Optimum acquisition of Raman spectra in pigment analysis with IR laser diode and pulsed UV irradiation

Abstract

AbstractFluorescence, due to the binding media, is the main problem that one encounters in the molecular analysis of artistic pigments using Raman spectroscopy. With the object of minimizing this problem, we propose the use of a semiconductor IR laser in Raman spectral acquisition and the application of local irradiation with a pulsed UV laser on the analysis zone. The Raman analysis with an IR source shows advantages compared to that with visible lasers, such as the reduction of fluorescence and its shot noise, although a price has to be paid in the form of the small intensity of the obtained spectra with the same acquisition time. Also, in this paper we demonstrate that controlled levels of pulsed UV radiation over the analyzed painting (pigment + binding media) can improve even more the Raman spectral quality obtained with the IR laser, which leads us to conclude that the local use of a pulsed UV laser, prior to Raman analysis with the IR source, can bring optimum quality results in pigment identification. The spectral quality of these experimental results for different pigments has been measured by calculating in each case the signal‐to‐noise ratio (SNR) of the corresponding spectra. From a quantitative point of view, in some practical cases (chromium yellow and ultramarine blue) an SNR improvement of 16 dB is achieved when pulsed UV irradiation and IR laser Raman analysis are used instead of a simple Raman analysis with a visible laser. Copyright © 2006 John Wiley & Sons, Ltd.