Analytical
Approach
X-ray Photoelectron Spectroscopy
F
ield archaeology uncovers a variety of physical remains that vary from simple stone artifacts, to complex metal alloys, to food residues on pottery vessels. Chemists have been applying their most sophisticated analytical techniques to these artifacts since the 18th century (J) to learn where the raw materials came from, how materials were manufactured, what diet ancient peoples consumed, or how alloy composition changed with time and place. To a large extent, these endeavors have involved bulk inorganic elemental analysis. Thus the bulk composition of a metal artifact defines its alloy the bulk composition of a glass artifact can place it in a chronological and geographical context and the bulk composition of a stone artifact such as obsidian can indicate the sourrp of the raw materials (i) Organic analysis also has been used although to a more rp-strirfed family of artifacts such as fnnd rpQidup*; anr1 dvp«
Today, the workhorses of bulk inorganic analysis are neutron activation analysis and inductively coupled plasma MS, but X-ray fluorescence and, earlier, atomic absorption have also contributed much. These methods detect a wide variety of elements on small samples with exeat sensitivity and with minimal harm to the artifact Nonetheless they cannot be used to examine elements with v252°C). Moreover, Paparazzo found that the more easily oxidized iron at the joint protected the lead from oxidation as demonstrated in Figure 5 Metallic Pb(0) is found at 137 eV and lead oxides (PbO and PbO ) at 139 eV The nonjoint portion of the pipe contains aDDreciable levels of oxidized Pb (see Figure 5a and 5b the latter rtf which was rthtained after Ar+ etchincr^ In contrast the material is primarily metallic lead at the 620 A
XPS complements existing analytical techniques for the study of materials of interest to the archaeologist. It has a number of particular strengths: Oxidation states are identified, all elements are measured simultaneously, and nonmetals that are poorly studied by standard methods may be readily observed by XPS. The technique is applicable to bom organic and inorganic materials. Because it is a surface method XPS can provide considerable information about vertical structure and Ar+ etching provides a means of sequential sampling as a function of depth Alternatively angle-dependent studies can provide similar information The technique requires very little sample (milligrams to micrograms) and can be nondestructive Offsetting these advantages are poor absolute sensitivity (elements must be present at the level of 0.1-0.5% to be detected, albeit in a very small sample), difficulties in quantitation associated with matrix matching, surface contamination (a particular part of the surface may not be representative of the rest of the surface or of the bulk), and difficulties with nondestructive analysis of large objects. With due consideration for these problems XPS provides an important method for the analysis of archaeological materials.
(7) Lambert, J. B.; McLaughlin, C. D. Archaeometry 1976,18, 169. (8) Scofield, J. U.J. Electron Spectrosc. Relat. Phenom. .976,8, 129. (9) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. .nterface Anal. 1981,3, 3,1. (10) Gillies, K. J. S.; Urch, D. S. Archaeometry 1983,25, 29. (11) Wilson-Yang, K. M.; Burns, G. Can. J. Chem. .987, 65,1058. (12) Braidwood, R. J.; Braidwood, L. S. Excavations in the Plains ofAntioch; Oriental Institute Publications, Vol. LXI; University of Chicago Press: Chicago, 1960. (13) Lambert, J. B.; McLaughlin, C. D.; Leonard, A., Jr. Archaeometry 1978,20,107. (14) Levanthal, E. T.; Thompson, M.J. Intern. Inst. Conserv. Can. Gr. 1971,3,16. (15) Lambert, J. B.; Xue, L.; Weydert, J. M.; Winter, J. H. Archaeometry 1990,32, 474 (16) Bruno, P., et al. Fresenius'J. Anal. Chem. 1994,350,168. (17) Bruno, P.; Caselli, M.; Curri, M. L.; Favia, P.; Laganara, C; Traini, A. Ann. Chim. 1997, 87, 539. (18) McNeil, R Literary Research 1988,13,137. (19) McNeil, R. J. In Archaeological Chemistry III; Lambert, J. B., Ed; Advances sn Chemistry Series No. 205; American Chemical Society: Washington, DC, 1984; pp 255-69. (20) Lambert, J. B.; McLaughlin, C. D. In Archaeological Chemistry II; Cartera G. F., Ed.. Advances in Chemistry Series No. 171; American Chemical Society: Washington, DC, 1978; pp 189-99. (21) Baschenko, O. A.; Nefedov, V. I,/. Electron Spectrosc. Relat. Phenom. 1991,53, 1. (22) Polak, M.; Baram, J.; Pelleg, J. Archaeometry 1983,25, 59. (23) Gillies, K. J. S.; Moos, G. ..; Urch, D. S. Revue dArcheometrie 1980, Vol. ol. (Suppi. 121), 121. (24) Ingo, G. M.; Scoppio, L.; Bruno, R; Bultrini, G. Mikrochim. Acta 1992,109, 269. (25) Paparazzo, E.Appl. Surf. Sci. 1994, 74, 61. (26) Paparazzo E.; Moretto, L.J. Electron Spectrosc. Relat. Phenom. 1995, 76, 65.
Joseph B. Lambert is Clare Hamilton Hall Professor of Chemistry at Northwestern University, wherreh combines rerearch interests in physical organic chemistry and archaeological chemistry. Charles D. McLaughlin reReferences ceived his doctoral degree in archaeological (1) Lambert, J. B. Traces sf the Past; Addison- and organic chemistry at Northwestern, Wesley: Reading, MA, 1997. worked for several years at Chevron Research (2) Carlson, T. A Photoelectron and Auger Spec-Co., and now pursues a career as a arofestroscopy; Plenum Press: New York, 1975. (3) Carlson, T. A. X-ray Photoelectron Spectros-sional sculptor. Catherine E. Shawl is a graduate student in inorganic chemistry yt Northcopy; Dowden, Hutchinsoo & Ross: Stroudsburg, PA, 1978. western University. Her rerearch interest is in (4) Practical lurface Analysis, Vol. 1: Auger the area of archaeological chemistry. Liang and X-ray Photoelectron Spectroscopy; 2ndXue a native of Shanghai China received his ed.; Briggs, D., Seah, M. P., Eds.; Wiiey: doctoral degrre in nrchaeological ala physical New York, 1990. chemistry at Northwestern nnd is now a nu(5) Hiifner, S. Photoelectron Spectroscopy; Springer-Verlag: Berlin, 1995. clear magnetic resonance si>ectroscotist at (6) Spoto, G.; Ciliberto, E. In Modern Analyti- Alcon Laboratories Inc Address corresponcal Methods in Art and Archaeometry; Cili- dence to Lambert Debt of Chemistry 2145 berto, E., Spoto, G., Eds; Wiley: New York, Sheridan Rd Evanston IL 60208 in press; Chapter 13.
Analytical Chemistry News & Features, September 1, 1999
