Quantifying Trace Elements in Historical Objects with SEM XRF

Studying the trace elements in brass objects provides valuable insights into their origin, historical use and the manufacturing techniques used to produce them. Elements such as lead, tin, iron, arsenic, nickel, antimony, silver, bismuth, and phosphorus—present in small amounts—can indicate whether the brass was intentionally alloyed, recycled, or contaminated during production. Micro-XRF on SEM (also known as SEM XRF) using XTrace 2 facilitates trace element identification and quantification down to 5 ppm.

Figure 1 shows a metal stamp investigated via micro-XRF on SEM, which can be dated back to the year of XianFeng (1851-1861) in the Qing Dynasty of China. The stamp is approximately 1 cm² in size with an irregular surface showing a relief depth of roughly 1.5 mm. Thanks to Bruker’s Rapid Stage which allows for fast, large area SEM XRF mapping, the area of interest is mapped in only 1 hour and without any stitching effects. Figure 2 shows elemental distribution maps of the stamp for Fe, Cu, Zn, Ca, Pb, Se and As.

From the elemental maps, we conclude that this stamp consists mainly of Cu and Zn, suggesting that the material is brass. The presence of inclusions is inferred from the inhomogeneous distribution of Zn. In addition, Ni, Ag, Mn, Sb, Co, Sn, Cr, Ti are identified in the spectrum, but with low signal-to-noise ratios. Note that Pb is often added to improve machinability, while Sn may indicate the recycling of bronze. As, Sb and Ag are typically inherited from copper ores and can help to determine the geographical origin of the raw materials.

The theoretical basis for standard-free sample quantification has been in place for nearly 70 years,¹: this is called fundamental parameter (FP) quantification. In this analysis four standard samples were selected and measured. The quantification is corrected with these standards, which we refer to as “standard-supported FP quantification

As shown in Figure 3, a relatively flat region with homogeneous elemental distribution was selected for quantification (marked in yellow). The quantification results are shown in table 1. After quantification, the main composition of this stamp was determined to be Cu and Zn, which occupy approximately 95%, while the rest of the sample contains approximately 3 wt% of lead with 1 wt% of Sn and Fe. The trace elements of Ag, Sb, Mn and Co are also quantified down to approximately 100 ppm in this case, which is close to the detection limit of electron-based detection methods.

The presence and ratios of these elements allow researchers to trace the provenance of brass, identify technological practices such as cementation or direct alloying, and even link artifacts to specific time periods or cultural traditions.

Figure 1a. A chinese metal stamp from the time of the Qing Dynasty.

Figure 1b. Elemental distributon of iron (Fe), copper (Cu) and zinc (Zn) within the stamp.

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Figure 2. Elemental distribution maps of the stamp: containing Cu and Zn as major elements and Fe, Cu, Zn, Ca, Pb, As, Se in minor/trace concentration.

Figure 3. The region of the stamp selected for for quantification (marked in yellow).

Table 1. Quantification result of selected region by standard-support FP quantification method.

References

1 - J. Sherman, The theoretical derivation of fluorescent X-ray intensities from mixtures, Spectrochimica Acta 7 (1955)