Elemental Analysis of Obsidian Artifacts by Proton Particle-Induced X-ray Emission K. K. Nielson,' M. W. Hill, N. F. Mangelson,* and F. W. Nelson Departments of Chemistry and Physics, Brigham Young University, Provo, Utah 84602
Proton particle-induced x-ray emission (proton PIXE) analysls has been used for the determlnatlon of minor and trace element concentrations in obsidian artifacts. Thin targets were made from the acid digests of 30 to 100 mg of flnely powdered obsldian samples. Concentrations are reported for K,Ca, TI, Ba, Mn, Fe, Zn, Ga, Rb, Sr, and Zr. Forty-nine obsidian artlfacts from Northern Campeche, Mexico, were analyzed. Methods used for the association of obsidian artifacts with geological sources are illustrated.
During t h e past decade ( I - 3 ) , elemental analyses of obsidian archaeological artifacts have been used i n studying artifac! provenience and ancient t r a d e relationships. X-ray fluorescence ( 4 ) ,emission spectrometry (5),and neutron activation analysis (6) have all been used in t h e artifact analyses. Most x-ray fluorescence analyses of obsidians have utilized wavelength dispersive methods ( 4 ) ;however, a recent report (7) describes t h e use of an energy dispersive method. A proton particle-induced x-ray emission (proton P I X E ) method of analysis has also recently been used for t h e analysis of artifacts from Southern Scandinavia (8). The use of protons to generate characteristic x-rays i n obsidian samples offers certain advantages over conventional photon-excited energy-dispersive methods. Proton PIXE analysis is especially useful for small samples ( 9 ) , and has reasonably uniform sensitivity and limit of detectability over a wide range of elements. Since artifact analysis should either be totally nondestructive or should require very small portions of an artifact, t h e small sample capability of proton PIXE is especially important. T h e purpose of the work reported here is t o present a sample preparation method for and typical results from prot o n PIXE analyses of obsidian artifacts. Analyses have been completed for 49 obsidian artifacts coming from three sites i n Northern Campeche, Mexico, where t h e r e are no obsidian flows. T h e artifacts were analyzed to provide a basis for study of ancient obsidian t r a d e relationships ( I O ) .
EXPERIMENTAL Characteristic x-rays from the obsidian samples were induced with 2MeV protons from the Brigham Young University Van de Graaff accelerator (High Voltage Engineering, Model AN-2000), while detection of the x-rays was accomplished with a Nuclear Semiconductor Model 101 Si(Li) x-ray spectrometer. Details of this analysis apparatus have been described elsewhere (11).A sandbath was assembled by placing sand in a stainless steel, 1000-watt constant-temperature-bath apparatus manufactured by Chicago Surgical and Electrical Co. with a 700-watt auxiliary heater. Weighings requiring a microbalance were made on a Cahn Model M-10 electrobalance. Five-ml Chemware Teflon beakers were used. An amalgamator used in reducing the obsidian particle size was manufactured by Crescent Dental Mfg. Co., and stainless steel and hardened steel powdering vials were obtained from Spex Industries Inc. Nuclepore filters used for target backings were 25-mm diameter with 0.4-pm pore size. Poly(viny1 chloride) films used for target
Present address, Battelle Northwest, Richland, Wash. 99352.
backings were fabricated from toluene-diluted Tygon paint obtained from U.S. Stoneware. Nitric acid was analytical reagent grade suitable for mercury determination, as obtained from J. T. Baker Chemical Co., and all other reagents were of ACS reagent grade. Calibration of the proton PIXE system for thin samples has been described separately (12).In order to use the thin sample calibration, samples were presented to the proton beam in a matrix thin enough to allow 2-MeV protons to pass through without being stopped. This requirement set a limit on the matrix thickness of several milligrams per square centimeter. Samples were generally mounted on a thin plastic film such as Mylar, poly(viny1chloride), or Nuclepore membrane filters, and were quantitatively enclosed within the uniform proton beam, which covered a 0.44 cm2 area in the center of the target. The artifact samples consisted of 30-100 mg of obsidian powder which had been hand ground from small artifact chips using a mullite mortar and pestle. The samples were collected under the auspices of the New World Archaeological Foundation. Techniques for weighing 0.1-0.2 mg samples of obsidian powder onto the required target area involved weighing the powder sample in a small aluminum container formed from household aluminum foil over a 2-mm diameter plastic rod. The obsidian powder was dispensed from the container to the appropriate area of the target film by gentle tapping of the forceps holding the container. The container was then weighed again to determine the weight of powder deposited on the target backing. Poly(viny1 chloride) (PVC) target backings were fabricated by depositing a drop of PVC diluted in toluene on the surface of deionized water held in a 16-cm diameter crystallization dish. The toluene rapidly evaporated, after which the resulting thin plastic film was mounted on an aluminum target frame by picking up the film from beneath the water surface with the frame. Similar techniques have been reported in fabricating thin Formvar target backings (9). Several approaches to target preparation were attempted. These included: (a) direct gluing of the weighed sample powder to a thin PVC film, (b) enclosing the weighed powder between two PVC films in a sandwich configuration, (c) dissolving the weighed powder with a drop of hydrofluoric acid on the target surface, and (d) dissolving the obsidian with hydrofluoric acid, evaporating, redissolving the residue with nitric acid, and depositing a drop of the resulting liquid on a Nuclepore target backing. The weight of sample deposited on the target was generally limited to several hundred micrograms of obsidian because of the small proton beam area. Samples glued directly to the target backing resulted in x-ray spectra with high Bremsstrahlung backgrounds due to the thick matrix left by the glue deposit. Nonuniformity of the obsidian particle sizes also contributed to inaccuracies in the resulting data. Similar difficulties were encountered with the PVC sandwich targets, since the powder tended to form a thick pocket when the two PVC films adhered together. Dissolving the powder in situ on the target surface reduced much of the background problem due to target thickness, and adequately adhered the resulting crystals to Nuclepore membrane filters. Little of the silicate matrix was removed by this method, however, and the particle sizes were not sufficiently reduced. Several samples were powdered with an amalgamator in hardened steel and stainless steel powdering vials to reduce the particle size. Although this appeared to solve the particle size problem, unacceptable levels of contamination were introduced into the small samples by the vials. The sample preparation method adopted consisted of dissolving the weighed (-10 mg) sample powder in 0.20 ml of concentrated hydrofluoric acid in a Teflon beaker. The resulting solution was then heated t o dryness in a 100 "C sandbath, eliminating most of the silicate matrix by evaporation of volatile silicon fluorides. The resultant residue was dissolved in 0.50 ml of dilute nitric acid, the solution weight was determined, and 5 ~1 portions of the solution were transferred to Nuclepore membrane filters using a plastic tipped micro-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976 * 1947
Table I. Concentration Range and Relative Standard Deviations Observed among Duplicate Analyses of Fortynine Obsidian Samples
Concentration range
Element K Ca Ti
Ba Mn Fe Zn Ga Rb Sr Zr
Re1 std dev, %
4.0-1.5 (%) 0.6-0.3 (%) 1000-400 (pprn) 1400-300 (pprn) 620-370 (ppm)
5.3 7.8 8.2 11.3 4.1
0.9-0.5 (%) 70-30 (ppm) 23-3 (ppm) 210-115 (pprn) 220-145 (pprn) 190-75 (ppm)
3.7 20.0 22.8 5.4 7.4 6.9
pipet. The weight of solution deposited on the filters was determined by weighing with an electrobalance. The latter step was carried out quickly to prevent weight loss by evaporation. After the sample spots dried in a laminar flow hood, the filters were used as targets for proton PIXE analysis. Two samples of each solution were analyzed to allow estimation of the precision of the method. Blanks were prepared in a similar manner, with no obsidian powder being placed in the Teflon
beakers. RESULTS A N D DISCUSSION
Concentrations of 11elements were tabulated for each of the analyses, as indicated in Table I. Relative deviations computed from the variances among duplicate analyses are also indicated there. A typical proton PIXE spectrum from an obsidian blade is shown in Figure 1,illustrating the peak and background magnitudes associated with the concentrations listed there. Peak intensities were corrected for both x-ray absorption and proton energy loss in the moderately thick target by methods described elsewhere ( 1 2 ) . The thickness of the target was estimated by the following procedure. The measured concentration of K in each sample was corrected to a value in the mid-range typical of obsidians by empirically adjusting the target thickness and applying absorption and energy-loss corrections. A similar procedure was followed for correcting the concentration of Fe and for the s u h of the concentrations of Rb, Sr and Zr. The target thickness used in the sample analysis was taken as the average of the three thickness values determined in this way.
Some sample thicknesses have been estimated by punching from a plastic sample support a small disk which included a deposited sample. Disks were weighed on a microbalance, and the sample weight was the difference between the disk weight and a blank disk weight. Sample weight and an estimate of sample area were then used to calculate the sample thickness in mg/cm2. Table I1 contains literature values (13) and data from the proton PIXE analysis of USGS rock standards AGV-1 (andesite), GSP-1 (granodiorite), and G-2 (granite). Thickness corrections were required for proper data analysis of these standard samples, and the methods used were similar to those discussed above for the obsidian samples. The proton PIXE values and the literature values agree reasonably well with a few exceptions. Potassium values from proton PIXE analysis are high and the values of Ca and Ti also tend to be high. These discrepancies may arise, in part, because the light elements are most sensitive to the large target-thickness corrections. Targets for these standards were two to three times thicker than the targets for obsidian artifact samples. The Zr value for AGV-1 is in good agreement with the literature value while Zr values for both G2 and GSP-1 are less than one-half the literature values. These results may be related to the facts that Zr in the AGV-1 andisite is homogeneously distributed through the rock structure while Zr in the G-2 and GSP-1 granites is found primarily in zircon which is present as zircon grains in the standard sample. Zircon is difficult to digest, and the low Zr values could be the result of incomplete digestion of the zircon. It could also be that the dense zircon grains were not properly mixed into the small sample (45 mg) taken. In contrast to granite, obsidian is a homogeneous solid solution and more nearly like the andesite than the granites. The Ba and Ti spectral peaks are more reliably resolved for obsidian samples than for these standards because, in the obsidians studied, the Ba concentrations were equal to or greater than the T i concentrations. These comparisons with standard rock samples indicate that the sample digestion and proton PIXE target preparation procedures are acceptable. The use of thinner targets or more exact target-thickness corrections would lead to better agreement between proton PIXE analysis and literature values for these standards. The concentrations given in Figure 1have precisions as are indicated in Table I, accuracies which are dependent on the
Table 11. Proton PIXE Analysis and Literaturea Values for Element Concentrations in USGS Rock Standards AGV-1, G-2, and GSP-1 AGV-1
Element (units)
G-2
GSP-1
Proton PIXE Flanagan" Proton PIXE Flanagana Proton PIXE Flanagana K (%) 3.4 f 0.2 2.40 6.3 f 0.5 3.74 6.5 f 0.2 4.99 Ca (%) 3.5 f 0.1 3.50 1.5 f 0.02 1.39 1.8 f 0.1 1.44 Ti (ppm) 6600 f 400 6190 2700 f 80 2780 4400 f 150 3990 Ba (ppm) 2600 f 2 1870 1900 f 150 1300 Cr (ppm) 18 f 6 12.2 763 230 f 10 263 320 f 10 331 Mn (ppm) 660 f 1 4.75 1.7 f 0.02 1.85 2.9 f 0.1 3.03 Fe (%) 4.4 f 0.02 Cu (ppm) 60 f 12 59.7 10 11.7 34 f 4 33 84 80 f 13 85 94 f 6 98 94 f 8 Zn (ppm) 67 160 f 20 168 190 f 10 254 63 f 1 Rb (ppm) 479 220 f 10 233 700 f 11 657 430 f 40 Sr (ppm) 225 120 f 20 300 190 f 30 500 Zr ( P P ~ ) 200 f 12 Ti (ppm)' 6300 f 400 6190 2900 f 78 2780 4500 f 200 3990 Ba (ppm) 1208 1208 1870 1870 1300 1300 a USGS values are accepted as those published by Flanagan (13). Average value and standard deviation of two determinations. Ti and Ba x-ray peaks interfere. These values are obtained by fixing Ba to the value reported by Flanagan ( 1 3 )and calculating Ti values. 1948
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13,NOVEMBER 1976
CHANNEL
Flgure 1. Proton PlXE spectrum of an obsidian blade
Sr
A
Dribilnocac
0 Santa Rosa Xtarnpak
Sa'n Bartolorno
L v ;B 80%
80%
Zr
0% Strontium
v
Rb
Figure 2. Relative concentrations of Rb, Sr, and Zr in 49 artifact samples compared to those reported for several possible obsidian sources (3). Points represent artifact data and dashed lines define the extent of source data for the sources indicated.
sample thickness used for correction calculations, and values which are in proper relative proportion to each other. Elemental concentrations in the artifacts were measured with sufficient accuracy and precision that each artifact could be associated with one of the several obsidian flows in highland Guatemala by comparison of relative element concentrations
in the artifacts with those in the obsidian source samples (3, 4).
Figure 2 is a triangular plot of the relative concentrations of Rb, Sr, and Zr. This representation has been used by others (14) and displays each of the three element concentrations as normalized to the sum of the three. Dashed lines enclose areas
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
1949
defined by obsidian source data (3, 4 ) for the sources indicated. Points plotted represent artifacts analyzed in this study. Concentrations of Mn, Fe, and Ba, compared in a similar way, support the assignment of sources inferred from Figure 2. In this study, obsidians were successfully analyzed by proton particle-induced x-ray emission analysis. The minor and trace element data from the analyses have proved useful for identifying the source of obsidian used for artifacts. ACKNOWLEDGMENT
The authors appreciate the help of G. F. Walters in the proton PIXE analyses. LITERATURE CITED R. F. Heizer, H.
Williams, and J. A. Graham, Contrib. Univ. Calif. Arch. Res. Fac.. 1. 89 (1965). N. Hammond, Science, 178, 1092 (1972). F. H. Stross, T. R. Hester, R. F. Heizer, and R. N, Jack in "Recent Advances in Obsidian Glass Studies: Archaeological and Geological Perspectives", R. E. Taylor, Ed., (in press). D. P. Stevenson, F. H. Stross, and R. F. Heizer, Archaeometry, 13, 17 (1971).
J. R. Can and C. Renfrew, Proc. Prehistoric SOC.,30, 110 (1964). F. Asaro, H. R. Bowman, and F. Stross in "Nuclear Chemistry Annual Report", D. L. Hendrie, C. F. Tsang, and A. Zalkin, Ed., Lawrence Berkeley Laboratory Report LBL-2368, 383 (1973). D. E. Nelson. J. M. D'Auria, and R. 8. Bennett, Archaeometry, 17, 85
(1975). (8) M. Ahlberg,R. Akselsson, B. Forkman,and G. Rausing, Archaeometry, 18, l(1976). (9) T. B. Johansson, R. E. VanGrieken, J. W. Nelson, and J. W. Winchester, Anal. Chem., 47, 855 (1975). (IO) F. W. Nelson, K. K. Nielson, N. F. Mangelson,M. W. Hill, andR. T. Matheny, Am. Antiq., to be published, 42, (1977). (11) N. F. Mangelson,M. W. Hili, K. K. Nielson, D. J. Eatough, J. J. Christensen, and R. M. Izatt, "Proton-Induced X-Ray Emission Analysis of Pima Indian Autopsy Tissues", Brigham Young University, Provo, Utah, 1975, to be published. (12) K. K. Nielson, M. W. Hill, and N. F. Mangelson in "Advances in X-Ray Analysis", Vol. 19, R. W. Gould, C. S. Barrett, J. B. Newkirk, and C. 0. Ruud, Ed., KendaWHunt Publishing Co., Dubuque. Iowa, 1975, p. 51 1. (13) F. J. Flanagan, Geochim. Cosmochim. Acta, 37, 1189 (1973). (14) J. A. Graham, T. R. Hester, and R. N. Jack, Contrib. Univ. Calf. Arch. Res. Fac., 16, 111 (1972).
RECEIVEDfor review February 9, 1976. Accepted July 15, 1976. This work was supported in part by an ASBYU College Council Research Grant.
Determination of Traces of Phosphate by Thin Layer X-ray Fluorescence Ghyslain Dub&' Gaston Boulay, and Frank M. Kimmerle" Laboratoire d'Analyse, Department de Chimie, Universite de Sherbrooke, Sherbrooke, Quebec
Microgram amounts of phosphorus can be determined after precipitation of orthophosphates as quinlne molybdophosphates. Subsequent redissolutlon and colorimetric analysls of the molybdenum blue, fiuorometrlc analysis of the qulnlne, AAS, and polarographic analysis of the molybdenum ion compare favorably with standard methods. However, x-ray fluorescence determination of phosphorus measured directly on a Millipore membrane proved to be most immune to interferences and necessitated a mlnimum of transfer steps. The relative standard deviatlon ranged from 0.5 to 5 % , the detection limit being less than 0.1 pg cm-2 for 100-s counting time, allowing thus the detection of less than 20 ppb P in a 50-ml sample.
Most sensitive analytical techniques for the determination of orthophosphates in aqueous solutions are based on the formation of the 12-molybdophosphorc acid (12-MPA). Spectrophotometric analysis of this yellow complex, or its reduction products, the molybdenum blue, in aqueous media, or (after extraction) in an organic solvent, constitutes the most widely accepted technique ( I ) . With the use of automatic analyzers, it should eventually be possible to reach acceptable levels of reproducibility and interlaboratory agreement below the 1 ppm range (2). Indirect determination of phosphates by the determination of an atom or molecule associated in a stoichiometric ratio with P in a readily isolated compound is also of interest. It then becomes essential to form a readily extracted complex, e.g., 12-MPA ( 3 ) , or a precipitate, e.g., AgzTlP04 ( 4 ) or ( C ~ ~ H Z ~ O Z(PM012040)2 N ~ H ~ ) ~( 5 ) .The preconcentration thus
Present address, Aluminium Company of Canada Limited, Analytical Centre, Arvida, QuBbec. 1950
affected as well as a possible stoichiometric ratio >> 1 often outweigh the disadvantages of additional manipulation required. Although x-ray fluorescence techniques have been used, the lower concentrations previously attained, e.g., 1to 10 mg (4)and 0.002% (6) were unsuitable for trace analysis. A rather complicated chemical separation technique suggested by Luke (7) allowed 1 to 40 pg P to be precipitated as Be3(P04)2 and detected as P. Leyden et al. ( 3 ) recently detected as little as 15 ppb P with a relative standard deviation, CT, of 3%. The 12-MPA complex was extracted with ethylacetate, adsorbed onto a specially treated silica gel, pressed into pellets using a cellulose binder, and analyzed for molybdenum. The direct method reported here uses the Kcr line of phosphorus and a very simple sample preparation technique. It relies on the very low solubility of the quinine phosphomolybdate complex studied by Kirkbright et al. ( 5 )and is applicable from 20 ppb to 3 ppm P using 50-ml sample volumes. EXPERIMENTAL Reagents and Materials. All chemicals used in this investigation were reagent grade, all solutions were prepared using deionized water, and all glassware was cleaned with sulfochromic acid followed by soaking in a nonphosphate detergent, Contrad-70. The stock phosphate solution, 25 fig P/ml, was prepared by dissolving 0.1258 g NaH2P04 2Hz0 and diluting to 1000 ml. Standard solutions were prepared daily by volumetric dilution. A stock molybdate solution, about 200 mM Mo, was prepared by dissolving 35.0 g (NH4)6Mo702~ 4H20 in about 500 ml water, adding sufficient HzS04 to obtain a final solution 1 1. 0.5 M H2SO4. A stock quinine solution, 20 mM quinine sulfate, was prepared by dissolving 8 g of quinine sulfate in a minimum of 50%sulfuric acid and diluting to obtain a final solution 500 mlO.05 M H2S04. Sample Preparation. The procedure employed for the precipitation of the phosphate as quinine molybdophosphate follows that proposed by Kirkbright et al. ( 5 )for the spectrofluorometric determination. To a 50.0-ml sample containing typically less than 0 pg P (as orthophosphate), is added 5 m15 M &Sod, 5 ml stock molybdate
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
