, the resin was conditioned by passing 100 mL 1 M HCl and next heated to 60 ◦ C. All the experiments were carried out at a flow rate of 3–4 mL min−1 .
2.2.
Instrumentation
Metal determination in the soil extracts and digests were carried out by ICP–AES using Spektroflame D (Spektro Analytical Instruments, Kleve, Germany) spectrometer. Operating conditions for the instrument are presented in Table 1-1. Spectral line used for Pt determination was according to [32]. The calibration of the spectrometer was done using freshly prepared solutions of Pt in concentration range 0.01–10 g mL−1 . Since all eluates were evaporated to incipient dryness and dissolved in a mixture of 2.5% HCl and 1.3% HNO3 the standard solutions for calibration were prepared the same way. Thus probable influence of the eluates on the signal was eliminated. The calibration curve was characterized by R2 = 99.2%. The residual standard deviation was about 2.3%. Calibration was checked twice daily with freshly prepared standard solutions. Previously the lack of matrix elements in the samples after ion-exchange has been proven [32] therefore the detection limit was determined as 3SD of the blank signal and amounts to 15 ng mL−1 . For evaluation the potential losses during the multiple evaporations of the eluates graphite furnace atomic absorption spectrometry of the organic phases before evaporation was used. The measurements were carried out on a PerkinElmer (Norwalk, CT, USA) Zeeman 3030 spectrometer with an HGA-600 graphite furnace (operating conditions given in Table 1-2).
Table 1-1 – Instrument operating parameters used for ICP–AES determination of Pt (Spectroflame D) Instrument Plasma power Cooling gas flow Auxiliary gas flow Nebuliser gas flow Nebuliser Sample uptake Wavelength [nm]
Spectroflame D 1200 W 14 L min−1 1 L min−1 0.8 L min−1 Cross-flow 2.5 mL min−1 214.423
121
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 1-2 – Instrumental operation conditions for PerkinElmer (Norwalk, CT, USA) Zeeman 3030 spectrometer with an HGA-600 graphite furnace Wavelength Slit with Signal type Measurement Dilution Type Pretreatment temperature Atomization temperature Lamp current
2.3.
265.9 nm 0.7 nm AA-BG Peak area 0.2% HNO3 Pyrolytic 1300 ◦ C (15, 10) 2500 ◦ C (0, 3) 7 mA
Description of the soil samples and analysis
Two soils: carbonic chernozem, typical for North Bulgaria and brown forest soil, typical for the Rhodopi mountains region were chosen for the experiment. Sampling was performed according to ISO 10381-1:2002 and ISO 10381-2:2002 in regions not directly affected by industrial or transport pollution. Sample preparation included air drying, sieving through 2-mm PTFE sieve, grinding and homogenization in a Teflon ball mill (ISO 11464:2006). pH of the original soils was determined according to ISO 10390:2005 and CEC according to ISO 23470:2007. After aqua regia digestion (ISO 11466:1995) the soil extracts were analyzed by ICP–AES at operating conditions shown in Table 1-1 and using the most intensive emission lines of the respective elements. The results are presented in Table 2.
2.4.
Preparation of spiked soil samples
Two kilograms of each type of soil were spiked with Pt catalytic converter. The body of a used catalytic converter (monolith)
was crushed and milled in a PTFE ball mill. The concentration of Pt in the converter material was determined after aqua regia digestion and ICP–AES measurement (instrumental parameters given in Kovacheva and Djingova) [32]. Ten grams of the autocatalyst were slurred in 50 mL aqua regia and heated in a sand bath for 2 h. The sample was filtered and diluted to 50 mL with bidistilled water and ICP–AES measurement was performed. The concentration of Pt was 750 g g−1 converter material. Two kilograms of each type of soil were mixed with the converter material (300 g were added to the brown forest and 140 g to the carbonic chernozem) and well homogenized. Another series of the two types of soil samples (2 kg) were spiked with different volumes of the stock standard solutions of PtCl4 2− and PtCl6 2− (1000 g mL−1 ) as follows: 56 mL and 14 mL of PtCl4 2− to the brown forest and carbonic chernozem and 42 mL and 18 mL of PtCl6 2− to the brown forest and carbonic chernozem soil, respectively (final concentrations given in Table 3) and well homogenized. The spiked soil samples, placed in flowerpots were watered regularly to keep the normal soil moisture. Two months later aliquots of 50 g of each soil samples were taken out, air dried and used for the experiments. The digestion of the soil samples for total Pt determination was performed with aqua regia digestion (as already described for the catalytic converter). In order to check the quantitative recovery of Pt total digestion was performed according to Djingova et al. [14] and the results are presented in Table 3. There is a very good agreement between introduced and determined total Pt in the samples.
2.5.
Preparation of soil extracts
In order to achieve quantitative recovery of the inorganic chloro complexes of Pt (PtCl4 2− , PtCl6 2− ) without species inter-
Table 2 – Characterization of the soil samples Element
Soil Brown forest
%
K Mg Fe
[g/g]
Cu Zn Na Cd Pb Sb As Sn Sr Y Ce Zr La Nd
Carbon [%]
Total C Inorganic C pH
(meqv 100 g−1 soil)
CEC
0.399 ± 0.001 0.816 ± 0.008 2.66 ± 0.03 47.8 ± 0.3 371 ± 12 166 ± 5 5.2 ± 0.2 37.6 ± 0.2 29.8 ± 0.6 28.2 ± 0.9 14.4 ± 0.1 73.8 ± 0.5 23.6 ± 0.2 89 ± 1 54 ± 2 40 ± 1 39 ± 1 3.735 0.029 7.2 31.1
Carbonic chernozem 0.126 ± 0.001 0.8433 ± 0.008 1.33 ± 0.01 68.5 ± 0.6 44.3 ± 0.4 65.75 ± 0.04 2.46 ± 0.01 22.6 ± 0.4 19.0 ± 0.1 24.8 ± 0.1 9.3 ± 0.3 56.6 ± 0.6 10.5 ± 0.3 42 ± 1 37 ± 2 17 ± 1 22 ± 1 1.147 0.845 8.0 23.4
122
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 3 – Concentration of platinum in soil samples spiked with different platinum species determined by ICP–AES Spiked Pt species
Added quantity of Pt [g g−1 ]
Brown forest (pH 7.2)
Pt-catalyst K2 PtCl4 ; K2 PtCl6
110 28; 21
114 ± 4 50 ± 2
Carbonic chernozem (pH 8.0)
Pt-catalyst K2 PtCl4 ; K2 PtCl6
50 7; 9
51 ± 2 16 ± 1
Soil type
conversion, an extraction of 3 g of soil samples with 20 mL 1 M HCl was performed by shaking for 2 h at 22 ± 3 ◦ C (two series of three replicates each). The extract was centrifuged at 1600 × g for 10 min. One of the series was used for direct determination of Pt and the second one for performing the procedure for concentration of Pt and separation of PtCl4 2− and PtCl6 2− .
3.
Results and discussions
While choosing the elution conditions two aspects have to be considered: (i) the platinum–chloro complexes should be efficiently eluted and (ii) the native species of the element should not be altered. The strong anion exchangers have a great selectivity to chloro complexes of Pt. Some of the platinum complexes are adsorbed too strongly to be eluted from the stationary phase. The use of eluents at elevated temperature, as well as several circulations of the eluent solution (e.g. 10 times thiourea, 60 ◦ C) for quantitative recovery of the studied metal in a small final volume has been proposed in a previous study [32]. In the present study the same approach was used for the quantitative elution of platinum species from the Dowex 1 × 10 column. To prevent the possible processes of hydrolysis and aquatation, the acidity range was chosen according to references [22,23,33] and was from 0.1 M HCl to 5 M HCl. According to Nachtigall et al. [22] and Nischwitz et al. [23], who investigated the long-term species stability in HCl media it can be concluded that 1 M HCl is the most suitable media to prevent reduction–oxidation processes.
3.1.
Ion-exchange procedure
The initial approach that has been chosen during the investigations was to find an appropriate complexing agent for elution of Pt(II) from the ion-exchange column and afterwards to reduce Pt(IV) remaining on the column with 10−3 M SnCl2 in 3 M HCl and elute it with the same complexing agent. [34–36].
3.2.
Determined quantity by ICP–AES [g g−1 ]
obtained using 5 M HCl as rinsing solution, 13.56% and 42.44%, and the highest ones using 1 M HCl, 54.68% and 58.30%, respectively. Similar results are obtained using 0.1 M HCl 50.12% and 52.68%. The results from the experiment show that the complexation between 2,2 bipyridyl and Pt(II) and Pt(IV) is slow and this reagent cannot be used for elution of Pt from the resin.
3.3.
Experiment with NaDEDTC
Another complexing agent, which reacts predominantly with Pt(II) is NaDEDTC. NaDEDTC forms neutral complexes with PtCl4 2− in acid solution as follows: Pt2+ + 2DEDTC− → Pt(DEDTC)2 . The formed complexes can be eluted with organic solvents. Pt(IV) reacts incompletely with possible reduction to the bivalent state [38]. Pt(IV)–dithiocarbamate complexes have a number of metal to ligand ratios, 1:1, 2:3, 1:2 and 1:3 and may contain chloride ions [38]. These complexes can be ionized in solution and the elution with apolar organic solvent is impossible [39]. NaDEDTC was used for solvent extraction of Pt(II) [40] and HPLC determination [39,41].
3.3.1. Elution of standard solutions of PtCl4 2− and PtCl6 2− with NaDEDTC To evaluate the eluted PtCl4 2− and PtCl6 2− complexes with NaDEDTC, two separate experiments were performed with standard solutions containing 30 ng mL−1 PtCl4 2− and 30 ng mL−1 PtCl6 2− in 1 M HCl. Ten milliliters of the standard solutions were passed through the column. The resin was rinsed with 10 mL 1 M HCl, after 3 mL 0.3 M NaDEDTC in water were added, circulating 4 times through the resin. The formed Pt(II)-DEDTC complexes were eluted with 10 mL MIBK. The elution profile of Pt(II) with MIBK is presented in Fig. 1.
Experiment with 2,2 -bipyridyl
It is known that 2,2-bipyridyl reacts with Pt(II) to produce Pt(bipy)Cl2 . In acidic solution 2,2-bipyridyl reacts also with Pt(H2 O)Cl3 − [37]. In our experiments first elution of PtCl4 2− was done with 5 mL 0.3 M 2,2-bipyridyl circulated 10 times at 60 ◦ C. Afterwards the inert Pt(IV) remaining on the column was reduced to Pt(II) by passing 2 mL 10−3 M SnCl2 in 3 M HCl and elution with bipyridyl was performed as before. Additionally the influence of the pH on the elution process was investigated by rinsing the resin with HCl with molarity 0.1, 1, 2, 4 and 5. The results from the measurement show extremely low recoveries. The lowest recoveries for PtCl4 2− and PtCl6 2− are
Fig. 1 – Elution profile of Pt(II) with MIBK.
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 4 – Recovery of PtCl4 2− and PtCl6 2− in the form of Pt(II)-DEDTC complexes eluted with MIBK from Dowex 1 × 10 column. Pt species
Input quantity [ng]
PtCl4 2− PtCl6 2−
300 300
Eluted quantity [ng] (n = 3) 293 ± 2 15.7 ± 0. 2
Recovery [%] 97.7 ± 0.7 5.2 ± 0.1
The same experiment was repeated with the standard solution of PtCl6 2− . MIBK was evaporated on a sand bath, 1 mL mixture of 37% HCl and 0.3 mL 65% HNO3 was added and evaporation to incipient dryness was done. The residue was dissolved in 1 mL mixture of 2.5% HCl and 1.3% HNO3 . The solution was diluted to 10 mL with bidistilled water and ICP–AES measurement was performed. Table 4 presents the results from the elution (mean of three replicates). The results from this experiment show that NaDEDTC reacts predominantly with PtCl4 2− (recovery 97.7%) and can be used for selective elution and separation of PtCl4 2− and PtCl6 2− .
3.3.2. Elution of complex standard solutions of PtCl4 2− and PtCl6 2− with NaDEDTC The described experiment with DEDTC was repeated with mixed standard solution of PtCl4 2− and PtCl6 2− in 1 M HCl. Five milliliters of this solution containing 50 ng mL−1 each of the species were passed through the resin. The resin was rinsed with 10 mL 1 M HCl and 3 mL 0.3 M NaDEDTC were circulating 4 times through the resin. The Pt(II)-DEDTC complexes were eluted with 10 mL MIBK. Afterwards reduction with SnCl2 of Pt(IV) to more labile Pt(II) ions was performed. Two milliliters of 10−3 M SnCl2 in 3 M HCl were passed through the column. Next 3 mL 0.3 M NaDEDTC were circulated 4 times through the resin. The elution of Pt(II)-DEDTC complexes thus formed was performed with 10 mL MIBK. The eluates, containing MIBK were evaporated on a sand bath, 1 mL mixture of 37% HCl and 0.3 mL 65% HNO3 were added and evaporation to incipient dryness was done. The residue was dissolved in 1 mL mixture of 2.5% HCl and 1.3% HNO3 . The solution was diluted to 5 mL with bidistilled water and ICP–AES measurement was performed. The data (mean of three replicates) are presented in Table 5. The low recovery for PtCl6 2− (56%) shows that the reduction of Pt(IV) with SnCl2 on the column is obviously not complete and this procedure cannot be used for quantitative separation of Pt(II) and Pt(IV).
123
3.3.3. Method for separation and determination of PtCl4 2− and PtCl6 2− The results from the above experiments prove that the elution with NaDEDTC is a successful approach for separation of Pt(II) however the reduction of Pt(IV) with SnCl2 is incomplete. Therefore change of the eluent for the complete recovery of Pt(IV) is necessary. Earlier in our laboratory a method for determination of Pt and Pd has been developed using ion-exchange concentration and elution with circulated thiourea at 60 ◦ C [32]. This approach was finally used to elute Pt(IV) from the column. Five milliliters of the complex solution, containing 50 ng mL−1 each of the species PtCl4 2− and PtCl6 2− (acidity 1 M HCl) was passed through the column. The resin was rinsed with 10 mL 1 M HCl and 3 mL 0.3 M NaDEDTC were circulated 4 times through the resin at 60 ◦ C. The Pt(II)-DEDTC complex was eluted with 10 mL MIBK. The eluate was evaporated in a sand bath and 1 mL mixture of 37% HCl and 0.3 mL 65% HNO3 were added. Evaporation to incipient dryness was performed and the residue was dissolved in 1 mL mixture of 2.5% HCl and 1.3% HNO3 . The solution was diluted to 5 mL with bidestilled water and ICP–AES measurement was performed. Afterwards elution of Pt(IV) was carried out using 4 mL 0.1 M thiourea in 0.1 M HCl circulated 10 times through the column at 60 ◦ C and ICP–AES measurement was performed [32]. Aliquots from the organic fractions (MIBK and thiourea) were measured by GFAAS for comparison and to control the effect from the evaporation steps. The results from the analysis of mixed standard solutions are presented in Table 6. The lower recovery for Pt(IV) 94 ± 4 might be explained by insignificant reduction of Pt(IV) to Pt(II) which recovery sometimes is over 100%. The achieved precision is 5% and the detection limit (3SD) amounts to 15 ng mL−1 . The results from the AAS measurement indicate that there is not a significant loss of Pt during the evaporation step of the organic phases before the ICP–AES measurement. The comparison of the proposed method for concentration and separation of PtCl4 2− and PtCl6 2 to existing ones [22–24] shows quantitative separation and very good recoveries for both species and only 2% of Pt(IV) is reduced to Pt(II) during the procedure. The detection limit is not better than the reported one but the measurements in the present study were done by ICP–AES and not ICP–MS [22–24] generally considered to be a much more sensitive method. An advantage in this respect however may be considered the lack of spectral interferences and no need for corrections for HfO+ as in ICP–MS [23].
4. Application of the method to spiked soil samples Table 5 – Recovery of PtCl4 2− and PtCl6 2− (after reduction with SnCl2 ) in the form of Pt(II)-DEDTC complexes eluted with MIBK from Dowex 1 × 10 column Species Input quantity Determined quantity Recovery [ng] [ng] (n = 3) [%] PtCl4 2− PtCl6 2−
250 250
236 ± 9 140 ± 7
94 ± 4 56 ± 3
The ion-exchange procedure was applied for the separation and preconcentration of PtCl4 2− and PtCl6 2− , extracted from soil samples spiked with converter material and from soil samples previously spiked with standard solutions of these ions (Table 3). This is a part of a long-term study on the conversion, oxidation, solubilization and immobilization of platinum group elements emitted in various forms (mainly from cat-
124
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 6 – Recovery of PtCl4 2− in the form of Pt(II)-DEDTC complexes eluted with MIBK and PtCl6 2− eluted with thiourea from Dowex 1 × 10 column, measured by ICP–AES and GFAAS Species
Input quantity [ng]
ICP–AES
GFAAS
Determined quantity [ng] (n = 3) PtCl4 2− PtCl6 2−
Recovery [%]
254 ± 8 236 ± 11
250 250
Determined quantity [ng] (n = 3)
102 ± 3 94 ± 4
264 ± 13 239 ± 11
Recovery [%] 106 ± 5 96 ± 4
The GFAAS measurements are before evaporation of the eluent.
alytic converters) to the environment. Therefore the results except the applicability of the proposed method give information on the probable processes that have taken place in the respective spiked soil. The results (mean of five replicates) from the analyses of soils spiked with crushed catalytic converter are presented in Table 7. These data show that oxidation of metallic platinum occurs and soluble Pt species are formed after a comparatively short period of time (2 months) in contact with the soil. The percentage of Pt(II) and Pt(IV) in the neutral soil are almost equal in contrast to the alkaline soil where the highest recovery for Pt(II) was achieved. The percentage of Pt(IV) is more than twice lower compared to Pt(II) in this soil type and lower compared to both Pt species in the neutral soil. It is worth mentioning that although the recovery for the investigated species differs significantly the total percentage of the investigated species is equal in both soils. This result might mean that the rate of dissolution of Pt from the autocatalytic converter is a process which does not depend on the characteristics of the investigated type of soils but on the contrary the oxidation state and degree of complexation are strongly dependent on soil characteristics as pH, redox potential, concentration of ligands, etc. This result is in good agreement with earlier investigations indicating that the transfer of Pt from soil to plant depends on soil characteristics [15]. Only a few speciation data exist, based on the standard solutions [22] or road dust samples where the concentration of single platinum species in road dust extracts was below the detection limit of the developed speciation method [23]. The present results are in good agreement with Nischwitz et al. who reported that the extraction of unspiked road dust samples with different extracting agents as HCl, Tris, EDTA, etc. yielded only low extracted amounts of less than 5% of total Pt due to the known fact that the major part of the Pt in road dust is presented in metallic form
[24]. Alt et al. reported 2.5–6.9% soluble platinum [42] and Lustig et al. found 3.9% [43], whereas Hill and Mayer established 10% to be water soluble in tunnel dust samples [44]. The reason why the street dust–Pt is oxidized more efficiently can be related to its nanocrystalline pore size and ultrafine dispersion of Pt in this matrix, which is in good agreement with other literature data [4,43,45,46]. Another reason can be the statement, previously reported, that the transformation of autocatalyst-emitted platinum into bioavailable species is a long-term process (the model samples are prepared 2 months before performing the experiment) [4,45]. A third possibility is that after oxidation, insoluble Pt species in the humic soil are formed [43,45]. An evidence for the latter is the behavior of Pt(II) and Pt(IV), spiked in the same type of soils and the results are presented in Table 8. It is obvious that there is a great difference in the extracted quantities of Pt(II) and Pt(IV) between the investigated types of soil. Similar to the soil samples spiked with converter material the percentage of soluble species of platinum does not differ significantly: 6.5% (Pt II) and 5.0% (Pt IV) in the neutral soil and the highest value is found for Pt(II) 22.3% in the alkaline soil. The percentage for Pt(IV) is again twice lower—12.6%. This result might be related to the organic carbon content in the soil samples—3.71% in the brown forest soil and 0.31% in the carbonic chernozem. The data indicate that a significant part of Pt-species formed from the originally water-soluble chloroplatinates (II, IV) is not soluble, not even in 1 M HCl. K2 PtCl4 and Na2 PtCl6 .6H2 O showed strong adsorption in the soil by recomplexation into water insoluble species. It seems to be possible that either insoluble platinum–humic acid complexes are formed or an adsorption onto the surface of clay minerals has taken place, as already described in the literature [43]. These data confirm the results of Nischwitz et al. who studied the behavior of Pt(II) and Pt(IV) in spiked road dust samples and reported that less than 60% of PtCl4 2− was
Table 7 – Extracted quantity and recovery of PtCl4 2− and PtCl6 2− from soil samples spiked with converter material Fraction
Soil type Brown forest [pH 7.2] Pt [g g−1 ] (n = 5)
2−
PtCl4 PtCl6 2−
1.19 ± 0.04 1.24 ± 0.04
Recovery [%] 1.08 ± 0.04 1.13 ± 0.04
Carbonic chernozem [pH 8.0] Pt [g g−1 ] (n = 5) 0.78 ± 0.02 0.34 ± 0.01
Recovery [%] 1.56 ± 0.04 0.68 ± 0.02
PtCl4 2− in the form of Pt(II)-DEDTC complexes was eluted with MIBK and PtCl6 2− was eluted with thiourea from Dowex 1 × 10 column. The recoveries are calculated in respect to the amount of Pt contained in the added converter material (Table 3).
125
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 8 – Extracted quantity and recovery of PtCl4 2− and PtCl6 2− from soil samples spiked with inorganic platinum species Fraction
Soil type Brown forest [pH 7.2] Pt [g g−1 ] (n = 5) 1.81 ± 0.05 1.05 ± 0.03
2−
PtCl4 PtCl6 2−
Carbonic chernozem [pH 8.0]
Recovery [%] 6.5 ± 0.2 5.0 ± 0.1
Pt [g g−1 ] (n = 5) 1.56 ± 0.05 1.13 ± 0.03
Recovery [%] 22.3 ± 0.71 12.6 ± 0.33
PtCl4 2− in the form of Pt(II)-DEDTC complexes was eluted with MIBK and PtCl6 2− was eluted with thiourea from Dowex 1 × 10 column. Recoveries are calculated in respect to the added amounts of the respective species (Table 3).
extracted and the spiked PtCl6 2− was totally transformed or retained by the matrix [24]. However, in the present study up to 12.6% of PtCl6 2− is recovered from the soils. This is one more strong indication that the behavior of emitted Pt is strongly dependent on the soil type and time of residence in the soil.
5.
Conclusions
A method for isolation and preconcentration of Pt and separation of Pt(II) and Pt(IV) is proposed. It is based on the different kinetics of Pt compounds using Dowex 1 × 10 anion-exchange resin and selective eluting agents. The recoveries of Pt species amount to (102 ± 3)% for Pt(II) and (94 ± 4)% for Pt(IV) at precision 5% and detection limit (3SD) of 15 ng mL−1 . Measurements may be performed by both ICP–AES and GFAAS. The application of the method to soil samples spiked with Pt species indicates difference in the behavior of Pt species in dependence on soil type.
references
[1] R. Barefoot, Environ. Sci. Technol. 31 (1997) 309. [2] T. Hees, B. Wenclawiak, S. Lustig, P. Schramer, M. Schwarzer, M. Schuster, D. Verstraete, R. Dams, D. Helmers, ESPR-Environ. Sci. Pollut. Res. 5 (1998) 105. [3] F. Zereini, B. Skerstupp, F. Alt, E. Helmers, H. Urban, Sci. Total Environ. 206 (1997) 137. [4] S. Lustig, Platinum in the environment. Car-catalyst emitted platinum: transformation behavior in soil and platinum accumulation in plants, Dissertation, Herbert Utz Verlag Wissenschaft, Munchen, 7, 1997. [5] M. Balcerzak, Analyst 122 (1997) 67R. [6] M.E. Farago, P. Kavanagh, R. Blanks, J. Kelly, G. Kazantzis, I. Thornton, P.R. Simpson, J.M. Cook, H.T. Delves, G.M. Hall, Analyst 123 (1998) 451. [7] K. Pyrzynska, J. Environ. Monit. 2 (2000) 99N. ¨ [8] E. Helmers, K. Kummerer, ESPR-Environ. Sci. Pollut. Res. 6 (1999) 29. ¨ [9] K. Kummerer, E. Helmers, P. Hubner, G. Mascart, M. Milandri, F. Reinthaler, M. Zwakenberg, Sci. Total Environ. 225 (1999) 155. [10] K. Ravindra, L. Bencs, R.V. Grieken, Sci. Total Environ. 318 (2004) 1. [11] R.R. Barefoot, Trends Anal. Chem. 18 (1999) 702. [12] L. Bencs, K. Ravindra, R.V. Grieken, Spectrochim. Acta, Part B 58 (2003) 1723.
[13] K.H. Ek, G.M. Morrison, S. Rauch, Sci. Total Envoron. 334 (2004) 21. [14] R. Djingova, H. Heidenreich, P. Kovacheva, B. Markert, Anal. Chim. Acta 489 (2003) 245. [15] R. Djingova, P. Kovacheva, G. Wagner, B. Markert, Sci. Total Environ. 308 (2003) 235. ¨ [16] G. Kollensperger, S. Hann, G. Stingeder, J. Anal. At. Spectrom. 15 (2000) 1553. [17] M.B. Gomez, M.M. Gomez, M.A. Palacios, J. Anal. At. Spectrom. 18 (2003) 80. [18] J.C. Ely, C.R. Neal, C.F. Kulpa, M.A. Schneegurt, J.A. Seidler, J. Jain, Environ. Sci. Technol. 35 (2001) 3816. [19] K. Kanitsar, G. Koellensperger, S. Hann, A. Limbeck, H. Puxbaum, G. Stingeder, J. Anal. At. Spectrom. 18 (2003) 239. [20] S. Artelt, O. Creutzenberg, H. Koch, K. Levsen, D. Nachtigall, ¨ ¨ U. Heinrich, T. Ruhle, R. Schogl, Sci. Total Environ. 228 (1999) 219. [21] M. Moldovan, S. Rauch, M. Gomez, M.A. Palacios, G. Morrison, Water Res. 35 (2001) 4175. ¨ [22] D. Nachtigall, S. Artelt, G. Wunsch, J. Chromatogr. A 775 (1997) 197. [23] V. Nischwitz, B. Michalke, A. Kettrup, J. Chromatogr. A 1016 (2003) 223. [24] V. Nischwitz, B. Michalke, A. Kettrup, Anal. Chim. Acta 521 (2004) 87. [25] S. Rauch, G.M. Morrison, Sci. Total Environ. 235 (1999) 261. [26] S. Rauch, M. Paulsson, M. Wilewska, H. Blanck, G.M. Morrison, Arch. Environ. Contam. Toxicol. 47 (2004) 290. ¨ [27] T.J. Einhauser, M. Galanski, B.K. Keppler, J. Anal. At. Spectrom. 11 (1996) 747. [28] W. Zhong, Q. Zhang, Y. Yan, S. Yue, B. Zhang, W. Tang, J. Inorg. Biochem. 66 (1997) 179. [29] B. Michalke, P. Schramel, J. Chromatogr. A 750 (1996) 51. ¨ [30] J. Messerschmidt, F. Alt, G. Tolg, Anal. Chim. Acta 291 (1994) 161. [31] F. Alt, J. Messerschmidt, G. Weber, Anal. Chim. Acta 359 (1998) 65. [32] P. Kovacheva, R. Djingova, Anal. Chim. Acta 464 (2002) 7. [33] C.И. иhcбyрг, Н.Aл Eзeрcкая, И.В. Прокофьeва, Н.В. Фeдорehко, В.И. Шлehcкая, Н.К. Бeльcкий, Aкадeмия Наyк, CCCР, Издаteльctво “Наyка”, Моcква (1972) 70. [34] B. Godlewska-Zyłkiewicz, Microchim. Acta 147 (2004) 189. [35] M. Balcerzak, Analysis 22 (1994) 353. [36] Z. Marczenko, S. Kus, M. Mojski, Talanta 31 (1984) 959. [37] D.E. Schwab, J. Rund, Inorg. Chem. 11 (1972) 499. [38] A. Hulanicki, Talanta 14 (1967) 1371. [39] B.J. Mueller, R.J. Lovett, Anal. Chem. 57 (1985) 2693. [40] J.T. Pyle, W.D. Jacobs, Anal. Chem. 36 (1964) 1796. [41] S.F. Wang, C.M. Wai, J. Chromatogr. Sci. 32 (1994) 506.
126
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
¨ [42] F. Alt, A. Bambauer, K. Hoppstock, B. Mergler, G. Tolg, Fresenius J. Anal. Chem. 346 (1993) 693. [43] S. Lustig, S. Zang, B. Michalke, P. Schramel, W. Beck, Sci. Total Environ. 188 (1996) 195. [44] R.F. Hill, W.J. Mayer, JEEE Trans. Nucl. Sci. 24 (1977) 2549.
[45] S. Lustig, S. Zang, W. Beck, P. Schramel, Mickrochim. Acta 129 (1998) 189. [46] D. Nachtigall, H. Kock, S. Artelt, K. Levsen, G. Wunsch, T. ¨ Ruhle, R. Schlogl, Fresenius J. Anal. Chem. 354 (1996) 742.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
Speciation of inorganic platinum–chloride complexes in spiked environmental samples by SPE and ICP–AES Valentina Ljubomirova ∗ , Rumyana Djingova Faculty of Chemistry, University of Sofia, 1 J. Bouchier Boulevard, 1164 Sofia, Bulgaria
a r t i c l e
i n f o
a b s t r a c t
Article history:
A method for the separation of tetrachloroplatinate PtCl4 2− , and hexachloroplatinate PtCl6 2− ,
Received 6 November 2007
by solid-phase extraction, using a Dowex 1 × 10 anion exchange resin is proposed. The
Received in revised form
sequential elution and separation of PtCl4 2− , and PtCl6 2− is achieved using selective com-
18 March 2008
plexing agents. The eluates, containing Pt(II) and Pt(IV) were analyzed by inductively coupled
Accepted 19 March 2008
plasma–atomic emission spectroscopy (ICP–AES). Recoveries of 102% for PtCl4 2− and 94% for
Published on line 27 March 2008
PtCl6 2− and detection limit of 15 ng g−1 were achieved. Using this method determination of Pt(II) and Pt(IV) in soil samples, spiked with different platinum species was performed. The
Keywords:
comparison with GFAAS determination showed a very good agreement.
Speciation
© 2008 Elsevier B.V. All rights reserved.
Tetrachloroplatinate Hexachloroplatinate Anoin-exchange ICP–AES GFAAS
1.
Introduction
The determination of total Pt contents of environmental and biomedical matrices has gained a considerable interest within the last decade, which is due to two main applications [1–8]. On the one hand, Pt compounds like cisplatin and carboplatin are used as anti-cancer drugs [9]. On the other hand, Pt containing vehicle exhaust catalyst (VECs) are introduced in the middle 1970s (USA-1975, Germany-1984, EU-1993). Platinum has been used as a catalytic converter material for the refinement of automobile exhaust gases [10–18]. The clinical application is characterized by relatively high concentrations of Pt species (up to the low mg L−1 range in whole blood of cancer patients) with a defined and locally limited distribution [9]. The emissions from VECs show exactly the opposite behavior. They occur in trace concentrations (in the low g kg−1 range in road dust) and are distributed nearly ubiquitously in the
∗
urbanized areas [3]. Moreover, Pt from VECs is emitted in the form of fine suspended particulate matter and therefore might be inhaled and accumulated by living organisms [19–21]. The high biochemical activity of some Pt species, e.g. the cytotoxicity and mutagenicity of cisplatin or the sensitizing effect of hexachloroplatinate bears the risk of significant negative effects on human health after chronic exposure even to trace amounts of Pt species. A detailed review on the health effects of Platinum Group Elements (PGE) was recently published by Ravindra et al. [10]. However, the (eco)toxicity and bioavailability of the emitted Pt are strongly dependent on oxidation state and the binding forms of Pt in the environmental matrix, i.e. the Pt speciation [22–24]. For example, elemental Pt and platinum dioxide are rather insoluble under environmental conditions while chloro–Pt complexes and further halogenated Pt compounds are well soluble in water and toxic due to their high allergenic
Corresponding author. Tel.: +359 88 5930 751; fax: +359 2 9625 438. E-mail addresses: valentina [email protected] (V. Ljubomirova), [email protected]fia.bg (R. Djingova). 0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2008.03.037
120
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
potential which is related to the halogen ligands and to the negative charge of these complexes [10]. A recent study concluded that platinum uptake mechanism depends on the form of platinum present in the environment: Pt(IV) being more likely to be taken up at a higher rate than Pt(II) by freshwater isopod Asellus aquaticus [25]. Another study with freshwater periphyton communities showed that the uptake did not depend on the oxidation state, Pt(II) exhibited higher toxicity to them compared with Pt(IV), perhaps because of the higher capacity of Pt(II) to bind to amino-acids and proteins [26]. The order of relative toxicity, determined by visual appraisal and in terms of oxidation states for different plant species, was Pt2+ > Pd2+ > Ru3+ ≈ Ru2+ ≈ Ir3+ > Pt4+ ≈ Os4+ Rh3+ . It was concluded that the toxic effects of Pt substantially depend on the type of plant species treated [10]. Studies of human tissue have shown that only Pt(II) binds to proteins in human blood, but Pt(IV) can be reduced to Pt(II) before binding [22,27,28]. All the above topics need further investigation to reach a better understanding of the behavior of Pt in the environment. Although the knowledge of Pt cycle in the environment is of great importance, studies on Pt speciation are rarely reported in the literature. A procedure for the analysis of several platinum species such as the anionic platinum–chloro complexes and some of their hydrolysis products based on ion chromatography was presented [22]. A modification of the chromatographic conditions was proposed by Nischwitz et al. to realize an on-line HPLC–ICP–MS hyphenation for the speciation of Pt(II) and Pt(IV) [23]. The method was validated with aqueous standard solutions as well as spiked road dust extracts with respect to an application for the characterization of Pt species in road dusts. The stability of the model species was investigated in mixtures with several extractants under variation of the pH as well as the concentration of the complexing agents and buffers respectively. Additionally the effect of energy supply by refluxing, sonication and microwave exposure was studied. The optimum conditions were applied for the extraction of platinum species from spiked and unspiked road dust samples [24]. In spiked samples PtCl6 2− was not recovered while in unspiked samples less than 5% from total platinum was extracted and the detection limit was not low enough to detect any soluble platinum complexes. Michalke and Schramel developed an on-line CE–ICP–MS method [29]. Messerschmidt et al. [30] and Alt et al. [31] applied gel permeation chromatography and preparative isotachophoresis combined with adsorptive voltametric measurements to platinum speciation in grass. All the described methods use hyphenated techniques for separation and quantification of Pt species. The aim of the present paper is to propose a method for ICP–AES determination of water-soluble species of Pt (PtCl4 2− and PtCl6 2− ) in environmental samples using anion-exchange procedure.
2.
Experimental
2.1.
Reagents and materials
(NH4 )2 PtCl4 (99%, Aldrich) and K2 PtCl6 (99%, Merck) were used for the preparation of stock solutions of PtCl4 2− and PtCl6 2−
(1000 g mL−1 ) in 1 M HCl. Diluted platinum metal solutions were daily freshly prepared in 1 M HCl. HCl (37%, p.a., Merck) and HNO3 (65%, p.a., Merck) were purified by sub-boiling distillation and stored in quartz vessel. The eluent solutions 2,2 -bipyridyl (p.a., Fluka), sodium-diethyldithiocarbamate (NaDEDTC) (p.a., Fluka) and thiourea (p.a., Fluka) were freshly made by dissolving 2.34 g 2,2 -bipyridyl, 3.38 g NaDEDTC, and 0.39 g of thiourea in 0.1 M HCl in a volume of 50 mL. The reducing solution of SnCl2 (99%, Fluka) was prepared by dissolving 2.84 g in 50 mL 3 M HCl. The elution of Pt-DEDTC complexes was performed with methylisobuthylketon (MIBK) (99%, Merck). Dowex 1 × 10 resin (Fluka) 100–200 mesh (ionic form: Cl− ) was used in the experiment. The resin was packed in a 1 cm × 3 cm glass column containing a glass frit at the bottom and a socket at the top. The resin was prepared by soaking in 1 M HCl overnight. For each ion-exchange experiment, the resin was conditioned by passing 100 mL 1 M HCl and next heated to 60 ◦ C. All the experiments were carried out at a flow rate of 3–4 mL min−1 .
2.2.
Instrumentation
Metal determination in the soil extracts and digests were carried out by ICP–AES using Spektroflame D (Spektro Analytical Instruments, Kleve, Germany) spectrometer. Operating conditions for the instrument are presented in Table 1-1. Spectral line used for Pt determination was according to [32]. The calibration of the spectrometer was done using freshly prepared solutions of Pt in concentration range 0.01–10 g mL−1 . Since all eluates were evaporated to incipient dryness and dissolved in a mixture of 2.5% HCl and 1.3% HNO3 the standard solutions for calibration were prepared the same way. Thus probable influence of the eluates on the signal was eliminated. The calibration curve was characterized by R2 = 99.2%. The residual standard deviation was about 2.3%. Calibration was checked twice daily with freshly prepared standard solutions. Previously the lack of matrix elements in the samples after ion-exchange has been proven [32] therefore the detection limit was determined as 3SD of the blank signal and amounts to 15 ng mL−1 . For evaluation the potential losses during the multiple evaporations of the eluates graphite furnace atomic absorption spectrometry of the organic phases before evaporation was used. The measurements were carried out on a PerkinElmer (Norwalk, CT, USA) Zeeman 3030 spectrometer with an HGA-600 graphite furnace (operating conditions given in Table 1-2).
Table 1-1 – Instrument operating parameters used for ICP–AES determination of Pt (Spectroflame D) Instrument Plasma power Cooling gas flow Auxiliary gas flow Nebuliser gas flow Nebuliser Sample uptake Wavelength [nm]
Spectroflame D 1200 W 14 L min−1 1 L min−1 0.8 L min−1 Cross-flow 2.5 mL min−1 214.423
121
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 1-2 – Instrumental operation conditions for PerkinElmer (Norwalk, CT, USA) Zeeman 3030 spectrometer with an HGA-600 graphite furnace Wavelength Slit with Signal type Measurement Dilution Type Pretreatment temperature Atomization temperature Lamp current
2.3.
265.9 nm 0.7 nm AA-BG Peak area 0.2% HNO3 Pyrolytic 1300 ◦ C (15, 10) 2500 ◦ C (0, 3) 7 mA
Description of the soil samples and analysis
Two soils: carbonic chernozem, typical for North Bulgaria and brown forest soil, typical for the Rhodopi mountains region were chosen for the experiment. Sampling was performed according to ISO 10381-1:2002 and ISO 10381-2:2002 in regions not directly affected by industrial or transport pollution. Sample preparation included air drying, sieving through 2-mm PTFE sieve, grinding and homogenization in a Teflon ball mill (ISO 11464:2006). pH of the original soils was determined according to ISO 10390:2005 and CEC according to ISO 23470:2007. After aqua regia digestion (ISO 11466:1995) the soil extracts were analyzed by ICP–AES at operating conditions shown in Table 1-1 and using the most intensive emission lines of the respective elements. The results are presented in Table 2.
2.4.
Preparation of spiked soil samples
Two kilograms of each type of soil were spiked with Pt catalytic converter. The body of a used catalytic converter (monolith)
was crushed and milled in a PTFE ball mill. The concentration of Pt in the converter material was determined after aqua regia digestion and ICP–AES measurement (instrumental parameters given in Kovacheva and Djingova) [32]. Ten grams of the autocatalyst were slurred in 50 mL aqua regia and heated in a sand bath for 2 h. The sample was filtered and diluted to 50 mL with bidistilled water and ICP–AES measurement was performed. The concentration of Pt was 750 g g−1 converter material. Two kilograms of each type of soil were mixed with the converter material (300 g were added to the brown forest and 140 g to the carbonic chernozem) and well homogenized. Another series of the two types of soil samples (2 kg) were spiked with different volumes of the stock standard solutions of PtCl4 2− and PtCl6 2− (1000 g mL−1 ) as follows: 56 mL and 14 mL of PtCl4 2− to the brown forest and carbonic chernozem and 42 mL and 18 mL of PtCl6 2− to the brown forest and carbonic chernozem soil, respectively (final concentrations given in Table 3) and well homogenized. The spiked soil samples, placed in flowerpots were watered regularly to keep the normal soil moisture. Two months later aliquots of 50 g of each soil samples were taken out, air dried and used for the experiments. The digestion of the soil samples for total Pt determination was performed with aqua regia digestion (as already described for the catalytic converter). In order to check the quantitative recovery of Pt total digestion was performed according to Djingova et al. [14] and the results are presented in Table 3. There is a very good agreement between introduced and determined total Pt in the samples.
2.5.
Preparation of soil extracts
In order to achieve quantitative recovery of the inorganic chloro complexes of Pt (PtCl4 2− , PtCl6 2− ) without species inter-
Table 2 – Characterization of the soil samples Element
Soil Brown forest
%
K Mg Fe
[g/g]
Cu Zn Na Cd Pb Sb As Sn Sr Y Ce Zr La Nd
Carbon [%]
Total C Inorganic C pH
(meqv 100 g−1 soil)
CEC
0.399 ± 0.001 0.816 ± 0.008 2.66 ± 0.03 47.8 ± 0.3 371 ± 12 166 ± 5 5.2 ± 0.2 37.6 ± 0.2 29.8 ± 0.6 28.2 ± 0.9 14.4 ± 0.1 73.8 ± 0.5 23.6 ± 0.2 89 ± 1 54 ± 2 40 ± 1 39 ± 1 3.735 0.029 7.2 31.1
Carbonic chernozem 0.126 ± 0.001 0.8433 ± 0.008 1.33 ± 0.01 68.5 ± 0.6 44.3 ± 0.4 65.75 ± 0.04 2.46 ± 0.01 22.6 ± 0.4 19.0 ± 0.1 24.8 ± 0.1 9.3 ± 0.3 56.6 ± 0.6 10.5 ± 0.3 42 ± 1 37 ± 2 17 ± 1 22 ± 1 1.147 0.845 8.0 23.4
122
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 3 – Concentration of platinum in soil samples spiked with different platinum species determined by ICP–AES Spiked Pt species
Added quantity of Pt [g g−1 ]
Brown forest (pH 7.2)
Pt-catalyst K2 PtCl4 ; K2 PtCl6
110 28; 21
114 ± 4 50 ± 2
Carbonic chernozem (pH 8.0)
Pt-catalyst K2 PtCl4 ; K2 PtCl6
50 7; 9
51 ± 2 16 ± 1
Soil type
conversion, an extraction of 3 g of soil samples with 20 mL 1 M HCl was performed by shaking for 2 h at 22 ± 3 ◦ C (two series of three replicates each). The extract was centrifuged at 1600 × g for 10 min. One of the series was used for direct determination of Pt and the second one for performing the procedure for concentration of Pt and separation of PtCl4 2− and PtCl6 2− .
3.
Results and discussions
While choosing the elution conditions two aspects have to be considered: (i) the platinum–chloro complexes should be efficiently eluted and (ii) the native species of the element should not be altered. The strong anion exchangers have a great selectivity to chloro complexes of Pt. Some of the platinum complexes are adsorbed too strongly to be eluted from the stationary phase. The use of eluents at elevated temperature, as well as several circulations of the eluent solution (e.g. 10 times thiourea, 60 ◦ C) for quantitative recovery of the studied metal in a small final volume has been proposed in a previous study [32]. In the present study the same approach was used for the quantitative elution of platinum species from the Dowex 1 × 10 column. To prevent the possible processes of hydrolysis and aquatation, the acidity range was chosen according to references [22,23,33] and was from 0.1 M HCl to 5 M HCl. According to Nachtigall et al. [22] and Nischwitz et al. [23], who investigated the long-term species stability in HCl media it can be concluded that 1 M HCl is the most suitable media to prevent reduction–oxidation processes.
3.1.
Ion-exchange procedure
The initial approach that has been chosen during the investigations was to find an appropriate complexing agent for elution of Pt(II) from the ion-exchange column and afterwards to reduce Pt(IV) remaining on the column with 10−3 M SnCl2 in 3 M HCl and elute it with the same complexing agent. [34–36].
3.2.
Determined quantity by ICP–AES [g g−1 ]
obtained using 5 M HCl as rinsing solution, 13.56% and 42.44%, and the highest ones using 1 M HCl, 54.68% and 58.30%, respectively. Similar results are obtained using 0.1 M HCl 50.12% and 52.68%. The results from the experiment show that the complexation between 2,2 bipyridyl and Pt(II) and Pt(IV) is slow and this reagent cannot be used for elution of Pt from the resin.
3.3.
Experiment with NaDEDTC
Another complexing agent, which reacts predominantly with Pt(II) is NaDEDTC. NaDEDTC forms neutral complexes with PtCl4 2− in acid solution as follows: Pt2+ + 2DEDTC− → Pt(DEDTC)2 . The formed complexes can be eluted with organic solvents. Pt(IV) reacts incompletely with possible reduction to the bivalent state [38]. Pt(IV)–dithiocarbamate complexes have a number of metal to ligand ratios, 1:1, 2:3, 1:2 and 1:3 and may contain chloride ions [38]. These complexes can be ionized in solution and the elution with apolar organic solvent is impossible [39]. NaDEDTC was used for solvent extraction of Pt(II) [40] and HPLC determination [39,41].
3.3.1. Elution of standard solutions of PtCl4 2− and PtCl6 2− with NaDEDTC To evaluate the eluted PtCl4 2− and PtCl6 2− complexes with NaDEDTC, two separate experiments were performed with standard solutions containing 30 ng mL−1 PtCl4 2− and 30 ng mL−1 PtCl6 2− in 1 M HCl. Ten milliliters of the standard solutions were passed through the column. The resin was rinsed with 10 mL 1 M HCl, after 3 mL 0.3 M NaDEDTC in water were added, circulating 4 times through the resin. The formed Pt(II)-DEDTC complexes were eluted with 10 mL MIBK. The elution profile of Pt(II) with MIBK is presented in Fig. 1.
Experiment with 2,2 -bipyridyl
It is known that 2,2-bipyridyl reacts with Pt(II) to produce Pt(bipy)Cl2 . In acidic solution 2,2-bipyridyl reacts also with Pt(H2 O)Cl3 − [37]. In our experiments first elution of PtCl4 2− was done with 5 mL 0.3 M 2,2-bipyridyl circulated 10 times at 60 ◦ C. Afterwards the inert Pt(IV) remaining on the column was reduced to Pt(II) by passing 2 mL 10−3 M SnCl2 in 3 M HCl and elution with bipyridyl was performed as before. Additionally the influence of the pH on the elution process was investigated by rinsing the resin with HCl with molarity 0.1, 1, 2, 4 and 5. The results from the measurement show extremely low recoveries. The lowest recoveries for PtCl4 2− and PtCl6 2− are
Fig. 1 – Elution profile of Pt(II) with MIBK.
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 4 – Recovery of PtCl4 2− and PtCl6 2− in the form of Pt(II)-DEDTC complexes eluted with MIBK from Dowex 1 × 10 column. Pt species
Input quantity [ng]
PtCl4 2− PtCl6 2−
300 300
Eluted quantity [ng] (n = 3) 293 ± 2 15.7 ± 0. 2
Recovery [%] 97.7 ± 0.7 5.2 ± 0.1
The same experiment was repeated with the standard solution of PtCl6 2− . MIBK was evaporated on a sand bath, 1 mL mixture of 37% HCl and 0.3 mL 65% HNO3 was added and evaporation to incipient dryness was done. The residue was dissolved in 1 mL mixture of 2.5% HCl and 1.3% HNO3 . The solution was diluted to 10 mL with bidistilled water and ICP–AES measurement was performed. Table 4 presents the results from the elution (mean of three replicates). The results from this experiment show that NaDEDTC reacts predominantly with PtCl4 2− (recovery 97.7%) and can be used for selective elution and separation of PtCl4 2− and PtCl6 2− .
3.3.2. Elution of complex standard solutions of PtCl4 2− and PtCl6 2− with NaDEDTC The described experiment with DEDTC was repeated with mixed standard solution of PtCl4 2− and PtCl6 2− in 1 M HCl. Five milliliters of this solution containing 50 ng mL−1 each of the species were passed through the resin. The resin was rinsed with 10 mL 1 M HCl and 3 mL 0.3 M NaDEDTC were circulating 4 times through the resin. The Pt(II)-DEDTC complexes were eluted with 10 mL MIBK. Afterwards reduction with SnCl2 of Pt(IV) to more labile Pt(II) ions was performed. Two milliliters of 10−3 M SnCl2 in 3 M HCl were passed through the column. Next 3 mL 0.3 M NaDEDTC were circulated 4 times through the resin. The elution of Pt(II)-DEDTC complexes thus formed was performed with 10 mL MIBK. The eluates, containing MIBK were evaporated on a sand bath, 1 mL mixture of 37% HCl and 0.3 mL 65% HNO3 were added and evaporation to incipient dryness was done. The residue was dissolved in 1 mL mixture of 2.5% HCl and 1.3% HNO3 . The solution was diluted to 5 mL with bidistilled water and ICP–AES measurement was performed. The data (mean of three replicates) are presented in Table 5. The low recovery for PtCl6 2− (56%) shows that the reduction of Pt(IV) with SnCl2 on the column is obviously not complete and this procedure cannot be used for quantitative separation of Pt(II) and Pt(IV).
123
3.3.3. Method for separation and determination of PtCl4 2− and PtCl6 2− The results from the above experiments prove that the elution with NaDEDTC is a successful approach for separation of Pt(II) however the reduction of Pt(IV) with SnCl2 is incomplete. Therefore change of the eluent for the complete recovery of Pt(IV) is necessary. Earlier in our laboratory a method for determination of Pt and Pd has been developed using ion-exchange concentration and elution with circulated thiourea at 60 ◦ C [32]. This approach was finally used to elute Pt(IV) from the column. Five milliliters of the complex solution, containing 50 ng mL−1 each of the species PtCl4 2− and PtCl6 2− (acidity 1 M HCl) was passed through the column. The resin was rinsed with 10 mL 1 M HCl and 3 mL 0.3 M NaDEDTC were circulated 4 times through the resin at 60 ◦ C. The Pt(II)-DEDTC complex was eluted with 10 mL MIBK. The eluate was evaporated in a sand bath and 1 mL mixture of 37% HCl and 0.3 mL 65% HNO3 were added. Evaporation to incipient dryness was performed and the residue was dissolved in 1 mL mixture of 2.5% HCl and 1.3% HNO3 . The solution was diluted to 5 mL with bidestilled water and ICP–AES measurement was performed. Afterwards elution of Pt(IV) was carried out using 4 mL 0.1 M thiourea in 0.1 M HCl circulated 10 times through the column at 60 ◦ C and ICP–AES measurement was performed [32]. Aliquots from the organic fractions (MIBK and thiourea) were measured by GFAAS for comparison and to control the effect from the evaporation steps. The results from the analysis of mixed standard solutions are presented in Table 6. The lower recovery for Pt(IV) 94 ± 4 might be explained by insignificant reduction of Pt(IV) to Pt(II) which recovery sometimes is over 100%. The achieved precision is 5% and the detection limit (3SD) amounts to 15 ng mL−1 . The results from the AAS measurement indicate that there is not a significant loss of Pt during the evaporation step of the organic phases before the ICP–AES measurement. The comparison of the proposed method for concentration and separation of PtCl4 2− and PtCl6 2 to existing ones [22–24] shows quantitative separation and very good recoveries for both species and only 2% of Pt(IV) is reduced to Pt(II) during the procedure. The detection limit is not better than the reported one but the measurements in the present study were done by ICP–AES and not ICP–MS [22–24] generally considered to be a much more sensitive method. An advantage in this respect however may be considered the lack of spectral interferences and no need for corrections for HfO+ as in ICP–MS [23].
4. Application of the method to spiked soil samples Table 5 – Recovery of PtCl4 2− and PtCl6 2− (after reduction with SnCl2 ) in the form of Pt(II)-DEDTC complexes eluted with MIBK from Dowex 1 × 10 column Species Input quantity Determined quantity Recovery [ng] [ng] (n = 3) [%] PtCl4 2− PtCl6 2−
250 250
236 ± 9 140 ± 7
94 ± 4 56 ± 3
The ion-exchange procedure was applied for the separation and preconcentration of PtCl4 2− and PtCl6 2− , extracted from soil samples spiked with converter material and from soil samples previously spiked with standard solutions of these ions (Table 3). This is a part of a long-term study on the conversion, oxidation, solubilization and immobilization of platinum group elements emitted in various forms (mainly from cat-
124
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 6 – Recovery of PtCl4 2− in the form of Pt(II)-DEDTC complexes eluted with MIBK and PtCl6 2− eluted with thiourea from Dowex 1 × 10 column, measured by ICP–AES and GFAAS Species
Input quantity [ng]
ICP–AES
GFAAS
Determined quantity [ng] (n = 3) PtCl4 2− PtCl6 2−
Recovery [%]
254 ± 8 236 ± 11
250 250
Determined quantity [ng] (n = 3)
102 ± 3 94 ± 4
264 ± 13 239 ± 11
Recovery [%] 106 ± 5 96 ± 4
The GFAAS measurements are before evaporation of the eluent.
alytic converters) to the environment. Therefore the results except the applicability of the proposed method give information on the probable processes that have taken place in the respective spiked soil. The results (mean of five replicates) from the analyses of soils spiked with crushed catalytic converter are presented in Table 7. These data show that oxidation of metallic platinum occurs and soluble Pt species are formed after a comparatively short period of time (2 months) in contact with the soil. The percentage of Pt(II) and Pt(IV) in the neutral soil are almost equal in contrast to the alkaline soil where the highest recovery for Pt(II) was achieved. The percentage of Pt(IV) is more than twice lower compared to Pt(II) in this soil type and lower compared to both Pt species in the neutral soil. It is worth mentioning that although the recovery for the investigated species differs significantly the total percentage of the investigated species is equal in both soils. This result might mean that the rate of dissolution of Pt from the autocatalytic converter is a process which does not depend on the characteristics of the investigated type of soils but on the contrary the oxidation state and degree of complexation are strongly dependent on soil characteristics as pH, redox potential, concentration of ligands, etc. This result is in good agreement with earlier investigations indicating that the transfer of Pt from soil to plant depends on soil characteristics [15]. Only a few speciation data exist, based on the standard solutions [22] or road dust samples where the concentration of single platinum species in road dust extracts was below the detection limit of the developed speciation method [23]. The present results are in good agreement with Nischwitz et al. who reported that the extraction of unspiked road dust samples with different extracting agents as HCl, Tris, EDTA, etc. yielded only low extracted amounts of less than 5% of total Pt due to the known fact that the major part of the Pt in road dust is presented in metallic form
[24]. Alt et al. reported 2.5–6.9% soluble platinum [42] and Lustig et al. found 3.9% [43], whereas Hill and Mayer established 10% to be water soluble in tunnel dust samples [44]. The reason why the street dust–Pt is oxidized more efficiently can be related to its nanocrystalline pore size and ultrafine dispersion of Pt in this matrix, which is in good agreement with other literature data [4,43,45,46]. Another reason can be the statement, previously reported, that the transformation of autocatalyst-emitted platinum into bioavailable species is a long-term process (the model samples are prepared 2 months before performing the experiment) [4,45]. A third possibility is that after oxidation, insoluble Pt species in the humic soil are formed [43,45]. An evidence for the latter is the behavior of Pt(II) and Pt(IV), spiked in the same type of soils and the results are presented in Table 8. It is obvious that there is a great difference in the extracted quantities of Pt(II) and Pt(IV) between the investigated types of soil. Similar to the soil samples spiked with converter material the percentage of soluble species of platinum does not differ significantly: 6.5% (Pt II) and 5.0% (Pt IV) in the neutral soil and the highest value is found for Pt(II) 22.3% in the alkaline soil. The percentage for Pt(IV) is again twice lower—12.6%. This result might be related to the organic carbon content in the soil samples—3.71% in the brown forest soil and 0.31% in the carbonic chernozem. The data indicate that a significant part of Pt-species formed from the originally water-soluble chloroplatinates (II, IV) is not soluble, not even in 1 M HCl. K2 PtCl4 and Na2 PtCl6 .6H2 O showed strong adsorption in the soil by recomplexation into water insoluble species. It seems to be possible that either insoluble platinum–humic acid complexes are formed or an adsorption onto the surface of clay minerals has taken place, as already described in the literature [43]. These data confirm the results of Nischwitz et al. who studied the behavior of Pt(II) and Pt(IV) in spiked road dust samples and reported that less than 60% of PtCl4 2− was
Table 7 – Extracted quantity and recovery of PtCl4 2− and PtCl6 2− from soil samples spiked with converter material Fraction
Soil type Brown forest [pH 7.2] Pt [g g−1 ] (n = 5)
2−
PtCl4 PtCl6 2−
1.19 ± 0.04 1.24 ± 0.04
Recovery [%] 1.08 ± 0.04 1.13 ± 0.04
Carbonic chernozem [pH 8.0] Pt [g g−1 ] (n = 5) 0.78 ± 0.02 0.34 ± 0.01
Recovery [%] 1.56 ± 0.04 0.68 ± 0.02
PtCl4 2− in the form of Pt(II)-DEDTC complexes was eluted with MIBK and PtCl6 2− was eluted with thiourea from Dowex 1 × 10 column. The recoveries are calculated in respect to the amount of Pt contained in the added converter material (Table 3).
125
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
Table 8 – Extracted quantity and recovery of PtCl4 2− and PtCl6 2− from soil samples spiked with inorganic platinum species Fraction
Soil type Brown forest [pH 7.2] Pt [g g−1 ] (n = 5) 1.81 ± 0.05 1.05 ± 0.03
2−
PtCl4 PtCl6 2−
Carbonic chernozem [pH 8.0]
Recovery [%] 6.5 ± 0.2 5.0 ± 0.1
Pt [g g−1 ] (n = 5) 1.56 ± 0.05 1.13 ± 0.03
Recovery [%] 22.3 ± 0.71 12.6 ± 0.33
PtCl4 2− in the form of Pt(II)-DEDTC complexes was eluted with MIBK and PtCl6 2− was eluted with thiourea from Dowex 1 × 10 column. Recoveries are calculated in respect to the added amounts of the respective species (Table 3).
extracted and the spiked PtCl6 2− was totally transformed or retained by the matrix [24]. However, in the present study up to 12.6% of PtCl6 2− is recovered from the soils. This is one more strong indication that the behavior of emitted Pt is strongly dependent on the soil type and time of residence in the soil.
5.
Conclusions
A method for isolation and preconcentration of Pt and separation of Pt(II) and Pt(IV) is proposed. It is based on the different kinetics of Pt compounds using Dowex 1 × 10 anion-exchange resin and selective eluting agents. The recoveries of Pt species amount to (102 ± 3)% for Pt(II) and (94 ± 4)% for Pt(IV) at precision 5% and detection limit (3SD) of 15 ng mL−1 . Measurements may be performed by both ICP–AES and GFAAS. The application of the method to soil samples spiked with Pt species indicates difference in the behavior of Pt species in dependence on soil type.
references
[1] R. Barefoot, Environ. Sci. Technol. 31 (1997) 309. [2] T. Hees, B. Wenclawiak, S. Lustig, P. Schramer, M. Schwarzer, M. Schuster, D. Verstraete, R. Dams, D. Helmers, ESPR-Environ. Sci. Pollut. Res. 5 (1998) 105. [3] F. Zereini, B. Skerstupp, F. Alt, E. Helmers, H. Urban, Sci. Total Environ. 206 (1997) 137. [4] S. Lustig, Platinum in the environment. Car-catalyst emitted platinum: transformation behavior in soil and platinum accumulation in plants, Dissertation, Herbert Utz Verlag Wissenschaft, Munchen, 7, 1997. [5] M. Balcerzak, Analyst 122 (1997) 67R. [6] M.E. Farago, P. Kavanagh, R. Blanks, J. Kelly, G. Kazantzis, I. Thornton, P.R. Simpson, J.M. Cook, H.T. Delves, G.M. Hall, Analyst 123 (1998) 451. [7] K. Pyrzynska, J. Environ. Monit. 2 (2000) 99N. ¨ [8] E. Helmers, K. Kummerer, ESPR-Environ. Sci. Pollut. Res. 6 (1999) 29. ¨ [9] K. Kummerer, E. Helmers, P. Hubner, G. Mascart, M. Milandri, F. Reinthaler, M. Zwakenberg, Sci. Total Environ. 225 (1999) 155. [10] K. Ravindra, L. Bencs, R.V. Grieken, Sci. Total Environ. 318 (2004) 1. [11] R.R. Barefoot, Trends Anal. Chem. 18 (1999) 702. [12] L. Bencs, K. Ravindra, R.V. Grieken, Spectrochim. Acta, Part B 58 (2003) 1723.
[13] K.H. Ek, G.M. Morrison, S. Rauch, Sci. Total Envoron. 334 (2004) 21. [14] R. Djingova, H. Heidenreich, P. Kovacheva, B. Markert, Anal. Chim. Acta 489 (2003) 245. [15] R. Djingova, P. Kovacheva, G. Wagner, B. Markert, Sci. Total Environ. 308 (2003) 235. ¨ [16] G. Kollensperger, S. Hann, G. Stingeder, J. Anal. At. Spectrom. 15 (2000) 1553. [17] M.B. Gomez, M.M. Gomez, M.A. Palacios, J. Anal. At. Spectrom. 18 (2003) 80. [18] J.C. Ely, C.R. Neal, C.F. Kulpa, M.A. Schneegurt, J.A. Seidler, J. Jain, Environ. Sci. Technol. 35 (2001) 3816. [19] K. Kanitsar, G. Koellensperger, S. Hann, A. Limbeck, H. Puxbaum, G. Stingeder, J. Anal. At. Spectrom. 18 (2003) 239. [20] S. Artelt, O. Creutzenberg, H. Koch, K. Levsen, D. Nachtigall, ¨ ¨ U. Heinrich, T. Ruhle, R. Schogl, Sci. Total Environ. 228 (1999) 219. [21] M. Moldovan, S. Rauch, M. Gomez, M.A. Palacios, G. Morrison, Water Res. 35 (2001) 4175. ¨ [22] D. Nachtigall, S. Artelt, G. Wunsch, J. Chromatogr. A 775 (1997) 197. [23] V. Nischwitz, B. Michalke, A. Kettrup, J. Chromatogr. A 1016 (2003) 223. [24] V. Nischwitz, B. Michalke, A. Kettrup, Anal. Chim. Acta 521 (2004) 87. [25] S. Rauch, G.M. Morrison, Sci. Total Environ. 235 (1999) 261. [26] S. Rauch, M. Paulsson, M. Wilewska, H. Blanck, G.M. Morrison, Arch. Environ. Contam. Toxicol. 47 (2004) 290. ¨ [27] T.J. Einhauser, M. Galanski, B.K. Keppler, J. Anal. At. Spectrom. 11 (1996) 747. [28] W. Zhong, Q. Zhang, Y. Yan, S. Yue, B. Zhang, W. Tang, J. Inorg. Biochem. 66 (1997) 179. [29] B. Michalke, P. Schramel, J. Chromatogr. A 750 (1996) 51. ¨ [30] J. Messerschmidt, F. Alt, G. Tolg, Anal. Chim. Acta 291 (1994) 161. [31] F. Alt, J. Messerschmidt, G. Weber, Anal. Chim. Acta 359 (1998) 65. [32] P. Kovacheva, R. Djingova, Anal. Chim. Acta 464 (2002) 7. [33] C.И. иhcбyрг, Н.Aл Eзeрcкая, И.В. Прокофьeва, Н.В. Фeдорehко, В.И. Шлehcкая, Н.К. Бeльcкий, Aкадeмия Наyк, CCCР, Издаteльctво “Наyка”, Моcква (1972) 70. [34] B. Godlewska-Zyłkiewicz, Microchim. Acta 147 (2004) 189. [35] M. Balcerzak, Analysis 22 (1994) 353. [36] Z. Marczenko, S. Kus, M. Mojski, Talanta 31 (1984) 959. [37] D.E. Schwab, J. Rund, Inorg. Chem. 11 (1972) 499. [38] A. Hulanicki, Talanta 14 (1967) 1371. [39] B.J. Mueller, R.J. Lovett, Anal. Chem. 57 (1985) 2693. [40] J.T. Pyle, W.D. Jacobs, Anal. Chem. 36 (1964) 1796. [41] S.F. Wang, C.M. Wai, J. Chromatogr. Sci. 32 (1994) 506.
126
a n a l y t i c a c h i m i c a a c t a 6 1 4 ( 2 0 0 8 ) 119–126
¨ [42] F. Alt, A. Bambauer, K. Hoppstock, B. Mergler, G. Tolg, Fresenius J. Anal. Chem. 346 (1993) 693. [43] S. Lustig, S. Zang, B. Michalke, P. Schramel, W. Beck, Sci. Total Environ. 188 (1996) 195. [44] R.F. Hill, W.J. Mayer, JEEE Trans. Nucl. Sci. 24 (1977) 2549.
[45] S. Lustig, S. Zang, W. Beck, P. Schramel, Mickrochim. Acta 129 (1998) 189. [46] D. Nachtigall, H. Kock, S. Artelt, K. Levsen, G. Wunsch, T. ¨ Ruhle, R. Schlogl, Fresenius J. Anal. Chem. 354 (1996) 742.