Università di Perugia, Center SMAArt, Perugia, IT

MOLAB Available facility: Mobile equipment collection

The MOLAB diagnostic equipment of the University of Perugia Center SMAArt (Scientific Methodologies applied to Archeology and Art) combines several point spectroscopic  techniques, using radiations from the X-rays to the infrared (IR), and imaging methods both from macro- to micro-scale. They include: X-ray fluorescence, mid- and near-FTIR, UV-Vis absorption spectroscopy, steady-state UV-Vis reflectance fluorescence (macro and micro), time resolved UV-vis fluorescence (Time Correlate Single Photon Counting)  Raman spectroscopy (macro- and micro-) uni-lateral NMR relaxometry,  atomic force microscopy, digital optical microscopy, UV-vis fluorescence, UV and near-IR  imaging, and others. These technique provide complementary information and all together permit thorough investigations of artworks, from their elemental to molecular composition. The non-invasivity permits to carry out measurements on a virtually infinite number of points Here a brief description of the portable spectroscopic tools for point measurements within the UNIPG-MOLAB portfolio.

X-ray fluorescence.The XRF equipment is made with a miniaturized X-ray generator EIS P/N 9910, equipped with a tungsten anode, and a SDD silicon drift detector (resolution of about 150 eV at 5.9 keV) cooled with a Peltier element. The instrument permits the detection of elements with Z higher than silicon (>14). Excitation parameters currently used are 38 kV and 0.05 mA; typical acquisition time 120 s. The distance artwork-probe working distance is fixed at 2 cm. The beam diameter at the object surface is around 4 mm.
Fiber optic mid-FT-IR. The spectrophotometer (JASCO VIR 9500) is equipped with a mid-infrared fiber optic sampling probe. The signal-to-noise ratio is very good in the range 900 to 4000 cm^-1 with the exception of the 2050-2200 cm^-1 region, and spectral resolution is of 4 cm^-1. The non-contact probe (4 mm diameter) is kept perpendicular to the painting surface (0°/0° geometry) at a distance of about 6 mm. Owing to the probe geometry, reflectance mid-FTIR spectra can present large distortions, both in band shape and absorption frequency, due to the presence of both specular and diffused components. Interpretation of spectra is possible through a wide specific database created ad hoc.
Fiber optic near-FT-IR. Reflectance spectra are recorded using a compact JASCO VIR 9600 spectrophotometer equipped with a fiber optic sampling probe. It is composed of a halogen lamp as source, a Michelson interferometer equipped with a CaF₂ beam splitter and an InGaAs detector. The Y shaped silica-glass fiber optics probe contains 14 fibers, 7 of which carry infrared radiation from the source to the sample, while the other 7 collect the radiation reflected off the surface. The investigated spectral range is 12500-4000 cm^-1 with a resolution of 4 cm^-1. The spot at the surface is of 4 mm diameter, the artwork-probe working distance is of about 6 mm.
Micro-Raman spectroscopy. The system can work with two optional laser sources (Nd:YAG at 532 nm; diode at 785 nm). The Olympus micro-probe is equipped with an attenuator which allows for the laser power tuning at the surface. The back-scattered Raman light is collected (at 180°) by an optical fiber and led to a Czerny-Turner polychromator of about 100 mm of focal length and to a Peltier cooled CCD detector. The spectral range is from 250 to 2000 cm-1 with a maximum spectral resolution of 8 cm-1. The measurement is contact-free with a spatial resolution of 0.1 mm² and the  output laser power can be modulated.
Micro-fluorescence. The system for micro-Raman can be arranged to work also as a micro-fluorimeter (l_exc. = 532 nm) permitting identification of red and blue colorants through a specifically developed fluorescence database.
UV-Vis absorption and steady-state fluorescence. Reflectance spectra are recorded through a system where the excitation source is a deuterium-halogen lamp (AvaLight-D(H)-S) coupled to a bifurcated bundle of quartz fibres that collect and transfer the reflectance signal to a high sensitivity Avantes CCD detector ( 200-1100 nm, spectral resolution 2 nm). The probe-head is usually positioned at 45° to reduce the specular component and an integrating sphere for diffuse reflectance measurements is also available. Calibration is performed by means of 99% Spectralon diffuse reflectance standard.
An analogous system is used to record fluorescence emission spectra, after appropriate excitation via a Xenon lamp (75 W, 240-600 nm emission), followed by a Jobin Yvon UV monochromator for selecting the appropriate excitation wavelength (spectral range 200-1100 nm, spectral resolution 25 nm).
Time-resolved fluorescence. The compact prototype for fluorescence decay-time measurements works with Time Correlate Single Photon Counting modality. It is composed of a pulsed source with inter-changeable diodes (emitting at 375 and 650 nm, picosecond time scale pulses), a photocathode detector (350-850 nm range), and a FluoroHub device containing a TAC (time-amplitude converter). A PC controls the acquisition system and the elaboration of data using Datastation and DAS6.2 software, respectively. A bifurcated fibre-optic system transfers the excitation light to the target point on a surface (spot  of 4 mm diameter) and a co-axial crown of fibres collects the emitted light transmitting the signal to the detector. Appropriate filters, positioned between the collecting fibres and the detector, permit the wavelength selection of the emitted light. The time resolution is 100 picoseconds.
NMR-MOUSE relaxometry. The portable instrument (Mobile Universal Surface Explorer by ACT GmbH, Roetgen,DE) consists of a unilateral NMR relaxometer purposely created for the study of cultural heritage unmovable objects. The portable device is equipped with a specific magnetic probe for 1H supplied by a Bruker Minispec console and with a laptop computer for the data acquisition.
NMR depth profiling. A second specific probe can generate a magnetic field of an area of 1 cm² and a thickness in the depth direction that can be selected from 200 to 2.3 microns. This probe, assembled on a movable micrometric lift, can be positioned step by step with a precision of 10 microns to scan the profile of the examined material through the interested depth. 

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UNIPG

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Name: Dr Costanza Miliani