Comparison of ATR–FTIR and O-PTIR Imaging Techniques for the Characterisation of Zinc-Type Degradation Products in a Paint Cross-Section

ATR–FTIR (attenuated total reflection–Fourier-transform infrared) microscopy with imaging is widely used in the heritage field to characterise complex compositions of paint cross-sections. However, some limitations include the need for ATR crystal contact with the sample and the inability to resolve particle size below the IR diffraction limit. Recently, a novel O-PTIR (optical-photothermal infrared) spectroscopy technique claimed to open a new avenue for non-invasive, efficient, and reliable analysis at sub-micron resolution. O-PTIR produces transmission-like FTIR spectra for interpretation, without the need to touch the sample, which are highly favourable attributes for analysing heritage samples. This paper reports the comparison of O-PTIR and ATR–FTIR techniques applied to a cross-section embedding a thin paint fragment that delaminated from a late 19th to early 20th-century oil portrait. The hazy paint fragment consisted of zinc soaps (both crystalline and amorphous), gordaite (NaZn(4)Cl(OH)(6)SO(4)·6H(2)O), and zinc lactate, that could not all be well-resolved with ATR–FTIR imaging. With O-PTIR analysis, the degradation compounds could be resolved at sub-micron resolution with an equivalent or better signal-to-noise ratio. This case study shows how the two techniques can be used to obtain comprehensive information at a broad level with ATR–FTIR and a detailed level with O-PTIR.