μ-XRF/μ-RS vs. SR μ-XRD for pigment identification in illuminated manuscripts

For the non-destructive identification of pigments and colorants in works of art, in archaeological and in forensic materials, a wide range of analytical techniques can be used. Bearing in mind that every method holds particular limitations, two complementary spectroscopic techniques, namely confocal mu-Raman spectroscopy (mu-RS) and mu-X-ray fluorescence spectroscopy (mu-XRF), were joined in one instrument. The combined mu-XRF and mu-RS device, called PRAXIS unites both complementary techniques in one mobile setup, which allows mu- and in situ analysis. mu-XRF allows one to collect elemental and spatially-resolved information in a non-destructive way on major and minor constituents of a variety of materials. However, the main disadvantages of mu-XRF are the penetration depth of the X-rays and the fact that only elements and not specific molecular combinations of elements can be detected. As a result mu-XRF is often not specific enough to identify the pigments within complex mixtures. Confocal Raman microscopy (mu-RS) can offer a surplus as molecular information can be obtained from single pigment grains. However, in some cases the presence of a strong fluorescence background limits the applicability. In this paper, the concrete analytical possibilities of the combined PRAXIS device are evaluated by comparing the results on an illuminated sheet of parchment with the analytical information supplied by synchrotron radiation mu-X-ray diffraction (SR mu-XRD), a highly specific technique.