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EAG Scientists Present Research at SciX 2024
EAG Senior Scientists Andrew Zipkin and Gideon Bartov recently presented a research poster at The Great Scientific Exchange 2024 conference in Raleigh, North Carolina detailing their work on compositional analysis and provenience studies of natural iron oxides pigments. These mineral pigments, commonly referred to as “ochre” by archaeologists and museum conservation experts, have become a major area of investigation in the archaeological sciences over the last twenty years. The most frequent research objective for archaeological ochre is determining the source of the pigment and that source’s relationship to the archaeological excavation or rock art paintings where the pigment was found in present day. This type of research is known as “sourcing” or “provenience” studies, and typically relies on measuring concentrations of elements in the ochre to develop a unique chemical “fingerprint” that allows scientists to match archaeological ochre with a geological deposit of origin on the landscape. Similar studies have been carried out for decades on other materials of archaeological interest like obsidian, pottery, and smelted metals.
The SciX poster, titled “Introducing “Full Scan” Glow Discharge Mass Spectrometry of Iron Oxides for Provenience Studies of Ochre Pigments”, builds on Zipkin’s dissertation and postdoctoral research on ochre provenience and Bartov’s recent method development work at EAG on oxide-specific data reduction procedures for Glow Discharge Mass Spectrometry (GDMS). The goal of this project was to apply GDMS, a technique with which EAG has decades of experience, to iron-containing oxide reference materials to determine how accurately and precisely the technique could measure a “full scan” of ~69 elements. Provenience studies of ochre often benefit from measuring as many elements as possible because it is usually not clear at the outset of a study which elements will be important for distinguishing among ochre sources. GDMS potentially offers major advantages over other techniques already used in ochre provenience because it is well-suited to the analysis of solid samples with minimal preparation, can easily measure up to 69 elements in under one hour per sample, and does not require running a calibration curve at the beginning of an analysis. Check out the conference abstract and the poster below to learn more about this latest method development project from the EAG Laboratories Purity Survey group.
Abstract Text
Provenience, or “sourcing”, studies of ochre pigments have proliferated since the mid-2000s and have become an accepted part of the archaeological sciences. Most such studies rely on elemental compositional data and multivariate statistical analysis to distinguish between sources of ochre and assign unknown archaeological pigments to their origin. Elemental concentration data must be precise and reproducible with consistent biases for samples of the same matrix, but absolute accuracy is not essential.
The most prominent techniques used for ochre compositional data have been SEM-EDS, EPMA, XRF, NAA, and variants of ICP-MS. This method development study represents the first application of Glow Discharge Mass Spectrometry (GDMS) to iron-bearing oxide reference materials using an oxide-specific set of relative sensitivity factors (RSFs). With further development, especially analysis of previously characterized geological and archaeological ochres, GDMS will be well-positioned to complement or replace the techniques noted above in the provenience analysis of ochres.
We analyzed five commercially available reference materials – NRC IOC-1 (65.66 wt% Fe), NIST SRM 692 (59.61 wt% Fe), USGS BCR-2 (9.63 wt% Fe), NIST SRM 679 (9.05 wt% Fe), and OREAS 25b (8.53 wt% Fe) – using a Nu Instruments Astrum GDMS (Ametek). The reference materials were analyzed using an acid-cleaned tantalum electrode. The instrumental conditions were set to 3.0 mA and 1.0 kV, with variable argon flow to maintain these conditions. Data reduction used oxide-specific RSFs developed in-house. As applied here, GDMS is a solid sampling technique that yields elemental concentration data similar to results from non-matrix matched calibration LA-ICP-MS, which has previously been successfully deployed in multiple ochre source discrimination studies.
There are 49 elements that have certificate values for at least two of the five reference materials. Of those 49 analyzed elements, 16 were measured within 10% recovery, 33 were within 25% recovery, and 48 were within a factor of 2 (100%) recovery of the certified values – the accepted bias range of the GDMS methodology used. Phosphorous had 215% recovery from the reference materials. More importantly, the relative external variability of 47 out of the 49 elements was better than 50%, with 26 being better than 25%.
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