Laser AblationâInductively Coupled Plasma ... - ACS Publications

Downloaded by UNIV MASSACHUSETTS AMHERST on September 11, 2012 | http://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch018

Chapter 18

Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Analysis of Ancient Copper Alloy Artifacts Laure Dussubieux Department of Anthropology, Field Museum of Natural History, Chicago, I L 60605

The elemental composition of copper-based archaeological artifacts and museum objects provides information that can address questions related to manufacturing technologies and metal circulation, and also may be helpful for authenticity verification. As the range of the materials investigated using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is growing wider due to technological improvements, we tested the performances of this analytical technique to determine copper alloy compositions. Standardization, detection limits and reliability of the method will be discussed. The application of our L A - I C P - M S protocol to the study of Matisse bronze sculptures will be presented.

Introduction Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is still a recent analytical technique that appeared in the middle of the 1980s and has yielded an increasing interest ever since (1). At the beginning of the 1990s, applications to determine the composition of archaeological or ancient artifacts

336

© 2007 American Chemical Society

In Archaeological Chemistry; Glascock, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


337 made of inorganic materials were developed (2). The wide range of elements detected with L A - I C P - M S , its low detection limits (in the range of the ppm or below) and the minimum damage caused to the artifacts make this technique particularly suitable to undertake provenance studies, to trace trade exchanges or to have a better understanding of ancient technologies. This technique does have some limitations: the material to be analyzed must be homogeneous and matrixmatched standard reference materials (SRMs) must be available. Natural and synthetic glasses were the first widely investigated materials using this technique since they meet the requirement for homogeneity and appropriate SRMs are easily accessible (3, 4, 5, 6). Metals are more problematic due to the existence of different phases or precipitates in some alloys. Nevertheless, analytical protocols have been developed and promising results presented for the compositional analysis of ancient gold (7, 8), silver (9) and iron alloys (10). Until now, little attention has been given to the analysis of ancient copper alloys with L A - I C P - M S . This type of material is usually analyzed with fast or instrumental neutron activation analysis ( F N A A or INAA), particle induced X ray emission (PIXE), X-ray fluorescence (XRF), inductively coupled plasmaatomic emission spectrometry or inductively coupled plasma-atomic absorption spectrometry (ICP-AES or ICP-AAS). Some of these techniques are destructive and involve extensive sample preparation, some measure only surface compositions, and some require access to a cyclotron or a reactor. L A - I C P - M S is not affected by any of these inconveniences. We propose here an analytical protocol for copper alloys using L A - I C P - M S and present its application to the study of Matisse bronze sculptures.

Instrumentation and Analytical Protocol Instrumentation and Parameters of Analysis The analyses were carried out at the Field Museum of Natural History in Chicago, IL. The instrumentation is a Varian inductively coupled plasma-mass spectrometer (ICP-MS) equivalent to the actual Varian 810 instrument. A New Wave UP213 laser is connected to the ICP-MS for direct introduction of solid samples. The Varian ICP-MS is a quadrupole mass spectrometer that takes advantage of a new technology. Instead of traveling linearly through the instrument, the ion beam is bent 90° by a series of mirrors and lenses before entering the quadrupole. This new design increases the sensitivity of the instrument 200 times compared to conventional quadrupole ICP-MS instrument without compromising on the analyte interferences or instrumental background signal (77).


338 The parameters of the ICP-MS are optimized to ensure a stable signal with a maximum intensity over the full range of element masses and to minimize oxides and doubly ionized species formation ( X 0 7 X and X ^ / X < 1 to 2 %). For that purpose the argon flow rate, the R F power, the torch position, the lenses, the mirrors and the detector voltages are adjusted using an auto-optimization procedure. Tests on copper SRMs show that a stable signal is obtained when each isotope is measured using the peak jumping mode and one point per peak, with a dwell time of 25,000 μϊ. The quadrupole mass spectrometer scans three times the mass range per replicate and accumulates 10 replicates for a total acquisition time of about 1 minute. For this application, 22 isotopes were selected (Table I).


+

+

Table I. Measured isotopes 9

2 4

Be Mg A1 Si

27

29

31p

"Cr Mn Fe Co Ni

5 5

6 5

66

Cu Zn As Se Ag

57

75

59

78

6 0

l 0 7

""Sn Sb Te 12,

,25

206, 207, 2 0 8 ρ ^ 209

B j

The New Wave UP213 laser operates at a wavelength of 213 nm. The sample chamber is a cylinder with a diameter of 6 cm and a height of 5 cm. Hence, the sizes of the samples are limited. A C C D camera connected to a computer allows for the visualization of the surface of the sample. Helium is used as a gas carrier at a flow rate of 0.50 1/min. Stability and sensitivity requirements for the signal are met when the laser operates at 70% of its maximum energy (0.2 mJ) and at a pulse frequency of 15 Hz. The single point analysis mode is selected with a laser beam diameter of 55 μηι. When a noncorroded surface is ablated, a 20-second pre-ablation time is set to be sure that possible surface contamination does not affect the results of the analysis and to eliminate the transient part of the signal. When the surface of the artifact is obviously corroded, two ablations are performed, at the same location. The laser beam diameter is set to 65 μπι for the first ablation and 55 μπι for the second one. The laser beam is focused at the bottom of the first crater before starting the second ablation. Only the signal acquired during the second ablation is assumed to be representative of the non-corroded copper alloy. The average of four measurements corrected from the blank is considered for the calculation of the concentrations.


339 Standardization To improve reproducibility of measurements, internal standardization is required to correct possible instrument drift or changes in the ablation efficiency. The isotope C u was selected as an internal standard because it is present in every sample at relatively high concentrations allowing for accurate measurement. Figure 1 shows signals after normalization. Quantitative results are obtained by comparing the signal intensity measured for a given element in a sample to the signal intensity for the same element in a S R M with certified concentrations. To prevent matrix effects, the compositions of SRMs must be as close as possible to those of the samples. We selected seven different SRMs with the largest number of elements and the widest range of concentrations as possible. Two SRMs are manufactured by the Centre de Développement des Industries de Mise en Forme des Matériaux in France (SRMs BIO and B12) and two others by the Bureau of Analysed Samples Ltd in England (SRMs 51.13-4 and 71.32-4). These four standards are bronzes which contain copper, tin, lead, zinc, nickel, phosphorus, iron, silica, manganese, arsenic, antimony, bismuth, aluminum, chromium and silver with concentrations ranging from about 100 ppm to a few percent. More elements were added to the previous list by using three SRMs from the National Institute of Standards and Technology: selenium and tellurium from S R M 500, beryllium from S R M C I 123 and cadmium, magnesium and selenium from S R M 1275. The concentrations of the elements present in the samples were calculated assuming that the sum of their concentrations, in weight percent, is 100% (J).


6 5

0.1 0.01 0.001 0.0001 0.00001 0.000001 0 1 2 3 4 5 6 7 8 9 1011 121314151617181920

replicates 6 5

Figure J. Signals of some elements normalized on the Cu signal


340

Evaluation of the Reliability of Our Analytical Protocol


To evaluate the performance of our method, the response linearity of the instrument was tested. We also determined the detection limits, reproducibility and accuracy.

Linearity As the concentrations of some elements in the samples may vary over several orders of magnitude, it is important to verify that the response of the instrument is linear over a large range of concentrations. For this purpose, all SRMs were analyzed and calibration curves were traced with the accumulated data. The response of the instrument is all the more linear as the R-square value or correlation coefficient of the calibration curve for a given element is close to one. For each element tested, the number of standards included in the calibration curve, the range of concentrations, the slope and the correlation coefficient of the calibration curves are reported in Table II. No calibration curve could be generated for elements that are present in only one S R M . The linearity of the instrumental response is satisfied for all elements.

Table II. Characteristics of the calibration curves

Al

Number of standards 4

Range of concentrations 0.02 - 7.30 %

4.0266

Element

Slope

Correlation coefficient 1

Ρ

3

0.02-0.53 %

0.6378

1

Μη

5

0.05-0.90%

9.133

0.9934

Fe

6

0.04-1.81 %

0.2492

0.9988

Ni

6

0.01 - 2 . 6 %

0.9386

0.9996

Zn

6

0.01-6.52%

1.9395

1

As

4

0.01-0.25%

1.7052

0.9989

Sn

5

0.01 - 9 . 5 7 %

3.0571

0.9988

Sb

4

0.01-1.14%

3.4331

0.9962

Pb

4

0.01-4.43%

18.718

1

Co

3

0.0001-2.3%

4.5457

1

Ag Cr

3

0.009-0.034%

5.5728

0.9886

2

0.001 - 0 . 0 5 %

0.12241

0.998


341


The Detection Limits The detection limits are calculated as three times the standard deviation obtained from the measurement of ten blanks. Ideally, we should have measured these detection limits from multiple measurements of a pure copper material to take into account the contribution of the copper matrix to the background, but we did not have such a material. This may imply a slight underestimation of the detection limits for some elements. The detection limits range from about 1 ppb for bismuth to 2 ppm for iron (Figure 2).

10 1 0.1 0.01 0.001 0-

M

C

ϋ

Ο

"ηΞ

a ™ Où Ό G Xi

ϋ

£

Figure 2. Detection limits in ppm.

Reproducibility Reproducibility is calculated as the relative standard deviation obtained from 10 measurements on the SRMs B10, Β12, 51.13-4 and 71.32-4 over several weeks. Reproducibility for the majority of the elements, in most SRMs, is better than 20 % and usually around 10 % (Figure 3). Phosphorus is a noticeable exception. The reproducibility of the measurements is better for phosphorus since the concentrations are higher. This observation applies to other elements such as arsenic. In S R M B10, arsenic has the lowest concentration and the poorest reproducibility. For some elements at low concentration, the dispersion of the results is more important.

Accuracy Accuracy is the relative deviation between the certified concentrations and the average concentrations corresponding to 10 measurements of the SRMs B10,



342

• 51.13-4 -71.32-4 ΔΒ10 XB12 —

Ê Δ Χ Χ

π ν

ϊ ι

ι< ,m ,

>-

C

t U i r i

χ -

ι

3

C

w

- • bl)

S τ

£ -τ

I

x •

ι

.û

ι

*J?

ι

Figure 3. Relative standard deviations calculated from 10 measurements for four SRMs.

• 51.13-4 -71.32-4 100%

ΔΒ10

XB12

10% 5

Δ

-

x

1%

0%

Δ

A

χ

Τ ~Ί

1

1—

— Γ

-

Figure 4. Relative deviation between the certified concentrations and the average concentrations measured by LA-ICP-MS for four SRMs.


343 Β12, 51.13-4 and 71.32-4 by L A - I C P - M S over several weeks. The deviations between certified and L A - I C P - M S concentrations are about 10% or less for the majority of the elements and for most of the SRMs (Figure 4).


Application of LA-ICP-MS to the Study of Matisse Bronze Sculptures The Baltimore Museum of Art hosts the Cone collection including many works by Matisse. Bronze sculptures by Matisse were cast using different methods (lost wax and sand cast) and in different foundries. Ann Boulton of the Baltimore Museum of Art initiated a project aimed at determining whether or not different compositions of Matisse bronze sculptures could be correlated to different manufacturing techniques or locations. The project started while the author was a post-doctoral fellow at the Smithsonian Center for Materials Research and Education, now Museum Conservation Institute, and involved other analytical techniques (72). The results presented here were obtained at the Field Museum of Natural History. We investigated eight Matisse bronze samples that were either small pieces of metal or shavings saved from the drilling of mounting bolts. A l l sculptures were made using lost wax casting except for The Serf, which was sand cast (Table III).

Table III. List of the sculptures studied using L A - I C P - M S and I C P - M S Acquisition number 1950.439 1950.437 1950.429 1950.93 1950.436 1950.435 1950.423 1950.422

Title

Cast date

Foundry

Venus in a Shell I Reclining Nude III Reclining Nude I (Aurore) The Serpentine Large Seated Nude Crouching Venus Madeleine I The Serf

1931 1929 1930

Claude Valsuani Claude Valsuani Claude Valsuani

1930 1930 1930 1925 1908

Claude Valsuani Claude Valsuani Claude Valsuani Claude Valsuani Bingen-Costenoble

Samples were analyzed using both L A - I C P - M S and ICP-MS of solutions to assess the impact on the results of the sampling by laser ablation. For ICP-MS analysis, less than 1 mg of material was dissolved in double distilled nitric acid. SRMs B10, Β12, 51.13-4 and 71.32-4 were prepared the same way, to obtain


344 Table IV. Characteristics of the calibration curves used for ICP-MS


Element Zn Sn Pb As Ag Bi

Number of standards 4 4 4 4 1 1

Range of concentrations 0.34-6.52% 0.27-9.57% 0.05-0.90% 0.01-0.25% 0.034% 0.05%

Slope 0.5805 4.1276 16.246 0.7189 7.5103 15.097

Correlation coefficient 0.9987 0.9774 0.9958 0.9923 N/A N/A

matrix-matched standard solutions. The isotope Cu was used as an internal standard. Table IV provides the characteristics of the calibration curves for zinc, tin, lead, arsenic, silver and bismuth. Using the same calculation method for both L A - I C P - M S and ICP-MS, the concentrations of the elements in the samples were calculated assuming that their sum, in weight percent, is 100%. Figure 5 compares the results obtained for the Matisse bronze samples with L A - I C P - M S and ICP-MS for six elements: zinc, tin, lead, arsenic, bismuth and silver. The best correlations between ICP-MS and L A - I C P - M S occur for silver, arsenic and tin. For zinc, lead and bismuth, the results by ICP-MS and L A - I C P M S concur quite well for some samples. For the other samples, the values are either over or underestimated. Immiscibility of lead in copper, even at low concentrations, could explain the non-agreement for some samples between the ICP-MS and the L A - I C P - M S results (75). However, zinc and bismuth are miscible in copper at the concentrations encountered in the Matisse bronze sculptures. The RSDs for each element of interest, calculated from the normalized signal intensities collected from the four ablations performed on each Matisse bronze sample range, on average, from 7% for zinc to 23% for bismuth. These same RSDs are much lower when considering SRMs: they range from 3% for zinc to 8% for bismuth. The difference in zinc and bismuth concentrations measured with ICP-MS and L A - I C P - M S seem to result from a slight heterogeneous distribution of the elements in the bronze. Under some alloying conditions, dendritic structures can form in metal (14). As a discrete sampling technique, laser ablation is sensitive to microstructural variations in the substrate. Performing a metallographic study of the samples may help to determine the source of the variations between ICP-MS and L A - I C P - M S results. The main goal of this study was to identify different copper alloy compositions and relate them to different foundries or casting techniques. When considering the tin and the zinc contents in the sculptures, three different groups


345

Zinc 20%

Tin

π

8%

15% «


S 10% y

y = 1.0499x R = 0.745

5% -

0% 0%

6

%

H

!

4%

~

2% ^

2

R = 0.9488

0% 5%

10%

0%

15% 20%

Lead

4%

6%

8%

Arsenic

1.5%

0.06%

*> 1.0%

/ 0.04%

d ϋ 0.5%

y = 1.2558x R = 0.8851 2

0.5%

2%

LA-ICP-MS

LA-ICP-MS

0.0% 0.0%

y = 0.9284x

2

1.0%

ï

y = 0.9878x R = 0.902

y 0.02% 0.00% 0.00%

1.5%

LA-ICP-MS

2

0.02% 0.04% 0.06% LA-ICP-MS

Silver

Bismuth

0.02%

0.01% C/5

C/5

S 0.01%

ι

ι

'

y = 1.0208x R = 0.9205

eu

A

2

V

0.00% 0.00%

0.01% LA-ICP-MS

0.02%

4 y = 0.8131x R = 0.6623

0.00% 0.00%

2

0.01% LA-ICP-MS

Figure 5. Comparison of concentrations (by weight percent) between LA-ICP-MS and ICP-MS


346 7

%

6% 5%

Ο ICP-MS • LA-ICP-MS

ι C. Valsuani, 1925

-I

ο


4% JBingenCostenoble, 3%1908 m

2% 1%

C. Valsuani, 1929-31

0% 0%

5%

10%

15%

20%

Zinc Figure 6. Zinc-tin binary diagram comparing concentrations measured by ICP-MS and LA-ICP-MS

are evident, regardless of the analytical technique employed: L A - I C P - M S or ICP-MS (Figure 6). The first group is comprised of 6 samples corresponding to sculptures produced using the lost wax technique from 1929 to 1931 by the C. Valsuani foundry. A second group (one sculpture, The Madeleine I) contains less zinc and more tin. It was cast by the same foundry (C. Valsuani) and with the same technique in 1925. The last group contains the sculpture The Serf. This sculpture has the lowest concentrations of tin and zinc. It was cast by the BingenCostenoble foundry, using the sand cast technique. The conclusions, derived from the results obtained by L A - I C P - M S and ICPM S are identical. This preliminary study shows that our parameters of analysis and more especially our parameters of ablation for L A - I C P - M S were found suitable to study Matisse bronze sculptures. These results are promising and further study should hopefully demonstrate the usefulness of L A - I C P - M S for the characterization of other types of copper alloys.

Conclusion The analysis of copper alloys using L A - I C P - M S can be successfully undertaken, but is complicated by the scarcity of appropriate solid standard reference materials containing the proper suite of elements of interest and the heterogeneity of the ancient or archaeological materials investigated. The first problem was overcome by multipying the number of SRMs to increase the range


347


of elements and concentrations. The comparison of ICP-MS and L A - I C P - M S results showed that laser ablation sampling can be sensitive to microstructural variations in the copper alloy but this did not affect the results obtained by L A ICP-MS in such a way that it leads to erroneous conclusions. If bulk analysis is the most appropriate elemental analytical approach for ancient copper alloys, we believe we have demonstrated, at least for the case of the Matisse bronze sculptures, that L A - I C P - M S analysis is a suitable technique to address questions related to bronze sculpture production.

Acknowledgments I am grateful to Ann Boulton from the Baltimore Museum of Art for the samples from the Matisse bronze sculptures and to Jia-Sun Tsang from the Museum Conservation Institute for having involved me in the Matisse bronze sculpture project. The L A - I C P - M S laboratory at the Field Museum of Natural History was built with funds from the National Science Foundation (grant No. 0320903), an anonymous donation, and the Anthropology Alliance.

References 1.

Günther, D.; Horn, I.; Hattendorf, B . Fresenius J. Anal. Chem. 2000, 368, 4-14. 2. Gratuze, B.; Giovagnoli, Α.; Barrandon, J. N.; Telouk, P.; Imbert, J. L . Rev. Archéom. 1993, 17, 89-104. 3. Gratuze, B. J. Archaeol. Sci. 1999, 26, 869-881. 4. Gratuze, B . ; Blet-Lemarquand, M.; Barrandon, J.-N. J. Radioanal. Nucl. Chem. 2001, 247, 645-656. 5. Robertshaw, P.; Glascock, M. D.; Wood, M.; Popelka, R. S. J. African Archaeology 2003, 1, 139-146 6. Neff, H . J. Archaeol. Sci. 2003, 20, 21-35. 7. Gondonneau, Α.; Guerra, M. F.; Cowell, M.R. In 32 Symposium of Archaeometry 2000, México City, 2001, C D - R O M . 8. Dussubieux, L.; Van Zelst, L . Appl. Phys. A, Mater. Sci. Process. 2004, 79, 353-356. 9. Devos, W.; Moor, C. J. Anal. At. Spectrom. 1999, 14, 621-626. 10. Devos, W.; Senn-Luder, M.; Moor, C.; Salter, C. Fresenius J. Anal. Chem. 2000, 366, 873-880. 11. Elliott, S.; Knowles, M.; Kalinitchenko, I. Spectroscopy 2004, 19, 1. 12. Dussubieux, L.; Pinchin, S. E.; Tsang, J.-S.; Tumosa, C. S. In 14 Triennial nd

th


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Meeting The Hague, ICOM Committee for Conservation, September 2005, Preprints Volume II, 2005; pp 766-773. 13. Turhan, H . ; Aksoy, M.; Kuzucu, V . ; Yildirim, M . M. J. Mater. Process. Technol. 2001, 114, 207-211. 14. Henderson, J. The Science and Archaeology of Materials. London: Routledge, 2000; p 219.


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