New Programs: Instrumental Neutron Activation Analysis (INAA) and Inductively-Coupled Plasma Mass Spectrometer (ICP-MS)

My research with Dr. Neff using instrumental neutron activation analysis (INAA) demonstrated surprising compositional diversity and lack of patterning of form and decoration with composition in ceramic assemblages from sites in the American Southwest (Larson et al. 1996; Neff et al. 1997; Neff and Larson 1997). We attribute this patterning in the ceramic data to selection for interaction without local specialization in agriculturally risky environments. The purpose of our long-term comparative analysis is to provide a larger and more detailed context for our earlier claims. We have argued that one contribution that INAA can make to scientific archaeology is to provide a means to monitor phenotypic characteristics of past human populations. Specifically, INAA data on ceramics monitor how humans used certain resources (clays and tempers) and how they moved materials (finished ceramics) across the landscape. Since evolutionary theory specifies a set of mechanisms that can produce change in human phenotypes, it must be possible to derive expectations from evolutionary theory about pattern in ceramic compositional data produced by INAA. Put another way, by giving us a means to monitor certain aspects of human phenotypes, INAA gives us a tool for testing the predictions about how evolutionary processes shaped the archaeological record.

An opportunity has arisen for archaeology students and faculty in the Department of Anthropology to become part of a strong program in Archaeometry in association with the School of Natural Sciences at CSULB. The research scientists in biology and geology departments with whom our students and we will collaborate are established, internationally recognized experts in the field of material analysis. Our role in the joint program will be to process a wide variety of archaeological materials for joint student and faculty projects using an inductively-coupled plasma mass spectrometer (ICP-MS). The ICP-MS would provide an extremely valuable complement to existing strength in instrumental neutron activation analysis (INAA) for archaeological applications. Like INAA, ICP-MS is capable of high-sensitivity, high-resolution, multi-element characterization of geological materials, including glass, metal, lithics, and ceramics, which are often the focus of archaeological provenance investigations (see Kennett et al in press). However, many elements whose nuclear properties prevent their determination or limit the resolution achievable with INAA can be determined with great precision using ICP-MS. As a result, addition of ICP-MS would roughly double the number of elements that could be reported from the analysis of archaeological materials using INAA. In addition, ICP-MS permits the direct determination of isotope ratios, and this would further enhance the provenance-determination capabilities.

Mallory-Greenough et al. (1998) summarize a series of comparisons between ICP-MS and INAA by stating that on the basis of precision and accuracy, number of elements, quantity of sample required and cost of analysis, ICP-MS represents a viable alternative to INAA. Compared to INAA, ICP-MS has greater sensitivity in about half the elements that can be determined in single-element standards (Parry 1991:Table 1.1) and greater sensitivity in about 85% of the elements determined in a silicate rock (Parry 1991:Table 1.2). Superior performance relative to INAA in the multi-element sample matrix is due to the relative freedom from spectral interference. Other techniques, such as ICP-AES, flame AAS, and furnace AAS, also do not achieve the typical sensitivity levels of ICP-MS in single-element solutions (Parry 1991:Table 1.1; Pollard and Heron 1996:Figure 2.5; Tykot and Young 1996), and the problem of spectral interference is equally severe. Precision of ICP-MS for most elements are in the 2 - 3% range (Pollard and Heron 1996:34), which compares favorably with the precision of INAA for most elements. Once samples are prepared, collection of compositional data via ICP-MS can be extremely rapid, a complete spectrum requiring only a few minutes to accumulate, and this contributes to lowering the per-sample cost of analysis. Another advantage of ICP-MS is that, besides providing elemental analysis, it also provides a capability to determine isotopic ratios, such as lead-isotope ratios used in metal provenance studies (e.g., Sayre et al. 1992). Precision, however, is much lower than with thermal ionization mass spectrometry (TIMS) used in most studies, and this has so far limited this kind of application of ICP-MS. Nonetheless, isotope ratio studies are a promising area for future archaeological applications of ICP-MS characterization.

One of the main disadvantages of ICP-MS compared to INAA for the analysis of silicate materials such as obsidian, chert, or ceramics, is that sample preparation is more time-consuming and requires greater care and expertise. Whereas preparation of silicate samples for INAA involves only powdering and encapsulation in polyethelene or quartz vials (Glascock 1992), silicate samples for ICP-MS analysis must be completely digested through the use of heat and/or concentrated acid attack (Bryan et al. 1996; Mallory-Greenough et al. 1998; Tykot and Young 1996). Means for analyzing solid samples via ICP-MS include slurry nebulization and laser ablation (Pollard and Heron 1996). The laser ablation approach offers the potential for spot analysis in a mode similar to an electron microprobe, but with dramatically enhanced results. If possible, it is our goal to outfit the CSULB ICP-MS with laser-ablation capabilities.

In sum, then, ICP-MS is a powerful analytical technique that is rapidly gaining popularity for compositional characterization of a wide range of materials, including archaeological artifacts. It is not, however, an alternative that will inevitably and easily replace other techniques, particularly INAA, that are currently used more widely in provenance studies. ICP-MS and INAA both have strengths and weaknesses, and thus, they should be seen as complementary analytical techniques in a comprehensive archaeometric characterization program.