Laser ablation coupled to mass quadruple spectometry (LAMQS) applied to ancient coins

Abstract

A Nd:Yag laser operating in second harmonic (532 nm), 3 ns pulse duration, 1-150 mJ pulse energy and 1-10 Hz repetition rate is employed to irradiate different coins placed in high vacuum chamber. The ablated material, in vapor phase, is analyzed by a mass quadrupole spectrome-ter in the range 1-300 amu with sensitivity up to 1 ppm. The “on line” analysis permits to investigate on the ele-ments, chemical compounds and isotopic ratios character-izing the coin composition. Focused and unfocused laser beams can be employed to irradiate sub-millimetric or tens millimetric areas, respec-tively. The first method is employed to determine the ele-mental depth profiles from surface up to about 0.1 mm. The second method is employed to analyze the average composition of the superficial patina layers. This micro-invasive technique, associated to the characteristic X-ray fluorescence emission due to 20 keV electron impact, was employed to analyze different coins, based on bronze and silver, from VI up to XX sec.