Hydrazine sulfate and formaldehyde solution. (rain. 37%, stabilised with methanol) were obtained from. Merck and were used without further purification.
Fresenius Z Anal Chem (1989) 335:813- 816
Only for private use
Fresenius Zeitschrilt [fir
© Springer-Verlag1989
Determination of platinum in biotic and environmental materials Part II: A sensitive voltammetric method*, ** K. Hoppstock 1, 2,3 F. Alt 1, K. Cammann 2 and G. Weber 1
1 Institut fiir Spektrochemie und angewandte Spektroskopie (ISAS), Bunsen-Kirchhoff-Strasse 11, D-4600 Dortmund 1, Federal Republic of Germany 2 Anorganisch chemisches Institut der Westf~ilischen Wilhelms Universitfit, Wilhelm-Klemm-Strasse 8, D-4400 Mtinster/Westf., Federal Republic of Germany 3 Present address: Max Planck Institut ffir Metallforschung, Stuttgart, Laboratorium f6r Reinststoffanalytik, Dortmund, Bunsen-Kirchhoff-Strasse 13, D-4600 Dortmund 1, Federal Republic of Germany
Bestimmung yon Platin in biotischen und umweltrelevanten Materialien Teil II: Eine nachweisstarke voltammetrische Methode Summary. A very sensitive procedure for the determination of baseline levels of platinum is presented. A high-pressure digestion step of the sample is followed by the determination via adsorption voltammetry and detection by a catalytic hydrogen wave. Interferences and blank value problems are discussed.
1 Introduction
The interest in the determination of extremely low amounts of platinum in biotic and environmental materials has increased rapidly with the introduction of automobile exhaust catalysts and the administration of platinum-containing drugs in cancer chemotherapy. The catalysts contain a catalytic active coating, mainly consisting of metallic platinum on different carrier materials. Although there are strict requirements in durability [1], the emission of catalyst material caused by abrasion for instance, cannot be avoided. Very few reliable data are available to estimate the emission-rate of noble metals [2] and up to now no research data have been published concerning the "regulated three-way catalyst" [3, 4]. The knowledge about the different platinum compounds (species) emitted is limited as are informations about their microbial biotransformation and their photochemical behaviour etc. On the other hand, it is the chemical species which primary induces the allergene potential and toxicity of this element, so that a wide spreaded investigation deficit is existing in this field. First of all, data about the influences of noble metals require precise and accurate information about their concentrations, even in the range of very few ng/kg. A multistage procedure for the determination of platinum in the gg/kg to * Dedicated to Prof. Dr. G. T61g on the occasion of his 60th birthday ** Part I: Microchim. Acta [Wien] 1988, III, 299-304 Olfprint requests to: K. Hoppstock
g/kg-range with extraction and detection by ET-AAS was presented in an earlier publication [5]. Hodge et al. [6] describe an elaborated procedure to determine platinum by ETAAS after ion-exchange separation, elution and re-extraction onto a single bead of ion-exchanger. In marine samples, a detection limit of about 15 pg is achieved. Radiochemical neutron activation analysis (RNAA), with its low samplethroughput and a determination-limit of 15 ng/kg [7] is only accessible to very few specialised laboratories. Therefore R N A A is not suitable as a routine-method, however it is particularly useful to check other methods. The deposition of platinum onto a glassy carbon electrode followed by anodic stripping voltammetry (DPASV) is not a very sensitive method (detection limit 0.04mg/kg) [8] and is therefore not capable of determining the baseline levels of platinum. Another voltammetric approach [9, 10] is based on the potential supported accumulation of a platinum complex (a condensation product of formaldehyde and hydrazine, so-called "formazone") at the surface of a hanging mercury drop electrode (HMDE). The electrochemically active complex lowers the hydrogen overpotential at the mercury electrode and thus produces a very sensitive catalytic current, which is measured in differential pulse mode. The goal of this present paper is to investigate this method with respect to real samples, especially decomposition solutions of biotic and environmental materials containing platinum in the ng/kg-range.
2 Experimental 2.1 Apparatus
Voltammetric measurements were done using a Metrohm Polarecord E 506 with VA-Timer E 608 and VA-Stand 663 equipped with a multimode working electrode, a glassy carbon auxiliary electrode and an Ag/AgC1/KC1 (3 tool/l) reference electrode. The measurements were carried out in 40 ml quartz glass crucibles (available from W. K. Becher, Mainz, FRG), which were inserted into commercially available polarographic vessels (see Fig. 1). The temperature was kept constant at 20°C. For the digestion, a high pressure asher (HPA, H. Kiirner, Rosenheim, FRG) equipped with
814
~ ¢ , 7 8
Pt-blanks of some quartz glass decomposition vessels with different history following different cleaning procedures T a b l e 2.
1
l' I
1
/
JJ
Vessel 1
6 h vapor cleaning with nitric acid 50 pg RDA, 8 h HNO3 RDA, 8 h HNO3 360 pg no Pt-signal Aqua regia aqua regia 5 h HNO3 no Pt-signal 88 pg RDA rinsed with bidistilled water 18 pg 35 pg
3 b 3
~2.,__._~0[ ¢30 40
Fig. 1. Device for the voltammetric determination of platinum (dimensions in [mini; a polarographic vessel; b quartz glass crucible
T a b l e 1.
HPA-temperature-program
Step 1 Step 2 Step 3 Step 4 Step 5
Vessel 2
Vessel 3
-Q
Temperature [°C]
20 30 30 60 -
20-120 120-120 120- 280 280- 280 280- 20
30 ml quartz glass vessels [15] was used. Up to 400 mg substance can be digested using 2 ml HNO3 and 200 ~tl HC1 (the temperature program is shown in Table 1). A vaporcleaning quartz apparatus was employed for vessel preconditioning. For preparation and storage of platinum standards only vapor-cleaned (HC1 and HzO) quartz glass flasks were used [18]. Other reagents and electrolyte solutions were stored in glass- or polypropylene-bottles.
2.2 Reagents Sulfuric acid (0.36 mol/1 and 1.0 mol/1) was obtained by diluting concentrated acid ( 9 5 - 9 7 % p.a., Merck, Darmstadt, FRG) with bidistilled water. Platinum standard solutions were prepared by diluting 1000 mg/1 Pt-standard (Aldrich No. 20.73-5 for AAS in 10% HC1) with 1 mol/1 sulfuric acid. Hydrazine sulfate and formaldehyde solution (rain. 37%, stabilised with methanol) were obtained from Merck and were used without further purification.
2.3 Stability of the prepared solutions The formaldehyde solution was stable for at least 6 months and the 0.01 mol/1 hydrazine sulfate solution for at least 2 months. In contrast to [17], the supporting electrolyte solution (0.36 tool/1 HzSO 4 containing 3 mmol/1 hydrazine sulfate and 6 mmol/1 formaldehyde) was found to be stable at room temperature for more than two weeks. Platinum standard solutions (1 - 5 gg/1 in 1 mol/1 sulfuric acid, stored in vapor-cleaned quartz glass flasks at room temperature) were prepared freshly every day, although they are stable for more than 14 days.
350 pg aqua regia 5 h HNO3 no Pt-signal
- = not determined RDA = cleaning decomposition within the HPA with 2 ml HNO3 and 0.5 ml HC1 HC1/HNO3 = vapor cleaning with HC1/HNO3 aqua regia = filled with aqua regia and stored for 60 min at ~45°C
3
Time [ m i n ]
100 pg 5 h HNO3 80 pg 14 h HC1
Results
and discussion
A very sensitive determination of platinum is possible using a catalytic hydrogen wave [9, 10]. A catalytic active platinum complex is accumulated on the surface of the H M D E and produces a very sensitive catalytic current at a potential of - 0 . 8 5 V by decreasing the hydrogen overvoltage. In pure solutions this method enables the determination of platinum down to 0.5 pg absolute. However, for the determination of platinum in the ng/kg range in real samples a decomposition procedure is necessary resulting in solutions, which contain an excess of nitric acid and maybe also other interferents. The complexing agent used here is a condensation product of formaldehyde and hydrazine generated in situ in 0.36 tool/1 sulfuric acid. It reacts with Pt(II) to form the 1:2 complex [Pt(NH2 - N = CH2)2] 2+. Platinum(IV) is reduced by hydrazine to Pt(II) and thus reacts similarly [9]. An electrolyte composition of 3 retool/1 hydrazine sulfate and 6 retool/1 formaldehyde in 0.36 tool/1 sulfuric acid at a temperature of 20°C was found to be optimal. The preconcentration potential of - 0 . 6 V was applied to a fresh mercury drop of medium size for 30 s while the solution was stirred with 2000 rpm. Then stirring was stopped and after 10 s the voltammogram was recorded in the DP mode (pulse height: - 5 0 mV) starting at - 0 : 6 V with a scan rate of - 10 mV/s. Despite the extreme trace level of the method no clean-room facilities were needed, because contamination by laboratory air is negligible. On the other hand, the vessels used for decomposition of the samples can be a source of severe contamination. Memory effects are observed, when vessels are used for decomposition of samples with different Pt-content. Simple vapor-cleaning of the vessels or even the special three-step cleaning procedure [5] is not sufficient if samples of ng/kg-level are to be analysed after decomposition of samples containing platinum in the mg/kg-range. Unpredictable blanks, varying over a wide range, are obtained from these vessels even after repeated drastic cleaning operations, such as cleaning decomposition (2 ml nitric acid and 0.5 ml hydrochloric acid using the HPA program). Results of different cleaning operations are shown in Table 2. To enable the work in this low concentration range, which is especially relevant for environmental samples, it is
815 7O 6O 5O l,O 30 2O I0 0
0
5
10
15
20
25
30
35
40 45 c HNO3(mrnoL/I)
Fig. 2. Variation of the background current with the nitric acid concentration Table 3. Platinum concentrations of some materials
Material (origin)
Filter dust [ng] (FhG Hannover) Corn [gg/kg] (TUV Stuttgart) Rye grass, control [ng/kg] (FhG Hannover) Rye grass, treated with automobile exhaust [ng/kg] NBS bovine liver, SRM 1577 [ng/kg] RNAA: 70 + 30 ng/kg [7]
Platinum AdsVA 187 148 80 +
ETAAS
7
214 167 70
320 -t-_ 80
-
1500 _+200
-
100 ± 40
-
necessary to use a vessel set only for these low concentrations. Containers made from synthetic, high-purity quartz glass, so-called "suprasil" (Heraeus Quarzschmelze, Hanau, FRG) proved to be optimal. Contamination of this material by noble metals during the manufacturing process is negligible. The metal-ion diffusion into the material is more slowly compared to normal quartz glass [11 - 13]. Using this material, vapor-cleaning with nitric acid for about 4 - 5 h is sufficient for preconditioning, leading to non detectable blank levels. Even trace amounts of surfactants interfere with the determination, so the digestion of organic matter must be complete. This demand is fulfilled by the technique of highpressure ashing described by Knapp [14, 15], using nitricand hydrochloric acid at temperatures of > 2 8 0 ° C [16]. However, nitric acid also interferes with the voltammetric determination of platinum. At concentrations of nitric acid > 2 retool/1 the background current is significantly increased at the relevant potential, resulting in a positive systematic error. In Fig. 2 the linear correlation between background current and nitric acid concentration is shown. Based on a tolerable nitric acid concentration of about 2 mmol/1 and a dilution of the decomposition solution to about 10 ml (as described in [5]) it is acceptable to add up to 10 ~tl of the diluted decomposition solution to the supporting electrolyte volume of 5 ml. Using this procedure, well interpretable voltammograms are obtained for samples with platinum contents of about >~ 30 l~g/kg (referring to a max. sample of 400 mg and using 2 ml HNO3). As only 10 gl are necessary
for a single determination, this procedure is well suited to serve as a reference method, while more than 99% of the sample is still left for another method. The overall recovery of platinum was found to be ~> 97% in all cases. A standard deviation of 3.7% (N = 7) was found for the determination of platinum in filter dust (see Table 3). The accuracy of the method was verified for several matrices (filter samples and corn) by ET-AAS (see Table 3). No interferences were observed for metal ions such as As, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn added at a concentration of 50 mg/1. As reported in [17], the other platinum metals do not interfere either. To extend the range of this method to the low ng/kgrange, the dilution step (only 10 lal are used out of 10 ml) should be avoided in order to use the whole digestion solution for a single determination. This requires to lower the nitric acid concentration to < 2 mmol/1. The usual evaporation with conc, sulfuric acid is not practicable but good success was achieved by using the following procedure: 200 ~tl sulfuric acid (subboiled) are added to the decomposition solution which is then heated with frequent shaking to about 200 ° C in a metal block until the volume is reduced to about 200 ~tl. Then 150 gl hydrochloric acid (subboiled) are added carefully and the solution is heated again. The addition of HC1 is repeated until no further formation of nitrose gases is visible. The solution is cooled to room temperature and diluted with bidistilled water to a volume of 10 ml. Then 5 gl of formaldehyde and 150 gl of hydrazine sulfate (0.01 tool/l) are added and after 10 rain the voltammogram is recorded. The recovery for pure solutions was found to be > 95%. Table 3 shows some results of real samples. 4 Conclusion
Only a few reliable data concerning baseline levels of platinum in biotic and environmental materials exist [6, 7, 17]. Considering these values, this level is estimated to range in the low ng/kg- and pg/1-region. In contrast to the procedures used by the cited authors the method presented here is relatively simple regarding time, labour and instrumental effort. Nevertheless the performance of the adsorption voltammetric technique requires a high standard by the user to recognize and to minimize interferences caused e.g. by surfactants and residues from decomposition acids. Future investigations will be carried out to improve the procedure. The decomposition step by high pressure ashing for example may be replaced by a cold plasma ashing system. Since only oxygen is used as reagent in this method, it can be assumed, that interferences by decomposition acids like nitric acid may be avoided. Most materials, specified in Table 3, are originated by participants of a combined research scheme "Edelmetallemissionen". The given values represent only single determinations. Any conclusions concerning the emission of platinum by automobile exhaust catalysts and the uptake by plants are not possible at the present state of the investigations. The utilization and interpretation of the results is reserved to the participants and the combined research scheme.
Acknowledgement. This project is supported by the Bundesminister fiir Forschung und Technologie as part of a combined research scheme "Edelmetallemissionen". Our sincere thanks are due to Dr. P. Tsch6pel (MPI fiir Metallforschung, Stuttgart, Laboratorium fiir Reinststoffe, Dortmund) for making available a polarographic working place.
816 References 1. Koberstein E (1984) Chemie in unserer Zeit 18:37-45 2. Malanchuk M, Barkeley N, Contner G, Richards M, Slater R, Burkart J, Yang Y (1974) Environmental toxicology research laboratory (NERC-Cincinatti). Interim Report EPA, p 9 1 - 98 3. Hertel RF (1986) Projektskizze, Untersuchungen zur Frage des Geffihrdungspotentials yon katalysatorbiirtigen Edelmetallemissionen. FhG Hannover 4. Rosner G, Hertel RF (1986) Staub Reinhaltung der Luft 46:281-285 5. Alt F, Jerono U, Messerschmidt J, T61g G (1988) Microchim Acta [Wien] III: 299 - 304 6. Hodge V, Stallard M, Koide M, Goldberg ED (1986) Anal Chem 58 : 616 -- 620 7. Zeisler R, Greenberg RR (1988) In: Br/itter P, Schramel P (eds) Trace element analytical chemistry in medicine and biology, vol 5. Walter de Gruyter, Berlin New York, p 297- 303 8. Kritsotakis K, Tobschall HJ (1985) Fresenius Z Anal Chem 320:156-158
9. Zhao Z, Freiser H (1986) Anal Chem 58:1498-1501 10. Wang J, Zadeii J, Lin MS (1987) J Electroanal Chem 237: 281-287 11. Heraeus Quarzschmelze GmbH, Hanau, pers6nliche Mitteilung 12. Kfirner H, AnalysentechnikRosenheim, pers6nliche Mitteilung 13. Firmenprospekt Heraeus Quarzschmelze GmbH 14. W/irfels M, Jackwerth E, Stoeppler M (1987) Fresenius Z Anal Chem 329:459-461 15. Knapp G (1984) Fresenius Z Anal Chem 317:213-219 16. Schramel P, Hasse S, Knapp G (1987) Fresenius Z Anal Chem 326:142-145 17. Van den Berg CMG, Jacinto GS (1988) Anal Chim Acta 211 : 129-139 18. Tsch6pel P, T61g G (1982) J Trace Micro Tech 1:1 --77
Received July 3, 1989
