Basics

Tin

Tin has ten stable isotopes and thus the largest number of all elements of the periodic table, resulting in a large atomic mass range of 12 u (112 to 124), over which the mass fractionation can be determined with high precision. Tin has as well as siderophile and chalcophile properties and is therefore particularly predestined to characterise processes involving metallic and sulphide phases. In the case of archaeometry, tin metal and bronze are such phases.
The most important tin mineral to human since prehistory is tin dioxide or cassiterite (SnO₂). To determine highly precise isotope ratios with a MC-ICP-MS, samples must be dissolved. For tin metal this is achieved with hydrochloric acid, whereas cassiterite is highly resistant to acids. Therefore, cassiterite samples must be thermally reduced to tin metal at first. While tin metal is pure enough to directly measure with the mass spectrometer, the sample solutions of bronzes need to be proceeded to ion chromatography in order to separate the tin from the sample matrix containing other elements (e.g. Cu, Pb, As) and thus to avoid interferences or contamination of the instrument during the measurement. To correct for mass discrimination in the mass spectrometer during measurement an antimony standard is added to the tin solution. To correct for further instrumental bias, the so-called standard-sampling-bracketing technique, where a reference standard with a known value, is used, which delivers the ẟ value relative to the standard. Because of the many possible different isotope ratios which can be calculated for tin, recently the nomenclature ẟSn, which considers the masses 116 to 124 in the slope of the linear regession in the graph of isotope masses against the calculated ẟ-values of multiple tin isotope ratios and the slope of this linear regression represents the fractionation in ‰ * u^-1.

Ion exchange chromatography of tin

Copper

Copper has only two isotopes (⁶⁵Cu and ⁶³Cu) and therefore the materials studied are characterised by the ⁶⁵Cu/⁶³Cu isotope ratio. The behavior of this element is controlled by its ability to occur in two oxidation states (Cu⁺ and Cu²⁺) and its chalcophile properties in both geological and archaeometry contexts. For archaeology, the use of copper is known since the Neolithic period (~12.000 BP), where it was first processed in native form, i.e. as a naturally occurring metal (solid copper). Since the Chalcolithic (~7000 BP), copper ores have been smelted and used to produce copper alloys such as arsenic or tin bronzes. Using the tin isotopic fingerprint in the respective materials, the origin or production technique can be investigated up to an extent.
Owing to the several possible oxidation states of Cu leads to a that its mineralogy and isotopic composition is complex, highly depending on chemical processes and environmental conditions. Copper deposits are formed under a huge span of conditions: over a wide temperature range from magmatic, hydrothermal to sedimentary and under reducing to oxidising conditions. This complexity is reflected by a large inter-depositional variability of the copper isotopic composition in copper deposits. Thus, individual deposits cannot be distinguished properly and copper isotope values often cannot be used for determining the origin of the source material alone. Nevertheless, there are significantly measurable differences in the Cu isotope composition of different archaeological objects, indicating the use of different raw material sources. With the help of the isotopic fingerprints, the allocation of individual objects to object groups can be investigated, which would help to interpret cultural relationships. Furthermore, in combination with elemental analysis and other isotope systems, it is eventually possible to distinguish between intentional and accidental alloys or to recognise mixtures of different types of metals.
Owing to the high Cu content of bronze artefacts (> 85 %) no purification previous to measurement is necessary. For mass-bias correction a Ni standard and the standard-sample-bracketing method is used against a reference standard with a known Cu isotope ratio. The isotope measurements are conducted at a MC-ICP-MS. The results are given as ẟ⁶⁵Cu.

Lead

In the first approximation, the isotope composition of all elements on earth is the same and unchangeable everywhere. For light elements, however, there are small changes due to the different behavior of the isotopes, especially in diffusion-controlled processes such as evaporation and condensation, owing to the high relative mass differences between the isotopes of light elements. For heavy elements like lead, the relative mass differences are too small for such effects. Only owing to the radioactive decay of some isotopes the isotope ratio of such heavy elements may change. For archaeometallurgy, isotope analysis of lead has proved particularly useful for determining the origin of metals, underlying the radioactive decay of uranium and thorium to lead. Therefore, three (namely ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) of the four stable lead isotopes are constantly being formed, called radiogenic. This lead mixes with the lead which was already present, thus the average isotopic composition of Pb has changed significantly since the formation of the earth.
During formation of a lead deposit the lead is geochemically separated from uranium and thorium by natural processes without any fractionation of the isotope ratios of lead . Consequently, the fixed lead isotope ratio only depends on the geological age and the composition of the parent rock. In any case, these lead isotope ratios usually varies in different deposits. This fact can be used for investigations of the origin, for instance, of the copper ores in case of bronze-artefacts. As mentioned before, since the lead isotope ratios do not fractionate by chemical reactions, the original value can be found unchanged in the finished products. This is regardless of processing of the ore such as preparation, smelting, refining and, if necessary, corrosion. The lead isotope method also works for other metals and materials, the prerequisite is that the lead as a secondary element together with the material in question comes from the same deposit and was not introduced as alloying component or contamination.
Nevertheless, there are some limitations to these methods. As stated before, the prerequisite for determining the origin is the assumption that the material originates from a single deposit site. If mixtures of materials from different deposit sites occurred, it is usually no longer possible to distinguish them. It is also noteworthy that in case of higher Pb contents owing to an addition of lead to the alloy, only the origin of the (added) lead and not of the base material can be determined. The identification whether lead was added intentionally or not depends on the material and on the experience gained from many thousands of analyses of cultural-historical objects.

Samples for lead isotope measurements are dissolved and the lead is separated by ion chromatographie from the matrix. Since it is a radiogenic isotope system it is given as the isotope ratio, corrected against a standard with a known value.

Quality Control

Ensuring high quality and reliability of our analyses is the constant focus of our laboratory. Therefore, we carry out control analyses on, where possible, certified reference materials for each series of measurements. The most important reference material for Pb is NIST SRM 981, a lead wire certified by the American Society for Testing and Materials in Washington DC, NIST SRM 976 for Cu and NIST 3161a for Sn isotope measurements. These are internationally used to check the accuracy of the analyses. In addition, several external reference solutions are used for a prove of reliability.