Carbonate Minerals and Other Samples Studied by Far IR ATR Spectroscopy

Significance of the Topic

The far‐infrared (far IR) region below 400 cm⁻¹ probes lattice and external vibrations of inorganic and heavier‐atom materials that are sensitive to crystal structure, polymorphism and intermolecular interactions. Historically, weak sources and room‐temperature detectors limited quality in this region, but modern FTIR instruments equipped with mid‐to‐far IR beamsplitters and advanced detectors now facilitate routine measurements. Attenuated Total Reflectance (ATR) simplifies sampling of neat solids and liquids without extensive preparation, making far IR ATR an attractive approach for rapid characterization and library matching.

Objectives and Study Overview

This application note aimed to demonstrate the capabilities of far IR ATR spectroscopy for a well‐characterized series of carbonate minerals (calcite, with Ca, Ba and Mn analogues) by comparing ATR‐corrected data to recent transmission measurements. Additional examples, including organic crystals (ascorbic acid), deuterated solvents (DMSO‐d₆) and metal oxides (CuO), were measured to illustrate the broader applicability of ATR in the far IR region.

Methodology and Instrumentation Used

Spectra were acquired on a Bruker VERTEX 70v vacuum FTIR spectrometer equipped with a solid‐state FIR beamsplitter and room‐temperature DLaTGS detector. A single‐crystal diamond ATR accessory (Platinum ATR) was installed for neat powder and liquid measurements. Experimental parameters included a 4 cm⁻¹ resolution, zero filling factor of 4, a mirror velocity of 2.5 kHz and accumulation of three scans per sample to improve signal‐to‐noise. Advanced ATR correction software in OPUS was applied using the crystal’s refractive index (2.4), an incidence angle of 45°, and an empirically adjusted effective number of reflections to compensate for dispersion and penetration depth effects.

• FTIR Spectrometer: VERTEX 70v with FM option
• Detector: DLaTGS room‐temperature
• Beamsplitter: solid‐state FIR
• ATR Accessory: Diamond Platinum ATR (Bruker A225/Q)
• Software: OPUS with Advanced ATR Correction and Curve Fit modules

Main Results and Discussion

Corrected ATR spectra of CaCO₃, BaCO₃ and MnCO₃ showed strong agreement with literature transmission peaks, with characteristic lattice modes shifted to agree within a few cm⁻¹ (e.g., calcite peaks at 111, 228 and 316 cm⁻¹). Barium carbonate exhibited prominent bands at 78, 153 and 203 cm⁻¹, while manganese carbonate displayed features at 160, 203 and 328 cm⁻¹. Organic ascorbic acid and DMSO‐d₆ spectra revealed clear far IR modes after ATR correction, and CuO presented well‐defined lattice vibrations at 538, 479, 322, 165 and 150 cm⁻¹, matching single‐crystal reflectivity data when corrected.

Benefits and Practical Applications

Applying ATR in the far IR region offers:

• Minimal sample preparation for solids and liquids
• Rapid access to lattice vibration information sensitive to crystal form and polymorphism
• Compatibility with direct library searching after ATR correction
• Unattended multi‐range measurements in vacuum to reduce water vapor interference

Future Trends and Applications

Advancements in detector technology, broader ATR crystal materials and enhanced software algorithms will further improve far IR ATR sensitivity and spectral fidelity. Integration with hyphenated techniques (e.g., microscopic ATR, temperature‐controlled stages) and expansion of spectral libraries for polymorph discrimination will extend applications in mineralogy, pharmaceuticals, materials science and quality control.

Conclusion

Far IR ATR spectroscopy, combined with advanced correction algorithms, provides a straightforward method for analyzing lattice modes in inorganic and organic materials. Corrected ATR spectra closely match transmission and reflectivity data, supporting the technique’s reliability for routine characterization, library matching and polymorph identification.

References

1. T. N. Brusentsova et al., "External Lattice Modes of Carbonate Minerals," American Mineralogist 95, 1515–1522 (2010).
2. K. Nishikida, "Advanced ATR Transformation," N & K Spectroscopy, Materials Science and Engineering, University of Wisconsin.
3. J. E. Bertie and H. H. Eysel, "Liquid ATR Effective Number of Reflections," Applied Spectroscopy 39, 382–401 (1985).
4. A. B. Kuz’menko et al., "Far‐IR Lattice Modes of CuO," Physical Review B 63, 094303 (2001).