Quantitative Analysis of Minium and Vermilion Mixtures Using Low-Frequency Vibrational Spectroscopy

Abstract

Low-frequency vibrational spectroscopy offers a compelling solution for the nondestructive and noninvasive study of pigments in historical artifacts by revealing the characteristic sub-200 cm–1 spectral features of component materials. The techniques of terahertz time-domain spectroscopy (THz-TDS) and low-frequency Raman spectroscopy (LFRS) are complementary approaches to accessing this spectral region and are valuable tools for artifact identification, conservation, and restoration. In this investigation of historical pigments, pure and mixed samples of minium (Pb3O4) and vermilion (HgS) were studied using a combination of THz-TDS and LFRS experiments to determine the limits of detection (LOD) and quantitation (LOQ) for each compound with both methods. The measurements were also supported using solid-state density functional theory simulations of the pigment structures and vibrations, enabling spectral peaks to be assigned to specific atomic motions in these solids. The THz-TDS LOD was found to be similar for both minium and vermilion at 6% by mass on average. In comparison, LFRS was found to be more sensitive to both pigments, particularly to the presence of vermilion with an LFRS LOD of 0.2%. These results demonstrate that low-frequency vibrational spectroscopy can be used for successful quantitative analysis of pigment mixtures and provide reliable new data for use in heritage science.

Terahertz Spectroscopic Analysis of the Vermilion Pigment in Free-Standing and Polyethylene-Mixed Forms

“…The two strong peaks at 38.4 and 42.6 cm –1 are clearly distinguished in our experiment carried out with a resolution of 0.43 cm –1 . According to a recent DFT calculation, the transverse optical modes corresponding to A 2 , E, and E symmetries should appear at 35.08, 46.52, and 90.61 cm –1 , respectively . The two experimentally observed peaks at 38.4 and 42.6 cm –1 fairly well match these DFT predictions.…”

“…Furthermore, most existing terahertz databases of pigments contain only room-temperature spectra where a substantial broadening of all spectral lines is inevitable. This is especially relevant to our efforts to connect the experimental data to density functional theory (DFT) calculations frequently performed on crystal structures stabilized at absolute zero without consideration of thermal broadening. ,,, Consequently, closely spaced or nearly overlapping absorption lines are not amenable to accurate mode assignments based on DFT. Additionally, the anharmonic nature of some lattice vibrational modes predicted by DFT needs to be unveiled experimentally in the absence of thermal broadening at elevated temperatures.…”

“…Here, we report on our terahertz spectroscopic study of vermilion (HgS, mercury sulfide), a representative pigment possessing a set of signature absorption bands in the terahertz range. A recent DFT calculation performed by Kleist and Korter predicted three absorption peaks at 35.08, 46.52, and 90.61 cm –1 , which were assigned to the lattice vibrational modes of the transverse optical (TO) type corresponding to A 2 , E, and E symmetries, respectively. Previous experiments indeed detected the presence of these three absorption peaks near the corresponding frequencies. ,,− However, the two lowest modes were not clearly resolved in some early measurements performed at low resolution due to the much weaker intensity of the second peak, which even led to some doubt about the reality of this peak itself.…”

“…A recent DFT calculation performed by Kleist and Korter predicted three absorption peaks at 35.08, 46.52, and 90.61 cm –1 , which were assigned to the lattice vibrational modes of the transverse optical (TO) type corresponding to A 2 , E, and E symmetries, respectively. Previous experiments indeed detected the presence of these three absorption peaks near the corresponding frequencies. ,,− However, the two lowest modes were not clearly resolved in some early measurements performed at low resolution due to the much weaker intensity of the second peak, which even led to some doubt about the reality of this peak itself. The detailed nature of intrinsic lineshapes and line broadening has not been seriously discussed either.…”

Solving the Computational Puzzle: Toward a Pragmatic Pathway for Modeling Low-Energy Vibrational Modes of Pharmaceutical Crystals

“…In recent years, with the advent of ultranarrow optical filters, low-frequency Raman (LFR) spectroscopy, in particular, has received increased attention. As it probes intermolecular vibrational modes related to the long-range order (i.e., crystallinity), this technique is especially useful for investigating solid-state properties, and has been adapted for qualitative and quantitative analysis of crystalline and amorphous drugs, including intricate kinetics modeling, , in situ solubilization studies , and probing of solid-dosage forms. , However, despite its broad application, the interpretation of LFR vibrational features is still challenging and requires elaborate (and often time/resource-intensive) computational analysis as highlighted by some of the recent studies. ,− Furthermore, there are a myriad of functionals and basis sets that can be chosen for calculations, which can complicate the effectiveness of modeling the low-wavenumber spectra.…”