Carbonate Minerals and Other Samples Studied by Far IR ATR Spectroscopy

Importance of the Topic

The far infrared (far IR) spectral region below 400 cm^–1 is critical for probing lattice and external vibrations of inorganic solids. These low-energy modes reveal crystal polymorphism, lattice dynamics and structural differences that are often invisible in mid-IR spectra. Recent advances in FTIR spectrometer design and detector sensitivity enable practical, high-quality far IR attenuated total reflectance (ATR) measurements. Such capability simplifies sample handling and accelerates analysis in mineralogy, materials science and industrial quality control.

Objectives and Study Overview

This application note examines a series of carbonate minerals and additional compounds using far IR ATR. The main goals are:

1. Demonstrate the ease and reliability of far IR ATR sampling versus traditional transmission methods.
2. Apply advanced ATR correction to recover true band positions and intensities.
3. Compare corrected ATR spectra with published transmission or reflectance data for validation.
4. Explore the method’s applicability to organic solids and liquids with far IR bands.

Methodology and Instrumentation

Powdered carbonates (CaCO₃, BaCO₃, MnCO₃) were measured neat on a diamond ATR crystal. Additional samples included ascorbic acid crystals, DMSO-d₆ liquid and cupric oxide (CuO) powder. Measurement parameters:

• FTIR spectrometer: VERTEX 70v with solid-state FIR beamsplitter, room-temperature DLaTGS detector
• Resolution: 4 cm^–1, zero filling 4 (≈1 point/cm^–1), mirror velocity 2.5 kHz
• Atmosphere: vacuum spectrometer or dry air/nitrogen purge to remove water vapor
• Scan average: three independent acquisitions (≈30 minutes) to improve signal-to-noise

Used Instrumentation

• FTIR System: INVENIO R / VERTEX 70v with BRUKER FM FIR-MIR option
• ATR Accessory: Platinum diamond single-crystal unit (Bruker A225/Q)
• Software: OPUS with Advanced ATR Correction and Curve Fit (second derivative)

Main Results and Discussion

After advanced ATR correction (refractive index n≈1.6 for carbonates, angle 45°, effective reflections empirically set to 0.56–1), major lattice bands aligned closely with transmission data:

• CaCO₃: corrected peaks at 111, 228, 316 cm^–1 vs. reference 110, 228, 319 cm^–1
• BaCO₃: features at 78, 137, 153, 173, 195 cm^–1 matching reported values
• MnCO₃: peaks at 160, 203, 209, 328 cm^–1 in agreement with literature
• Ascorbic acid: key bands at 292, 350, 448, 566 cm^–1 recovered
• DMSO-d₆: liquid bands at 263, 308 cm^–1
• CuO: lattice modes at 150, 165, 322, 479, 538 cm^–1 matched reflectivity data

Effective correction restored band shapes, reduced long-wavelength distortion and enabled second-derivative peak deconvolution of overlapping features.

Benefits and Practical Applications

The far IR ATR approach provides:

• Minimal sample preparation: neat solids and liquids without pellet pressing or mulls
• Rapid spectral acquisition and real-time library searching
• Ability to study moisture-sensitive or reactive samples under vacuum or dry purge
• Quantitative band position recovery for phase identification and polymorph screening

Industries such as mineral exploration, cement production, pharmaceuticals and polymer development benefit from fast far IR fingerprinting.

Future Trends and Potential Applications

Continued instrument development will improve detector sensitivity below 50 cm^–1 and enable sub-ambient temperature studies. Potential areas of growth include:

• In situ monitoring of solid–solid phase transitions under controlled temperature and pressure
• High-throughput screening of battery materials and perovskite structures
• Integration with microscopy for spatially resolved far IR imaging
• Advanced chemometric analysis of multicomponent systems

Conclusion

Far IR ATR spectroscopy combined with advanced software correction offers a streamlined route to high-quality lattice vibration spectra. Results closely match traditional transmission and reflectance data, validating the technique for mineralogical and materials analysis. The approach reduces sample handling time and opens new possibilities for rapid fingerprinting across inorganic and organic systems.

References

1. T.N. Brusentsova et al., American Mineralogist 95, 1515–1522 (2010)
2. K. Nishikida, Advanced ATR-Transformation, N & K Spectroscopy LLC, University of Wisconsin
3. J.E. Bertie and H.H. Eysel, Applied Spectroscopy 39, 382–401 (1985)
4. A.B. Kuz’menko et al., Physical Review B 63, 094303 (2001)