Laser ablation is a special sampling technique that is mostly used on solids. A laser beam is focused to a diameter of only a few µm (2-100 µm [stationary setup] or 55-120 µm [portable setup]) on the sample surface. This creates a physical plasma, i.e. an electrically charged gas, at this position. The material heats up very strongly – yet limited to the diameter of the laser focus - so that some part of the material is detached. This process is defined as an ablation process.

In the case of the stationary UV laser device (Resonetics 50-E with an ATL SI-300 laser, λ = 193 nm ArF), this is coupled with the analytical instrument, an ICP mass spectrometer (Thermo Fisher ICAPQ with CCT [collision cell technology]); the ablated material is transported to the mass spectrometer by a carrier gas. In the portable laser setup (Bright Solutions Wedge HB 532, λ = 532 nm in combination with a custom ablation unit from the ETH Zurich), however, the ablated material is first collected on membrane filters on site, for example in a museum, and then transported to the laboratories. There, the material is dissolved from the filter with various acids depending on the material under investigation and the filter used. In a further step, the chemical and/or isotopic composition is subsequently determined by means of ICP mass-spectrometry.

The stationary LA-ICP-MS offers various advantages over conventional analysis in solution, inter alia the processing of very small samples, in-situ measurements with high sensitivity (e.g. of solder joints) and a spatially resolved analysis (e.g. of inclusions, "coatings", gold plating), as well as the possibility of relatively fast serial analyses with high sensitivity. With the stationary system, a wet-chemical digestion is not required, rendering the method particularly suitable for materials that are not easily dissolved. Since this is not necessary, the sample material is usually almost completely preserved and is therefore available for further analyses (e.g. isotope analyses).

In case of the stationary system, the maximum dimensions of the object to be analyzed are limited by the size of the ablation chamber. With the portable LA setup, the quasi-non-invasive sampling for chemical and/or isotopic analysis can also take place outside the laboratory. In addition, the sampling of large objects is possible. Other mobile methods, such as X-ray fluorescence analysis, are completely non-invasive but are much less sensitive. Among other things, such analyses are usually not sufficient to answer questions regarding the origin and authenticity of objects. In addition, laser ablation can examine much deeper layers of the material (approx. 100-200 µm). The commercial offer of the mobile LA method currently represents a unique feature of CEZA.

Figure 1 – The mobile laser ablation setup (schematics). Draft: Moritz Numrich (CEZA); Photograph of the Lockenring: W. P. Tolstikow – M. J. Trejster, Der Schatz aus Troja. Stuttgart, Zürich 1996/97, 60, Kat.-Nr. 35.

Figure 2 – The mobile laser ablation setup. Photograph: Elka Duberow (CEZA).

Quantitative chemical analyses may generally be accomplished for most elements in the mass range from ⁷Li to ²³⁸U. In general, the elements to be analyzed depend on the sample material and the research question. Most often the lack of reference material is the limiting factor. In the case of the stationary LA-ICP-MS, gold, silver, copper alloys and iron are routinely analyzed with detection limits in the ppb and ppm range. But also other materials such as glass are examined in our laboratory, while the portable LA system, as mentioned above, is currently primarily used for the analyses of gold and silver objects.

While the described chemical steps are carried out in a clean room, the samples are taken with the described laser ablation systems in a special laser laboratory. However, the mobile laser system can be used outside of these laboratories without limitations.

Die Analyse antiker Goldmünzen durch mobile Laserablation und ICP-MS: Eine Untersuchung zu den Kosōn-Stateren aus Siebenbürgen