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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
chlorinated dibenzofurans were present in every sample. This suggests that these compounds are formed universally during incineration processes although the combustible material and incineration conditions may vary.
LITERATURE CITED
(9) I. W. Davies, R. M. Harrison, R. Perry, D.Ratnayaka, and R. A. Wellings, Environ. Sci. Techno/., IO. 451 (1976). (,o) A. di Domenics, F. Merli, L. Boniforte, I. Camoni, A . Di Muccio, F. Toggi, L. Vergori, G. Colii, G. Elli, A. Gorni, P. Gtassi, G. Invernizzi, A. Jemma, L. Luciani, F. Cattabeni, L. De Angelis, G. Galli, C. Chiabrando, and R. Fanelii. A n d . Chem., 51, 735 (1979). F. W. Karasek, D. W. Denney, K. W. Chan, and R. E. Clement, Anal. Chem., 50, 82 (1978). W. A. Aue, C. R. Hastings, and S. Kapila, J . Chromatogr., 77, 299 (12) (1973). (13) L. C. Dickson, "Software Improvements for GC/MS/Caiculator Used In Trace Organic Analysis of Environmental Samples", Department of Chemistry Report, University of Waterloo, Waterloo, Ont., April 1979. (14) R. E. Clement, "Application of Computer Techniques to the Collection and Analysis of Analytical Data", M.Sc. Thesis, University of Waterloo, Waterloo, Ont., August, 1976.
F. N. Rubel, "Incineration of Solid Wastes, Pollution Technology Review No. 13, Noges Data Corp., Park Ridge, N.J., 1974, p 4. H. Freeman, Environ. Sci. Techno/., 12, 1252 (1978). D. A. Vaughan, P. D. Miller, and W. K. Boyd, in "Resource Recovery Through Incineration", The American Society of Mechanical Engineers, New York, 1974, p 187. K. Olk, P.L. Verrneulen, and 0. Hutzinger, chemosphere, 6, 455 (1977). H. R. Buser, A. Bosshardt, C. Rappe, and R. Lindehl, Chemosphere, 7, 417 . , . ,(1!27R\ . . ,. H. R. Buser, H. Bosshardt, and C. Rappe, Chemosphere, 7, 109 (1978). The Chlorinated Dioxin Task Force, Michigan Division Dow Chemical,
RECEIVED for review Julv 10. 1979. AcceD
R L. Rawls: Chem. Eng. News, 57 (7) 23 (1979).
of the Environment, Air Resources Branch
- -
Elemental Analysis of Thick Obsidian Samples by Proton Induced X-ray Emission Spectrometry Peter Duerden," D. D. Cohen,' Eric Clayton, and J. R. Bird Australian Atomic Energy Commission Research Establishment, Private Mailbag, Sutherland, NS W 2232, Australia
W. R. Ambrose Depatfment of Prehistory, Research School of Pacific Studies, Australian National University, P.O. Box 4, Canberra, ACT 2600, Australia
B. F. Leach Department of Anthropology, University of Otago, Dunedin, New Zealand
Proton induced X-ray emission is shown to be sultable for the analysis of thick obsldian samples and artifacts with no special treatment other than washing prlor to mounting in a sample chamber vacuum system. X-ray spectra observed unfiltered and with plastic or pinhole filters are compared. Using a pinhole filter and a single measurement of approximately 4min duration followed by thick target yield calculations, a fiR to the observed spectra gives concentrations of such elements as K, Ca, Ti, V, Mn, Fe, Rb, Y, Sr, Zr, Nb, Ta, and Pb. Results for selected source samples from the Pacific region show that the technique provides a sultable method for distinguishing between many of the sources.
Many analytical techniques have been used t o study the composition of obsidian from different source regions. In 1976, Nielsen e t al. ( I ) reported the use of Proton Induced X-ray Emission (PIXE) for such measurements. However, they used samples that were specially prepared from acid digests of 30 to 100 pg of finely powdered obsidian, a portion of which was evaporated to dryness on Nucleopore membrane filters. Because of the importance of rapid, nondestructive techniques for the study of artifact collections, we have investigated the use of P I X E measurements on thick samples with no treatment other than washing. Present address, A u s t r a l i a n I n s t i t u t e of Nuclear Science a n d Engineering, P r i v a t e M a i l Bag, Sutherland, N.S.W. 2232, Australia. 0003-2700/79/0351-2350$01.00/0
Proton induced y rays have been shown to be suitable for nondestructive analysis of obsidian artifacts ( 2 ) ,b u t when measuring X-ray yields, the greater effects of absorption in the sample and variations in surface condition raise doubts about their suitability for elemental analysis. P I X E studies of thick samples have shown that, for samples which are closely related in composition, accurate determination of many elements is possible (3)and this experience has been used for developing methods for the study of obsidian. The classification of obsidian artifacts by their chemical similarity to known sources has been used extensively in the study of trade and migration in regions such as the Mediterranean, the Americas, Japan, and Oceania ( 4 ) . We are participants in an investigation of obsidian from the latter region, with particular interest being centered on the South West Pacific and source samples are available to us from all known flows in this region. The majority of samples studied to date are from the Admiralty Group, New Britain, the d'Entrecasteaux Islands, the New Hebrides, and New Zealand. In addition, samples are available from Easter Island, Hawaii, Indonesia, Australia, and other locations. Previous work on the characteristics and geographic distributions of volcanic glasses in Oceania has been reviewed by Smith, Ward, and Ambrose ( 5 )* Data from about 40 source samples are used in this report. Although not included, measurements have also been made on a considerable number of artifacts to confirm that the methods give satisfactory results for samples of various ages which have been subjected to a variety of treatments prior to collection for study. An additional parameter of importance 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2351
is t h e angle a t which the irradiated portion of t h e sample surface rests in relation to the direction of the incident beam and emergent X-rays. Mounting techniques have been used which minimize variations in this angle. The results show that artifacts can be effectively characterized by this technique.
EXPERIMENTAL Apparatus. A 2.26- or 2.5-MeV proton beam was used to irradiate targets which were loaded on a 1-m long target holder, the position of which could be varied by a computer-controlled stepping motor. X-rays emitted by the target material were detected by an intrinsic silicon detector placed beneath the target chamber (via a 25-pm Be window). Filters could be placed in the path of the X-rays between the target chamber and the detector which has an active area of 50 mm2 and depletion depth of 5 mm. The resolution was measured to be 285 eV at the Mn K a 5.9-keV line and 325 eV at the Zr K a 15.77-keV line. The target chamber was insulated (>70 MQ) from the rest of the accelerator beam line and acted as a Faraday cup for charge collection and integration to give a measure of the total proton dose to the target. The obsidian samples varied in size from small chips to pieces approximately 50 mm in diameter and 20 mm thick. Larger samples could be mounted individually within the chamber without using the target mover. The samples were positioned so that an approximately flat surface was pressed against the mounting plate which was at an angle of 45’ to both the incoming proton beam and to the emitted X-rays. The majority of samples irradiated had smooth, cleaved surfaces, but a number were ground flat for comparison. The only target preparation was a washing of the samples in distilled water and Mallinckrodt Nanograde benzene using ultrasonic agitation. A carbon filament for flooding the insulating targets with electrons was attached to the top of the chamber. During measurements, the pressure within the chamber was approximately 0.5 mPa or less. P r o c e d u r e . The experiment was carried out in three stages. (1)Using no X-ray filter other than the 25-pm Be windows in the target chamber and detector housing and the intervening 3 cm of air. In this case the detection efficiency could be represented by: for 1.7 < E , < 20 keV t = 100 exp[-19.93 E;2835] (1) The 50 pg/cm2 gold electrode on the detector is also accounted for in this expression. Using a 20-nA proton beam and an aperture over the detector such that only one sixteenth of its area was in use, 3000 counts/s were observed and runs of 5 - p C livetime corrected total charge took approximately 4 min. (2) Using an additional 3 mm of Perspex to absorb low energy X-rays (including those from the major elements Si, K, and Ca) so that a higher beam current could be used and higher count rates obtained for X-rays from heavier elements. The detection efficiency was then: t = 100 exp[-1176 Ei3] for 6 19, but a least squares fit shows that an overall normalization factor of 15% has to be applied to reduce the calculated yields to match the measurements. Similar results have been reported by Liebert et al. (7) who measured a detector efficiency that is 13% lower than that stated by the manufacturer. They attributed this to a geo-
2352
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2353
2.26 MeV
EpZ
- CALCULATED EXPT VALUES
0
0
e
'\
x
k
&-mn io
L _ -
25
30
35
I
2 45 40
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ZOF ELEMENT
Figure 3. Calculated and measured X-ray yields for various elements in the BCR-1 standard sample
MT BAO
1
t
The variation in yield as a function of outgoing angle was also calculated. It is compared with measurements ic Figure 4 for a 2.26-MeV proton beam, the yield for each element being normalized to the mean of values observed with the target at 35", 4 5 O , and 55' to the X-ray detector direction. Large changes can occur for low 2 elements. Ratios of adjacent elements are less affected than estimates of individual element concentrations. With the sample mounting techniques used, it was estimated that h5" variation would be possible and this would give a variation in the yield from Si of f15%. The Si / K ratio would, however, vary by only *2?70. As a further check on the yield calculations, these have been carried out for up to nine elements and compared with published concentrations in Pacific obsidian ( 5 and refermces therein). Fourteen different types of obsidian were compared and the average ratio of measured to expected yields was 0.80 f 0.07. This is in satisfactory agreement with the efficiency normalization discussed above. Further information on the data analysis techniques used is to be published.
RESULTS AND DISCUSSION
IU
0 0
k 0
BO
12 0
16 0
20 0
x-Rm ELlERGY [ K E L I Figure 2. Observed and fitted X-ray spectra obtained using different filters. Top, no filter. Middle, 3 mm Perspex. Bottom, pinhole filter. Note t h e different X-ray energy scales metrical factor correcting for losses due to apertures or annular dead areas inside the detector housing. The enhancement of the X-ray production of an element A by excitation from the radiation of an element B within the sample matrix has been considered using the calculational procedure outlined by Reuter et al. (8). For the range of obsidian samples studied, the maximum contribution was 1to 2% of the yield of Mn Kn X-rays and this effect has not been considered further in the analysis.
Element concentrations derived from t h e cbserved peak areas and the necessary normalization factor are listed in Table I for 32 obsidian sources from the Pacific region. The standard errors on the element concentrations included in Table I are obtained for each element by adding in quadrature the individual standard errors for the counting statistics, for the e..perimental reproducibility, and for the absolute normalization. For most elements, the standard errors are dominated by the uncertainty in t h e absolute normalization, estimated t o be *lo%, as the statistical uncertainty in the peak areas is much less than this. Some of the more significant interelement ratios are listed in Table 11. No significant difference was observed between results from cleaved or ground surfaces and repeated measurements on some samples over several months showed variations of approximately 7% for element concentrations and less than the statistical uncertainty for values of element ratios. The majority of the source samples studied can be distinguished from each other, either by direct examination of the elemental concentrations or by comparing t h e interelement ratios. Results for K, Ca, Ti, Fe, Sr, Rb, and Zr are particularly useful for source identification. However, in some cases, other elements are important indicators. For example, yttrium concentrations are high for the Easter Island samples; a high concentration of arsenic is measured in the Weta (N.Z.) sample; Ga is high in both the Easter Island and Pitcairn samples; N b and V are not detected in most samples, b u t have a very high concentration in t h e Santa Cru;! samples and, together with Ta, are clear indicators for this obsidian Some of t h e elements listed in Tables I and I1 require
2354
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Table 11. Interelement Concentration Ratios A1 2
A9 A8 Tikopia Dire Mt. Bao Talasea Hospital Bitokara Admin Garua Voganakai Pilu Waisisi Iaupolo Igwageta Gaua Vanualava Umrei A Pam Lakou Umrei B Te Mamavai Manuga Orito Hala'uta Tefito Weta Cook's Bay Pitcairn Kermadecs Hawaii Dobu
K/Ca
Fe/Ti
Si/KU
Fe/Rb
0.178 0.123 0.399 0.189 3.58 3.55 3.57 3.39 3.49
3.05 2.80 2.00 14.7 6.3
0.73
1810 3590 1120 792 136.0 140.0 151.0 168.0 140.0
0.61
0.91 0.87 0.47 0.49 0.52 0.50 0.47
6.6
6.6 6.5 7.1
136.0
4.64 4.23 4.46
6.3 6.7
1.88
7.0
0.45 0.45 0.45 0.82
4.36 4.72 3.49
10.2 19.8 10.3
0.25 0.30 0.43
6.8
6.86 0.179 0.43 1 9.63
20.8 2.70 2.11 20.5
3.12 0.142 7.40
0.47 1.46 1.23 0.45
13.7 12.2
0.28 2.51 0.30
16.1
The Si/K ratio is obtained from peak areas,
90.0 132.0 115.0 218.0 74.2 74.2 198.0 195.0
e
OUTGOING "coM"G%X - R A Y
DETECTOR
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-
CALCULATED o x MEASURED VALUES
0.2.
6
'
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14
'
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'
22
Z OF
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' 26 ELEMENT '
'
'
20.2 13.3 169 56.0 54.5 66.7
66.5 60.6
55.7 41.5 58.3 52.5 58.8 24.3 35.4 111.0
Sr/Rb
Sr/Zr
Rb/Zr
Y/Zr
Nb/Zr
4.35 14.1 5.23 3.31 3.77 3.74 3.74 4.19 3.69 3.85 2.26 2.67 2.51 5.18 0.57 0.59 1.79 0.85 0.47 0.25
0.039 0.079 0.062 0.705 1.55 1.45 1.65 1.66 1.60 1.58 1.04
0.009 0.006
0.45 0.43 0.32
1.66
1.18
1.15 0.72 0.188 0.279 1.01 0.276 0.183 0.156 0.179 0.162 0.025 0.018
'
42
'
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0.21 0.41 0.39 0.44 0.40 0.43 0.41 0.46 0.44 0.46 0.27 0.33 0.48 0.56 0.33 0.39 0.62 0.30 0.36 0.111 0.100 2.0 0.67 5.1 1.08
0.110 0.20 0.13 0.12
0.11
1.66 2.08 0.073
0.13 0.070 0.080 0.16 0.085 0.11
0.07 0.09 0.12 0.18 0.043 0.036 0.063 0.10
0.076 0.107 0.060
0.212 0.175 0.168 0
0.16 0.76 0.18 0.069 0.43 0.044 0.07 9
0.067 0.088 0.074 0.066
0.087 0.085 0.216 0.086
The precision of elemental analysis by the nondestructive technique described here, is poorer than can be achieved by PIXE analysis of carefully prepared samples. Much of this loss of precision arises from two factors. The angular variation of attenuation of low energy X-rays affects the determination of the lighter elements which, being major constituents of obsidian, are of less importance in characterization. Also, the variability of some samples can have an important effect and sufficient measurements must be made to test this factor. Nevertheless, this nondestructive technique provides an excellent method for studying similarities and differences between obsidian samples. The extent of the differences observed for the samples studied from the Pacific region confirm the fact t h a t rapid, multielement determination by proton irradiation of complete specimens, including artifacts, is a viable method for obsidian characterization.
SAMPLE
0.4-
16.1
63.7 41.1 83.7 51.6 160.0 47.7 0.60 116 41.5 0.45 24.9 225.0 0.23 240.0 24.2 0.18 3260 6510 13.1 26.1 986.0 665.0 6.02 8.93 0 0 15.3 77.4 0.577 74.5 80.3 0.54 470.0 0.61 0.068 51.8 3390 666.0 16.9 3.32 27.9 0.115 208.0 0.015 183.0 0 0 21.8 not from elemental concentrations. 106.0
PROTON
0.6 -
Fe/Zr
46
Flgure 4. Variation of detected X-ray counts with change in angle of the sample surface relative to the direction of the incident proton beam
ACKNOWLEDGMENT We acknowledge the assistance of the staff of the Lucas Heights 3-MeV accelerator.
LITERATURE CITED careful fitting of all peaks in the spectrum to unfold the multiple peaks for each element and, in the case of an unfiltered spectrum, possible sum peaks and escape peaks accompanying the strongest element concentrations. When the requirement is to distinguish the various sources, rather than do a complete elemental analysis, it is satisfactory to consider only the stronger peaks and those weaker peaks which are not affected by such interference. An intercomparison of raw counts in 17 pulse height intervals (which are selected to include the most prominent peaks in the X-ray spectrum) is almost as satisfactory for this purpose as a complete spectrum analysis. Calculated correlation coefficients using interelement ratios, element concentrations, or just the pulse height window data provide a simple method for testing the similarity of pairs of samples.
( 1 ) Neiisen, K. K.; Hill, M. W.; Mangelson, N. F.; Nelson, F. W. Anal. Chem. 1976,4 8 , 1947. (2) Bird, J. R.; Russell, L. H.; Scott, M. D.; Arnbrose, W . R. Anal. Chem. 197a,50,2082. (3) Cohen, D. D.; Duerden, P.; Clayton, E. J. Aust. At. Energy Comm. report E468, 1979. (4) "Advances in Obsidian Glass Studies"; Taylor, R. E., Ed., Noyes Press: Park Ridge, N.J., 1976. (5) Smith, I. E. M.; Ward, G.K.; Ambrose, W. R. Archaeol. Phys. Anthropol. Oceania 1977, XII, 173. (6) Willis, R. D.; Walter, R. L. Nucl. Instrum. Methods 1977, 141, 231. (7) Liebert, R. B.; Zabei, T.; Miljanic, D.; Larsen, H.; Valcovic, V.; Phillips, G. C . Phys. Rev. 1973,A 8 , 236. (8) Reuter, W.; Lurio. A,; Cardone, F.; Ziegler, J. J. J. Appl. Phys. 1975, 46. 3194.
RECEIVED for review June 21,1979. Accepted August 31,1979. Support received from the Australian Institute of Nuclear Science and Engineering.
