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

In addition to visual characteristics, a less manipulable criterion for aut henticity verification of silver antiques is given by trace and minor eleme nt patterns in the silver alloy. The analytical method used to analyse prec ious silver antiques should not visibly damage the object and should enable the determination of impurities in the ppm-0.5% range. Using laser ablatio n inductively coupled plasma mass spectrometry (LA-ICP-MS), visible damage can be restricted to an acceptable minimum. Because most antique silver obj ects are too large to fit into a normal laser ablation cell, an alternative cell design was used that allows a direct, virtually non-destructive analy sis of entire antique silver objects. This cell is placed upon the object t o be analysed. A micro-amount of the object is then ablated through an aper ture in the bottom of the cell. The 100 mu m wide craters are almost invisi ble 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 standar d materials were used for external calibration. The crater-to-crater repeat ability of the normalized signals in a homogeneous silver sample was below 10% RSD (Iz = 3) for most elements. Detection limits lie within the sub-ppm to 2 ppm range. The accuracy was validated with comparative ICP-MS measure ments after digestion and with X-ray fluorescence (XRF) measurements. The a nalysis of eight antique silver objects, including one forgery, illustrates the application of the method.