Spark Source Mass Spectrometry in Archaeological Chemistry

4 Spark Source Mass Spectrometry in Archaeological Chemistry A. M. FRIEDMAN and J. LERNER

Downloaded by CORNELL UNIV on October 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0171.ch004

Chemistry Division, Argonne National Laboratory, Argonne, IL 60439

Techniques for analysis and sample preparation have been developed for using spark source mass spectrometry (SSMS) to study archaeological samples. Comparative studies of neutron activation and SSMS on identical samples have been made. The technique is used to determine the ores of origin of two series of early Peruvian artifacts.

nalysis of metal artifacts has long been a major tool i n archaeological studies. Since by their very nature metal samples are electrically conducting, they are also well suited for analysis by spark source mass spectroscopy ( S S M S ) . For several years we investigated the use of this technique and have applied it to archaeologically interesting samples. Figure 1 is a diagram of the spectrograph. The sample is formed into two small electrodes (2 X 2 X 10 mm each), and a pulsed radiofrequency spark is induced between them, volatilizing and ionizing some of the material. The resulting ion beam is accelerated, defined, mass-analyzed, and monitored. The individual masses then are deposited on a photographic emulsion. A series of exposures are made in which the intensity varies over a factor of 10 ; the plate then is developed and scanned by a microdensitometer. This data is fed into a computer which is programmed to take into account elemental isotopic ratios, sensitivity factors, and machine parameters and which then generates an analysis of the electrode material. In general we have determined at least 11 elements i n each sample with sensitivities of the order of 0.1 ppm or better. The SSMS is particularly well suited for the analysis of archaeological samples. Relatively small amounts of material are required for the electrodes (ca. 100 mg) yet the photographic plate records simultaneously all elements, including those not specifically sought. Thus it is 6

This chapter not subject to U.S. copyright. Published 1978 American Chemical Society

Carter; Archaeological Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


4.

FRIEDMAN AND LERNER

Spark Source Mass Spectrometry

MAGNETIC

Figure 1.

71

ANALYSER.

Mass spectrograph

possible to re-examine the plate at a later time to check for unexpected abundances or to verify that a specific element is indeed absent. Since a pulsed R F source is used for sparking rather than a continuous arc, the electrodes remain relatively cool. Tests have shown that the concentration of even volatile elements (e.g., mercury) is unaffected by the sparking. Furthermore, the radiofrequency interruptions (ca.

Figure 2. Sample electrode preparation apparatus


72

ARCHAEOLOGICAL

Table I.

Results of Analysis of CAS


CAO

Element

ppma actual

ppma exp.

Sb Bi Cr Ga Pb Ag Sn

261 152 269 319 123 277 257

282.6 98.1 495.2 799.6 114.7 312.8 292.1

CHEMISTRY—II

actual exp.

actual exp.

xioo

ppma actual

ppma exp.

92.4 154.9 54.3 39.9 107.2 88.6 88.0

22.4 24.3 36.7 36.4 15.4 35.3 26.8

59.7 22.4 142.3 187.0 26.3 52.6 57.3

xioo 37.5 108.5 25.8 19.7 58.6 67.1 46.8

500 kHz) cause the spark to move erratically over the surface of the electrodes—a desirable feature for samples that are rarely uniformly homogeneous. While the analyses may be relatively imprecise, in general high orders of accuracy are not required for most archaeological applications of SSMS. The entire process of electrode preparation, spectrometry, plate development, scanning, and data card punching takes about one day per sample.

Table II. Abundance (ppm)

Error (%)

100-1000 10-100 4-10 1-3

15 20 50 80

Table III. Mass Sample

Ag

Co

Cr

34-21-35AW 34-21-35L 34-21-35Z 43-21-16D 66-21-00 30-05-10As 90-20-01-S 16-ll-50Es 16-12-58Hs

77 150 42 3.3 45 44 0.26 84 98

0.38 ND ND 1.1 0.63 0.60 14 7.6 21

0.35 0.35 0.12 5.3 0.32 0.50 1.4 0.41 0.54

Comparison of SSMA and

Spectrometry

0

Fe

Sb

Sc

0.14 0.66 200 0.26 320 170 ND 170 19 0.36 1.5 470 1.8 0.30 55 32 1.3 0.42 26 0.64 0.61 7200 0.28 0.22 0.21 27000 320

° N D = Not Detected.


Se 0.89 3.1 1.4 10.0 2.4 4.1 2.0 0.78 7.2

4.

73


FRIEDMAN AND LERNER

C A Series of Copper Standards


CA6 ppma actual

ppma exp.

2.61 2.74 3.67 4.56 2.15 3.53 3.21

5.28 1.95 8.10 45.98 2.65 5.80 4.40

CA8

CA7 actual exp.

xioo

ppma actual

ppma exp.

49.4 140.5 45.3 9.9 81.1 60.9 73.0

1.57 0.91 2.44 2.73 1.23 2.36 1.61

4.70 1.47 7.58 25.93 2.56 5.30 3.11

actual exp.

xwo 33.4 61.9 32.2 10.5 48.1 44.5 51.8

ppma actual

ppma exp.

0.52 0.61 1.22 2.73 0.92 0.12

0.86 0.29 3.28 14.19 1.02 3.82 1.08

—

actual exp.

xwo 60.5 210.3 37.2 19.2 90.2 3.1

—

Sample Preparation It is clearly desirable to minimize the amount of material withdrawn from an archaeological sample for analysis. I n some cases it has been possible mechanically to shear off slivers of metal that can serve directly as electrodes. I n other cases the material is somewhat granular but can be compressed i n a die to form cohesive electrodes. However, under many circumstances there is no alternative to melting and casting, and for this the apparatus shown diagrammatically i n Figure 2 was developed. The sample is maintained under a nitrogen atmosphere at the base of the quartz melting tube. Application of an R F current to the induction coil melts the material. Suction is immediately applied to the top of the inner quartz tube to draw a layer of metal over the tantalum wire core; contact with the cold quartz walls solidifies the sample immediately. Active stirring of the molten bead by the R F field and subsequent rapid chilling to produce a fine-grained solid help to promote homogeneity within the electrodes. The entire melting and solidification process requires only a few seconds and consumes roughly 25-100 m g of sample. After further cooling the inner quartz tube is removed and broken to release an electrode of appropriate size and shape. Only a small N A A (concentrations in PPMA) Neutron

Activation

0

Ag

Co

Cr

Fe

Sb

Sc

Se

120 140 180 7.2 45 78 ND 8.8 540

ND 0.55 0.52 0.14 ND 0.47 0.26 3.2 24

ND ND ND 1.5 0.38 1.9 ND ND ND

ND 1400 960 570 100 720 250 240 32000

1.5 ND ND 0.24 0.44 3.6 0.71 0.52 1300

0.034 0.51 0.22 0.38 ND 0.06 0.30 0.022 ND

ND 0.56 0.16 ND ND 0.86 0.15 ND 0.25


74

ARCHAEOLOGICAL

CHEMISTRY

II

amount of etching and pre-sparking is required to remove surface contamination. Although the use of a tantalum core precludes analysis for this element in the sample, the determination would be suspect in any case because virtually the entire ion source is constructed of tantalum.


Data Analysis W h i l e the sensitivities for each of the elements are relatively uniform, there are differences caused by variations in ionization efficiency, volatility, and overlap of isotopic mass lines. These effects require the calibration of the technique with a variety of standards covering a range of concentrations, matrix materials, and other trace elements. Table I contains a set of analyses of copper matrix standards and compares the spark source mass spectrograph results to the true values. A series of experiments was performed on standard electrodes containing a variety of elements and yielded the average errors shown in Table II. The errors are a function of elemental abundance, and in the p p m range the analyses are in general only roughly correct. A set of samples was analyzed by neutron activation analysis ( N A A ) and spark source mass spectrometry. Some of these results are shown in Table III. The differences correspond to the range of accuracies shown in Table II except possibly for some of the iron results. In those cases it was felt that the N A A samples were contaminated. Application to Archaeology In previous reports (1,2) the origins of copper samples have been traced on the basis of NAA-determined impurities to three kinds of ore: native copper ( C u metal) ores, oxidized ( C u O or C u C 0 ) ores, and reduced ( C u S or C u M S ) ores. This data has been used as the basic parameter of a computer program (1) which calculates the probability of a sample originating in a given ore type. The program generates this probability as a linear combination of uncorrelated individual impurity distributions. This calculation has been described in Refs. 1 and 2, and the Fortran listing is available on request. W e have been investigating a similar scheme set up on the basis of SSMS-determined impurities; presumably the classification might be even more definitive since more trace elements can be estimated. The mass spectrometer data for 44 type I ores is presented in Figures 3 and 4; for comparison the N A A data for 214 type I ores is shown in Figure 5. In general, both the location of the peaks and the width of the distributions for corresponding elements are quite similar. Since there is reasonable agreement between the two methods of analysis, the SSMS data was processed using the program described 3

2


4.

FRIEDMAN AND LERNER

MASS TYPE


75

MRSS S P E C TYPE 1 PS

SPEC 1 AG

CD LJ Q_

-80


LQG

CONCENTRATION MASS TYPE

-7.0

LBG

-B.D

-5-0

-t.O

-3-0

-ZO

-LO

CQNCENTRflTIQN MRSS S P E C TYPE 1 CQ

SPEC 1 BI

LJ Q_

HL LQG

LBG

CBNCENTRflTIBN MASS TYPE

Figure 3.

MASS TYPE

SPEC 1 CR

LQG CONCENTRATION

CQNCENTRflTIQN

'

LQG

SPEC 1 FE

CQNCENTRATIQN

Impurity distribution in type I ores—SSMS data

above. Of the 44 type I ore samples only two (5%) were classified as other types. Establishment of SSMS probability tables for the three ore types and the use of 11 trace elements should result in improved identification of the sources of copper.


76

ARCHAEOLOGICAL MASS TYPE

SPEC 1 PB

CHEMISTRY MASS TYPE

II

SPEC 1 SB

CJ

J1ML


"LOG

CBNCENTRFIfI8 N MASS TYPE

LQG

SPEC 1 SC

CQNCENTRflTIQN MASS TYPE

LBG

C 8NCENTRAfI8 N MASS TYPE

SPEC 1 SE

LBG C B N C E N T R A T I B N

SPEC 1 SN

n m 1111 m. n LBG C B N C E N T R A T I B N

Figure 4.

Impurity distribution in type I ores—SSMS data

In addition, a series of early Peruvian artifacts were analyzed and assigned to ore types by the program. The results are shown i n Table I V . The early samples have a high probability of being simple type I ores, and the latter are apparently more complex mixtures.


4.

FRIEDMAN AND

TYPE

LJ Q_

1.1


0.0

-••0

-«.0

-7.0

LBG

1 AG

j

Of CJ

. J

S.O

-5-0

-4.0

77


LERNER

TYPE

1 CB

CJ CE LJ eo.o CJ LJ Q_

-II

-t.0

-|-0

CBNCENTRATIBN TYPE

0.0

-10.0

-«.0

. 1

-0-0

-7.0

LBG

-0-0

-4.0

-4.0

-1-0

-CO

-1.0

CBNCENTRATIBN

1 FE

TYPE

1 HG

u

en

CJ LJa.o CJ

CJ

at.

cz

LJ Q_

LJ Q_

-10.0

-0.0

-i.O -7.0

LBG

-S.O

-5.0

-4.0

-1.0

-t.0

-L0

CBNCENTRATIBN TYPE

0-0

-10.0

-0.0

-0.0

-7.0

LBG

ffkLu

o -4.o -i.o -eo

CBNCENTRATIBN

1 SB

TYPE

CJ

1 SC

CJ CH E-

z

Uei.o CJ

on

LJ •_

•>nfThJWjH.. -10.0

-1.0

-0-0

-7.0

LBG

-i.O -S.0

-4.0

-0.0

-t-0 -1.0

CBNCENTRATIBN

Figure 5.

0-0

-10.0

-0-1

-4-1

-7.0

LBG

-»••

LttL. -4.0

-0.1

-0-8

-14

•••

CBNCENTRRTIBN

Impurity distribution in type I ores—NAA data

These results imply a sharp change i n metallurgical patterns between the late intermediate and late horizon periods, and we have now started to study a series of Moche artifacts obtained from Gary Vesculius i n order to investigate this transition further. W h e n the artifacts i n this set are grouped into broad time periods, our preliminary results indicate that these also show a transition between the use of type I and type II copper ores. Of the seven artifacts believed to originate before 500 A D , 57% had a high probability ( > .95) of being made from type I ores, and 4 3 %


78

ARCHAEOLOGICAL

CHEMISTRY—II

Table IV. Selected Moche Results and Localized Chronological Table Sample No.

Era


4645 4891-32 169897 5712-16 A1936-32 2200 170082

Typel

early intermediate (Moche) early intermediate (Moche) early intermediate (Moche) early intermediate (Moche) late horizon or late intermediate late horizon (Inca Bronze)

?

Dates 1450-1532 1200-1450 80CM200 0-800

Era late horizon late intermediate middle horizon early intermediate

91.01 80.69 71.30 93.95 23.24 12.07 25.77

Type 2 4.56 10.49 27.77 5.74 38.72 24.59 33.52

Typei 4.43 8.83 0.94 0.31 38.07 63.35 40.71

Style Inca Chiamu, late Chancay, lea Coast Tialuanco, Hicara Mochica, early Lima, Nazca

had a high probability ( > .95) of being made from type II ores. Of the 15 artifacts believed to have been made between 500 and 1000 A D , 40% had a high probability( > 0.9) of being made from type I ores, and 60% had a high probability (0.7-.95) of being made from type II ores. Of the seven artifacts studied which were made after 1000 A D , only one (or 13% ) had a high probability (0.9) of being made from type I ores, and the remainder (87%) had high probabilities (0.6-.95) of being made from type II ores. Since the artifacts were chosen at random from the set, this variation in ore types should indicate the general trend, but at present the sampling is still too small for a definite conclusion. However, these two examples do indicate the general utility of the method. Acknowledgments Without the efforts of George Lamich and Argonne Undergraduate Honors Program students Cathy Richardson, Charles Shuey, Robert Bowman, and Gary Sanford this report would not have been possible. We also wish to thank the Field Museum for making available the Moche samples. Work performed under the auspices of the USERDA.

Literature Cited 1. Friedman, A. M., Conway, M., Kastner,J.,Milsted,J.,Metta, D., Fields, P., Olsen, E., Science (1966) 152, 1504. 2. Bowman, R., Friedman, A. M., Lerner,J.,Milsted,J.,Archaeometry (1975) 17, 157. RECEIVED

September 19, 1977.


Spark Source Mass Spectrometry in Archaeological Chemistry

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