Portable XRF

X-ray Fluorescence (XRF) is based on the study of the interaction of X-rays with a material in order to determine its elemental composition, and represents an effective method for the qualitative and quantitative analysis of the chemical composition of the material under investigation.

The application of XRF to the study of illuminated manuscripts provides information on the composition of inorganic phases (such as pigments, inks and metallic components), avoiding any contribution from organic substances, such as binders or any surface layers of an organic nature. Additional information from the deeper regions of the parchment, which are characteristic of processing or preparation procedures, can also be obtained.

In the present study, XRF analyses were carried out with a portable Thermo Scientific Niton XL3t GOLDD spectrometer (Thermo Fisher Scientific, East Greenbush, NY, USA) from the Department of Chemistry of the University of Torino. The spectrometer is equipped with an Ag target with a maximum current of 100 μA and voltages between 8 and 50 kV. The detector is a Large Drift Detector (LDD) with a surface area of 25 mm² and an energy resolution of 135 eV at the Mn Kα line. The geometry used is 30°/30°, with a working distance of 2 mm along the normal to the detector/source plane.

Two acquisition modes were adopted, METALS and MINING, both with measurement times of 120 s. Each analysis involved four sequential measurements at four different voltages (main, Al/Fe filter, 40 kV; low, Cu filter, 20 kV; high, Mo filter, 50 kV; light, no filter, 8 kV) and three different filters, in order to improve the material response in the energy range of the spectrum. The analysis was carried out on an elliptical area with a maximum diameter of 3 or 8 mm, which was monitored by a CCD camera. The data obtained were compared with the acquisition of a background at the same live time and with calibration standards (NIST, SGT, CORNING, MBH).

All the measurements were performed three times and the results averaged. All the acquired spectra were processed with the commercial software bAxil (Brightspec NV, Belgium), derived from the academic software QXAS (IAEA).

Portable XRF setup at the Biblioteca Nazionale Vittorio Emanuele III of Naples.

A mixed analysis algorithm was applied, based on a fundamental parameter (FP) approach to the matrix. Matrix interactions, as well as the effects of secondary fluorescence and self-absorption, were taken into account using a system of non-linear equations that made it possible to minimize the correlation coefficients. Although quantitative data are obtained, their representativeness has to be carefully considered, due to the impossibility of predicting the different layer thicknesses and the possible influence of the back of the pages.

All results were subjected to a threshold of 0.01%, considered as the lower limit of detection (LLD) for all elements (0.05% was applied for aluminum). This is a conservative limit, given the instrumental sensitivity of tens of ppm (parts per million). In some cases, the extent of the interferences was such as to prevent quantification, as in the case of the simultaneous presence of sulfur and lead.

References

Aceto, M., Agostino, A., Boccaleri, E., Crivello, F. and Garlanda, A.C., 2006. Evidence for the degradation of an alloy pigment on an ancient Italian manuscript. Journal of Raman Spectroscopy, 37(10), pp.1160-1170.
Aceto, M., Agostino, A., Boccaleri, E. and Garlanda, A.C., 2008. The Vercelli Gospels laid open: an investigation into the inks used to write the oldest Gospels in Latin. X-Ray Spectrometry, 37(4), pp.286-292.
Aceto, M., Agostino, A., Fenoglio, G., Baraldi, P., Zannini, P., Hofmann, C. and Gamillscheg, E., 2012. First analytical evidences of precious colourants on Mediterranean illuminated manuscripts. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 95, pp.235-245.
Aceto, M., Agostino, A., Fenoglio, G., Capra, V., Demaria, E. and Cancian, P., 2017. Characterisation of the different hands in the composition of a 14th century breviary by means of portable XRF analysis and complementary techniques. X-Ray Spectrometry, 46(4), pp.259-270.

Keywords: Portable, XRF