- Email: [email protected]
Ancient artefacts and modern analytical techniques – Usefulness of laser ablation ICP-MS demonstrated with ancient gold coins
Nuclear Instruments and Methods in Physics Research B 181 (2001) 723±727
www.elsevier.com/locate/nimb
Ancient artefacts and modern analytical techniques ± Usefulness of laser ablation ICP-MS demonstrated with ancient gold coins Stephan A. Junk
*
Lehrstuhl f ur Archaometallurgie, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, Freiberg D-09596, Germany
Abstract Laser ablation (LA) is a sampling technique which is well suited for the characterization of precious archaeological artefacts. Using the example of Celtic gold coins ± so-called ``Regenbogensch usselchen'' ± the bene®ts of LA attached to an inductively coupled plasma mass spectrometry (ICP-MS) with a sector ®eld mass spectrometer and multiple Faraday detectors are reviewed. It will be shown that osmium isotopic ratios determined by this method are a valuable guide for investigations on the provenance of Celtic gold. The precision and accuracy achieved with LA±ICP-MS are discussed. Corrections for chemical interferences and mass bias are exempli®ed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.Eh; 82.80 Keywords: LA±ICP-MS; Osmium; Isotopic ratios; Celtic gold
1. Introduction Cooperation between humanities and science brings up questions which on ®rst sight seem to be strange for an analytical chemist. In the present case the archaeological question is: What do ancient Greeks and Celts of the third and second century BC have in common? Where did they satisfy their demand of gold? This results in the question for the scientist: How can the provenance of gold be determined? Generally, provenance determination should be possible by comparing trace element concentra-
*
Tel.: +49-373-139-2899; fax: +49-373-139-2489. E-mail address: [email protected] (S.A. Junk).
tions of artefacts and ores [1], but previous attempts in this direction have not been conclusive for alloys. Unlike the Greeks, the Celts left only insucient written testimony concerning their gold supplies. Reports stem from people and nations who were in ± often war-like ± contacts with Celts and thus have a political and social bias. Hence we have a fair knowledge of the Greek gold sources, which is not the case for Celtic gold. There are some speculations about the ore provenance [2±4] based on the general composition [5] and platinum inclusions often found in Celtic gold coins of the second century BC. Conclusive evidence is however scarce. The platinum group metal inclusions in these coins contain Os. This is important for two
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 6 6 - 4
724
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
reasons. Firstly, 187 Os in natural Os is a radiogenic product of 187 Re. Secondly, Os and Re show a dierent geochemical behaviour. In terrestrial rocks, Re behaves as an incompatible element and is enriched in the earth's crust relative to the mantle. Since Os remains in the mantle this results in a variation over three orders of magnitude in Re/Os abundance ratios in terrestrial samples [6]. Therefore, long-term and relatively large geological fractionations of Re and Os produce dierent contents of 187 Os in Os. This means, that there are frequently exceptional geological specimens, in which Os has an isotopic composition outside the normal, e.g. in [7] reported range. Since the isotopic composition depends on the ore source, it is very promising for provenancing of the gold placer deposits used for Celtic gold coins. The examination of the platinum group inclusions in Celtic gold coins should thus exemplify some applications and limits of laser ablation (LA) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). This article provides a short and selective introduction. It will not cover LA mechanism and elemental fractionation discussed in [8±10]. Extended lectures, technical details, and applications of LA and ICP-MS can be found elsewhere, e.g. [1,11±16]. 2. Materials and instrumentation 2.1. Samples The investigated Celtic gold coins are so-called ``Regenbogensch usselchen'' (rainbow cup coins) of the Wallersdorf hoard (Pr ahistorische Staatssammlung M unchen, 3 coins of the found 368 Celtic gold staters). They contain inclusions of platinum group metals [3]. To demonstrate abundance eects, a second set of 20 specimens (0.5± 4 mm) from a placer deposit (Nishny Tagil, Russia, Lagerst attenkundliche Sammlung Freiberg) is used. Isobaric interferences are shown using a 0.3 mg/l Os solution from a 1 g/l stock solution (FLUKA) in 2% HNO3 , and the same solution spiked with 0.03 mg/l Re (1 g/l stock solution, FLUKA) and a wolframite sample from Sadisdorf (Erzgebirge, Germany (Fe, Mn)WO4 ).
2.2. Instrumentation LA was performed with a Microprobe II laser ablation device (VG, Nd:YAG laser, 266 nm, pulse duration 3 ns, energy output to sample up to 4 mJ, ablation spot sizes: 5 lm for coins, up to 100 lm for placer deposit samples). Liquids were nebulized using a MCN 6000 (Cetac). The aerosols generated by LA and nebulizer were transferred by an Ar stream to the MC-ICPMS (VG Axiom, multicollector version with 9 Faraday cup detectors, resolution 400). The signals were measured at m=z 183 (W), 184 (W + Os), 185 (Re), 186 (W + Os), 187 (Re + Os), 188 (Os), 189 (Os), 191 (Ir) and 193 (Ir). 3. Results and discussion 3.1. General The main advantage of LA is the possibility of direct sampling with low sample consumption. However, this can be a disadvantage. On the one hand, no sample preparation is needed. On the other hand, matrix interferences due to lack of separation may cause some problems, such as isobaric interferences. In addition, if a small signal of an analyte is measured adjacent to a large peak from the matrix, the tailing of the large peak into the smaller peak, the so-called abundance sensitivity, may be a further problem. 3.2. Interferences and corrections Interferences and the presence of isobaric nuclides are the principal factors that in¯uence the accuracy of all LA±ICP-MS analyses. This is illustrated by a setup, in which isobaric interferences can easily be switched on or o: At ®rst a solution of Os was nebulized, then the same solution spiked with Re was nebulized and this aerosol mixed with the aerosol from the LA of the wolframite. The 187 Os/186 Os ratios are reported with the standard deviation (for 120 measurements) of the last digit in parentheses. For the ®rst run without interferences, the accepted true value was 1.007(1).
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
The second run with the interferences deviated from this value. The LA produced a 186 W signal which was about 8.8 times of the 186 Os signal, and a 187 Re signal from the spiked solution of about 1/5th of the 187 Os signal. The not corrected ratio of these signals at m/z 187 and 186 was 0.1(2) which diers from the accepted 187 Os/186 Os value without interferences. Therefore, the isobaric interferences of 186 W and 186 Os as well as 187 Re and 187 Os must be corrected. For m=z 186 the 186 W signal was calculated with the measured 183 W signal and the natural abundances of its isotopes [7]. Then, this 186 W part was subtracted from the (W + Os) signal at m=z 186. Likewise the signal at 187 was corrected for the 187 Re using the measured 185 Re signal. These corrections resulted in an 187 Os/186 Os ratio of 0.73(4). This result still diers from the value of the pure sample. Thus, the systematic error of the mass bias due to mass fractionation in the ICP-MS [11] has to be considered in the same manner as for the pure sample. In the experiment described above, the mass bias was about 1.4% per mass unit. Since the signal of 186 W was about 8.8 times of the 186 Os signal, and the contribution of 186 W was calculated from 183 W, the corrected signal at 186 had a bias of 1:4 %= u 8:8 186±183u 37%. Therefore, mass bias corrections are critical, especially if chemical or isobaric interferences have to be considered. This mass bias is usually corrected by measurement of a well known isotopic ratio. Natural Os has a constant 188 Os/189 Os ratio and can therefore be used for this purpose [17]. Using an exponential fractionation law and the IUPAC best measurement values [7] this results in an 187 Os/ 186 Os ratio of 0.99(1). Compared with the accepted true value, this shows that using this procedure, chemical interferences can be eciently corrected within certain limits.
725
This will be exempli®ed by the placer deposit samples from Nishny Tagil, Russia. The results are illustrated in Fig. 1. There are two points out of the expected con®dence limits for the normal 186 Os/188 Os ratio [7]. The anomalous ratios can be explained by the matrix (see a mass spectrum of the corresponding specimens in Fig. 2). Both samples consisted of platinum and contained only traces of Os. Since 186 Os is a radiogenic product of
Fig. 1. Some Os isotopic ratios of Nishny Tagil placer ores. Results from 20 samples with 95% con®dence limits are shown. The lines represent the value with upper and lower 95% con®dence limits as expected for the normal 186 Os/188 Os ratio [7]. The two points out of these limits can be explained by Pt matrix eects.
3.3. Abundance sensitivity As mentioned before, no sample puri®cation is performed with LA. The isobaric interferences shown here are only one resulting problem to be considered. Another one is the abundance sensitivity of the mass spectrometer.
Fig. 2. Mass spectrum of placer deposit sample outside the con®dence limits: MS shows Pt isotopes as main signals, Os is only a trace component. Measured at a resolution of 400.
726
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
the a-decay of 190 Pt, deviations can be expected [18]. In addition the con®dence limits of one deviating value are worse than for the rest of the samples. As can be seen in Fig. 2, there is a signi®cant signal overlap at a low resolution of 400. The tails of the large Pt signals lead to a higher and also unstable background, e.g. for the small 189 Os signal. This can be avoided by sample puri®cation, but obviously LA excludes this step. One possible solution is to avoid the overlap of neighbouring signals, i.e. to improve the abundance sensitivity or the resolution of the mass spectrometer. A higher resolution usually results in lower detector signals and in a worse precision. Another way is the measurement and subtraction of the background around the signal in question, e.g. at m=z 0:5. This was done for the values shown in Fig. 1. The procedure is feasible and gives acceptable results, provided that no very large signals are nearby, which would reduce the precision. 3.4. Celtic coins If the above mentioned requirements are met, the determination of Os isotopic ratios of platinum group inclusions in ancient artefacts works well. Fig. 3 shows some results for three Regenbogensch usselchen from Wallersdorf (Germany). The mean 187 Os/186 Os ratio is about 1. This value
Fig. 3. Some Os isotopic ratios of three Celtic coins (so-called ``Regenbogensch usselchen'', Wallersdorf) plotted as squares, triangles and diamonds, respectively. 95% con®dence limits as in Fig. 1.
can be compared with other gold deposits, e.g. the Grasberg deposit (Indonesia, Freeport-McMoRan Copper & Gold claims it to be the world's richest deposit of copper and gold). The 187 Os/186 Os ratios in this deposit are about 5±11 [19]. Thus, it seems to be possible to dierentiate between gold sources. But since this work is still in progress, it is too early and beyond the scope of this publication to provenance these Regenbogensch usselchen. For these specimens, the amount of Os in the inclusions was small and limited. The inclusions usually had diameters between 5 and 30 lm, in rare cases up to 200 lm. This aected the detector signals and hence the resulting uncertainties. For these specimens, the variations between the inclusions in each coin are comparable to the variation within a single ore source (Fig. 2) and do not indicate a mixture of dierent ore sources. But they indicate that metallurgical gold processing did not homogenize the isotopic composition of the inclusions. This is important for provenancing, since alloying and minting did not disturb the isotopic patterns. 4. Conclusions LA±ICP-MS is a promising tool for analysis. The main advantages are low sample consumption and the possibility of highly localized sampling. Sample contamination is usually not a problem since there is no chemical sample preparation. Surface contaminations of the samples need to be considered, but they can be easily removed by laser preablation prior to analysis. If some possible error sources are considered, LA allows to determine concentrations or isotopic ratio distributions even on tiny sample areas. The determination and distribution of Os isotopic ratios in ancient artefacts may provide answers to archaeological questions such as: Where does Celtic coin gold come from? Acknowledgements This work is supported by the Forschungszentrum J ulich, project BEO21-04040323-
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
03PE9FRE7. For the loan of the Celtic coins, I thank the Prahistorische Staatssammlung (M unchen) and for the placer deposit samples the Lagerst attenkundliche Sammlung (Freiberg).
[8] [9]
References [1] A. Gondonneau, M.F. Guerra, in: S.M.M. Young, A.M. Pollard, P. Budd, R.A. Ixer (Eds.), Metals in Antiquity, Archaeopress, Oxford, 1999, p. 262. [2] H. Moesta, P.R. Franke, Antike Metallurgie und M unzpr agung: ein Beitrag zur Technikgeschichte, Birkh auser Verlag, Basel, 1995. [3] U. Stegen, R. Gebhard, G. Lehrberger, G. Morteani, in: A. Oddy, M. Cowell (Eds.), Metallurgy in Numismatics, Vol. 4, Royal Numismatic Society, London, 1998, p. 202. [4] U. Zwicker, in: A. Oddy, M. Cowell (Eds.), Metallurgy in Numismatics, Vol. 4, Royal Numismatic Society, London, 1998, p. 171. [5] A. Hartmann, Jahrbuch des R omisch-Germanischen Zentralmuseums, Mainz 32 (1985) 660. [6] T. Hirata, R.W. Nesbitt, Earth Planet. Sci. Lett. 147 (1997) 11. [7] K.J.R. Rosman, P.D.P. Taylor, Isotopic compositions of the elements 1997, Technical Report, International Union
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
727
of Pure and Applied Chemistry, Inorganic Chemistry Division Commision on Atomic Weights and Isotopic Abundances, Subcommittee for Isotopic Abundance Measurements, 1997. S.M. Eggins, L.P.J. Kinsley, J.M.G. Shelley, Appl. Surf. Sci. 127±129 (1998) 278. R.E. Russo, X.L. Mao, H.C. Liu, J.H. Yoo, S.S. Mao, Appl. Phys. A 69 (1999) S887. O.V. Borisov, X.L. Mao, A. Fernandez, M. Caetano, R.E. Russo, Spectrochim. Acta, Part B 54 (1999) 1351. A. Montaser (Ed.), Inductively coupled plasma mass spectrometry, Wiley-VCH, New York, 1998. L.M. Cabalin, D. Romero, J.M. Baena, J.J. Laserna, Fresenius' J. Anal. Chem. 365 (1999) 404. J.S. Becker, H.-J. Dietze, Fresenius' J. Anal. Chem. 365 (1999) 429. D. G unther, R. Frischknecht, H.-J. M uschenborn, Fresenius' J. Anal. Chem. 359 (1997) 390. I. Horn, R.L. Rudnick, W.F. McDonough, Chem. Geol. 167 (2000) 405. J.S. Becker, H.-J. Dietze, Int. J. Mass Spectrom. 197 (2000) 1. T. Hirata, M. Hattori, T. Tanaka, Chem. Geol. 144 (1998) 269. A.D. Brandon, M.D. Norman, R.J. Walker, J.W. Morgan, Earth Planet. Sci. Lett. 174 (1999) 25. R. Mathur, J. Ruiz, S. Titley, S. Gibbins, W. Margotomo, Earth Planet. Sci. Lett. 183 (2000) 7.
www.elsevier.com/locate/nimb
Ancient artefacts and modern analytical techniques ± Usefulness of laser ablation ICP-MS demonstrated with ancient gold coins Stephan A. Junk
*
Lehrstuhl f ur Archaometallurgie, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, Freiberg D-09596, Germany
Abstract Laser ablation (LA) is a sampling technique which is well suited for the characterization of precious archaeological artefacts. Using the example of Celtic gold coins ± so-called ``Regenbogensch usselchen'' ± the bene®ts of LA attached to an inductively coupled plasma mass spectrometry (ICP-MS) with a sector ®eld mass spectrometer and multiple Faraday detectors are reviewed. It will be shown that osmium isotopic ratios determined by this method are a valuable guide for investigations on the provenance of Celtic gold. The precision and accuracy achieved with LA±ICP-MS are discussed. Corrections for chemical interferences and mass bias are exempli®ed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.Eh; 82.80 Keywords: LA±ICP-MS; Osmium; Isotopic ratios; Celtic gold
1. Introduction Cooperation between humanities and science brings up questions which on ®rst sight seem to be strange for an analytical chemist. In the present case the archaeological question is: What do ancient Greeks and Celts of the third and second century BC have in common? Where did they satisfy their demand of gold? This results in the question for the scientist: How can the provenance of gold be determined? Generally, provenance determination should be possible by comparing trace element concentra-
*
Tel.: +49-373-139-2899; fax: +49-373-139-2489. E-mail address: [email protected] (S.A. Junk).
tions of artefacts and ores [1], but previous attempts in this direction have not been conclusive for alloys. Unlike the Greeks, the Celts left only insucient written testimony concerning their gold supplies. Reports stem from people and nations who were in ± often war-like ± contacts with Celts and thus have a political and social bias. Hence we have a fair knowledge of the Greek gold sources, which is not the case for Celtic gold. There are some speculations about the ore provenance [2±4] based on the general composition [5] and platinum inclusions often found in Celtic gold coins of the second century BC. Conclusive evidence is however scarce. The platinum group metal inclusions in these coins contain Os. This is important for two
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 6 6 - 4
724
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
reasons. Firstly, 187 Os in natural Os is a radiogenic product of 187 Re. Secondly, Os and Re show a dierent geochemical behaviour. In terrestrial rocks, Re behaves as an incompatible element and is enriched in the earth's crust relative to the mantle. Since Os remains in the mantle this results in a variation over three orders of magnitude in Re/Os abundance ratios in terrestrial samples [6]. Therefore, long-term and relatively large geological fractionations of Re and Os produce dierent contents of 187 Os in Os. This means, that there are frequently exceptional geological specimens, in which Os has an isotopic composition outside the normal, e.g. in [7] reported range. Since the isotopic composition depends on the ore source, it is very promising for provenancing of the gold placer deposits used for Celtic gold coins. The examination of the platinum group inclusions in Celtic gold coins should thus exemplify some applications and limits of laser ablation (LA) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). This article provides a short and selective introduction. It will not cover LA mechanism and elemental fractionation discussed in [8±10]. Extended lectures, technical details, and applications of LA and ICP-MS can be found elsewhere, e.g. [1,11±16]. 2. Materials and instrumentation 2.1. Samples The investigated Celtic gold coins are so-called ``Regenbogensch usselchen'' (rainbow cup coins) of the Wallersdorf hoard (Pr ahistorische Staatssammlung M unchen, 3 coins of the found 368 Celtic gold staters). They contain inclusions of platinum group metals [3]. To demonstrate abundance eects, a second set of 20 specimens (0.5± 4 mm) from a placer deposit (Nishny Tagil, Russia, Lagerst attenkundliche Sammlung Freiberg) is used. Isobaric interferences are shown using a 0.3 mg/l Os solution from a 1 g/l stock solution (FLUKA) in 2% HNO3 , and the same solution spiked with 0.03 mg/l Re (1 g/l stock solution, FLUKA) and a wolframite sample from Sadisdorf (Erzgebirge, Germany (Fe, Mn)WO4 ).
2.2. Instrumentation LA was performed with a Microprobe II laser ablation device (VG, Nd:YAG laser, 266 nm, pulse duration 3 ns, energy output to sample up to 4 mJ, ablation spot sizes: 5 lm for coins, up to 100 lm for placer deposit samples). Liquids were nebulized using a MCN 6000 (Cetac). The aerosols generated by LA and nebulizer were transferred by an Ar stream to the MC-ICPMS (VG Axiom, multicollector version with 9 Faraday cup detectors, resolution 400). The signals were measured at m=z 183 (W), 184 (W + Os), 185 (Re), 186 (W + Os), 187 (Re + Os), 188 (Os), 189 (Os), 191 (Ir) and 193 (Ir). 3. Results and discussion 3.1. General The main advantage of LA is the possibility of direct sampling with low sample consumption. However, this can be a disadvantage. On the one hand, no sample preparation is needed. On the other hand, matrix interferences due to lack of separation may cause some problems, such as isobaric interferences. In addition, if a small signal of an analyte is measured adjacent to a large peak from the matrix, the tailing of the large peak into the smaller peak, the so-called abundance sensitivity, may be a further problem. 3.2. Interferences and corrections Interferences and the presence of isobaric nuclides are the principal factors that in¯uence the accuracy of all LA±ICP-MS analyses. This is illustrated by a setup, in which isobaric interferences can easily be switched on or o: At ®rst a solution of Os was nebulized, then the same solution spiked with Re was nebulized and this aerosol mixed with the aerosol from the LA of the wolframite. The 187 Os/186 Os ratios are reported with the standard deviation (for 120 measurements) of the last digit in parentheses. For the ®rst run without interferences, the accepted true value was 1.007(1).
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
The second run with the interferences deviated from this value. The LA produced a 186 W signal which was about 8.8 times of the 186 Os signal, and a 187 Re signal from the spiked solution of about 1/5th of the 187 Os signal. The not corrected ratio of these signals at m/z 187 and 186 was 0.1(2) which diers from the accepted 187 Os/186 Os value without interferences. Therefore, the isobaric interferences of 186 W and 186 Os as well as 187 Re and 187 Os must be corrected. For m=z 186 the 186 W signal was calculated with the measured 183 W signal and the natural abundances of its isotopes [7]. Then, this 186 W part was subtracted from the (W + Os) signal at m=z 186. Likewise the signal at 187 was corrected for the 187 Re using the measured 185 Re signal. These corrections resulted in an 187 Os/186 Os ratio of 0.73(4). This result still diers from the value of the pure sample. Thus, the systematic error of the mass bias due to mass fractionation in the ICP-MS [11] has to be considered in the same manner as for the pure sample. In the experiment described above, the mass bias was about 1.4% per mass unit. Since the signal of 186 W was about 8.8 times of the 186 Os signal, and the contribution of 186 W was calculated from 183 W, the corrected signal at 186 had a bias of 1:4 %= u 8:8 186±183u 37%. Therefore, mass bias corrections are critical, especially if chemical or isobaric interferences have to be considered. This mass bias is usually corrected by measurement of a well known isotopic ratio. Natural Os has a constant 188 Os/189 Os ratio and can therefore be used for this purpose [17]. Using an exponential fractionation law and the IUPAC best measurement values [7] this results in an 187 Os/ 186 Os ratio of 0.99(1). Compared with the accepted true value, this shows that using this procedure, chemical interferences can be eciently corrected within certain limits.
725
This will be exempli®ed by the placer deposit samples from Nishny Tagil, Russia. The results are illustrated in Fig. 1. There are two points out of the expected con®dence limits for the normal 186 Os/188 Os ratio [7]. The anomalous ratios can be explained by the matrix (see a mass spectrum of the corresponding specimens in Fig. 2). Both samples consisted of platinum and contained only traces of Os. Since 186 Os is a radiogenic product of
Fig. 1. Some Os isotopic ratios of Nishny Tagil placer ores. Results from 20 samples with 95% con®dence limits are shown. The lines represent the value with upper and lower 95% con®dence limits as expected for the normal 186 Os/188 Os ratio [7]. The two points out of these limits can be explained by Pt matrix eects.
3.3. Abundance sensitivity As mentioned before, no sample puri®cation is performed with LA. The isobaric interferences shown here are only one resulting problem to be considered. Another one is the abundance sensitivity of the mass spectrometer.
Fig. 2. Mass spectrum of placer deposit sample outside the con®dence limits: MS shows Pt isotopes as main signals, Os is only a trace component. Measured at a resolution of 400.
726
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
the a-decay of 190 Pt, deviations can be expected [18]. In addition the con®dence limits of one deviating value are worse than for the rest of the samples. As can be seen in Fig. 2, there is a signi®cant signal overlap at a low resolution of 400. The tails of the large Pt signals lead to a higher and also unstable background, e.g. for the small 189 Os signal. This can be avoided by sample puri®cation, but obviously LA excludes this step. One possible solution is to avoid the overlap of neighbouring signals, i.e. to improve the abundance sensitivity or the resolution of the mass spectrometer. A higher resolution usually results in lower detector signals and in a worse precision. Another way is the measurement and subtraction of the background around the signal in question, e.g. at m=z 0:5. This was done for the values shown in Fig. 1. The procedure is feasible and gives acceptable results, provided that no very large signals are nearby, which would reduce the precision. 3.4. Celtic coins If the above mentioned requirements are met, the determination of Os isotopic ratios of platinum group inclusions in ancient artefacts works well. Fig. 3 shows some results for three Regenbogensch usselchen from Wallersdorf (Germany). The mean 187 Os/186 Os ratio is about 1. This value
Fig. 3. Some Os isotopic ratios of three Celtic coins (so-called ``Regenbogensch usselchen'', Wallersdorf) plotted as squares, triangles and diamonds, respectively. 95% con®dence limits as in Fig. 1.
can be compared with other gold deposits, e.g. the Grasberg deposit (Indonesia, Freeport-McMoRan Copper & Gold claims it to be the world's richest deposit of copper and gold). The 187 Os/186 Os ratios in this deposit are about 5±11 [19]. Thus, it seems to be possible to dierentiate between gold sources. But since this work is still in progress, it is too early and beyond the scope of this publication to provenance these Regenbogensch usselchen. For these specimens, the amount of Os in the inclusions was small and limited. The inclusions usually had diameters between 5 and 30 lm, in rare cases up to 200 lm. This aected the detector signals and hence the resulting uncertainties. For these specimens, the variations between the inclusions in each coin are comparable to the variation within a single ore source (Fig. 2) and do not indicate a mixture of dierent ore sources. But they indicate that metallurgical gold processing did not homogenize the isotopic composition of the inclusions. This is important for provenancing, since alloying and minting did not disturb the isotopic patterns. 4. Conclusions LA±ICP-MS is a promising tool for analysis. The main advantages are low sample consumption and the possibility of highly localized sampling. Sample contamination is usually not a problem since there is no chemical sample preparation. Surface contaminations of the samples need to be considered, but they can be easily removed by laser preablation prior to analysis. If some possible error sources are considered, LA allows to determine concentrations or isotopic ratio distributions even on tiny sample areas. The determination and distribution of Os isotopic ratios in ancient artefacts may provide answers to archaeological questions such as: Where does Celtic coin gold come from? Acknowledgements This work is supported by the Forschungszentrum J ulich, project BEO21-04040323-
S.A. Junk / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 723±727
03PE9FRE7. For the loan of the Celtic coins, I thank the Prahistorische Staatssammlung (M unchen) and for the placer deposit samples the Lagerst attenkundliche Sammlung (Freiberg).
[8] [9]
References [1] A. Gondonneau, M.F. Guerra, in: S.M.M. Young, A.M. Pollard, P. Budd, R.A. Ixer (Eds.), Metals in Antiquity, Archaeopress, Oxford, 1999, p. 262. [2] H. Moesta, P.R. Franke, Antike Metallurgie und M unzpr agung: ein Beitrag zur Technikgeschichte, Birkh auser Verlag, Basel, 1995. [3] U. Stegen, R. Gebhard, G. Lehrberger, G. Morteani, in: A. Oddy, M. Cowell (Eds.), Metallurgy in Numismatics, Vol. 4, Royal Numismatic Society, London, 1998, p. 202. [4] U. Zwicker, in: A. Oddy, M. Cowell (Eds.), Metallurgy in Numismatics, Vol. 4, Royal Numismatic Society, London, 1998, p. 171. [5] A. Hartmann, Jahrbuch des R omisch-Germanischen Zentralmuseums, Mainz 32 (1985) 660. [6] T. Hirata, R.W. Nesbitt, Earth Planet. Sci. Lett. 147 (1997) 11. [7] K.J.R. Rosman, P.D.P. Taylor, Isotopic compositions of the elements 1997, Technical Report, International Union
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
727
of Pure and Applied Chemistry, Inorganic Chemistry Division Commision on Atomic Weights and Isotopic Abundances, Subcommittee for Isotopic Abundance Measurements, 1997. S.M. Eggins, L.P.J. Kinsley, J.M.G. Shelley, Appl. Surf. Sci. 127±129 (1998) 278. R.E. Russo, X.L. Mao, H.C. Liu, J.H. Yoo, S.S. Mao, Appl. Phys. A 69 (1999) S887. O.V. Borisov, X.L. Mao, A. Fernandez, M. Caetano, R.E. Russo, Spectrochim. Acta, Part B 54 (1999) 1351. A. Montaser (Ed.), Inductively coupled plasma mass spectrometry, Wiley-VCH, New York, 1998. L.M. Cabalin, D. Romero, J.M. Baena, J.J. Laserna, Fresenius' J. Anal. Chem. 365 (1999) 404. J.S. Becker, H.-J. Dietze, Fresenius' J. Anal. Chem. 365 (1999) 429. D. G unther, R. Frischknecht, H.-J. M uschenborn, Fresenius' J. Anal. Chem. 359 (1997) 390. I. Horn, R.L. Rudnick, W.F. McDonough, Chem. Geol. 167 (2000) 405. J.S. Becker, H.-J. Dietze, Int. J. Mass Spectrom. 197 (2000) 1. T. Hirata, M. Hattori, T. Tanaka, Chem. Geol. 144 (1998) 269. A.D. Brandon, M.D. Norman, R.J. Walker, J.W. Morgan, Earth Planet. Sci. Lett. 174 (1999) 25. R. Mathur, J. Ruiz, S. Titley, S. Gibbins, W. Margotomo, Earth Planet. Sci. Lett. 183 (2000) 7.