Determination of impurities in antique silver objects for authentication by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)

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Abstract
In addition to visual characteristics, a less manipulable criterion for authenticity verification of silver antiques is given by trace and minor element patterns in the silver alloy. The analytical method used to analyse precious silver antiques should not visibly damage the object and should enable the determination of impurities in the ppm-0.5% range. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), visible damage can be restricted to an acceptable minimum. Because most antique silver objects are too large to fit into a normal laser ablation cell, an alternative cell design was used that allows a direct, virtually non-destructive analysis of entire antique silver objects. This cell is placed upon the object to be analysed. A micro-amount of the object is then ablated through an aperture in the bottom of the cell. The 100 µm wide craters are almost invisible on an antique silver object. The analytes Zn, Cd, Sn, Sb, Au, Pb and Bi were measured. Signals were normalized to the Ag signal and silver standard materials were used for external calibration. The crater-to-crater repeatability of the normalized signals in a homogeneous silver sample was below 10% RSD (n=3) for most elements. Detection limits lie within the sub-ppm to 2 ppm range. The accuracy was validated with comparative ICP-MS measurements after digestion and with X-ray fluorescence (XRF) measurements. The analysis of eight antique silver objects, including one forgery, illustrates the application of the method.

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