FTIR spectroscopy determines the vibrations of molecules in a substance that absorbs infrared light at a particular wavelength. These are augmented by ATR-FTIR, in which a crystal (typically diamond, ZnS, or Ge) is in contact with the sample, and the IR beam is internally reflected via the crystal. The penetration depth of the evanescent wave generated is limited to a few microns in the sample, making it suitable for analyzing surface layers or thin coatings on top. The ATR-FTIR application is beneficial for crystalline samples and heterogeneous, rough, or opaque samples, which cannot be well analyzed using conventional transmission methods.
Methodology of ATR-FTIR
In an ATR-FTIR, the instrument, the sample is pressed against an internal reflection crystal. The crystal is illuminated with light of an infrared wavelength, and the internally reflected light produces an evanescent wave, which interacts with the surface of the sample. Vibrational modes of sample molecular bonds provide absorbances at wavelengths. The resulting spectrum serves as a molecular fingerprint, enabling the identification of both functional groups and chemical compositions. Solids, viscous liquids, gels, or films can be coated directly using an elaborate preparation.
Instrumentation of ATR-FTIR
In ATR-FTIR systems, an FTIR spectrometer contains an ATR accessory. The ATR unit is composed of a high-refractive-index crystal (e.g., diamond, ZnS crystal, or germanium), a pressure clamp to contact the sample, and optical elements to deliver the beam. A mid-IR detector is typically used to record spectra, which are then baseline-corrected. Peaks are identified and matched to libraries using specialized software. There are also handheld ATR-FTIR units that can be used to analyze materials in the field.
Strengths of ATR-FTIR
ATR-FTIR is a rapid and precise surface analyzer that requires minimal or no sample preparation. It is highly effective on a wide range of substances, including powders, polymers, composites, and biological materials. It is also sensitive to its surface and hence suited to the detection of coating, contaminants, or thin films. ATR-FTIR is qualitative-semi-quantitative, reproducible, and non-destructive.
Limitations of ATR-FTIR
Although ATR-FTIR is ideal for analyzing surfaces, the depth of penetration of this technique is superficial, typically 1-5 microns, and thus not suitable for bulk characterization. Sample pressure, the quality of contact, and roughness can determine the results. Also, very reflective or very soft samples can be a challenge when it comes to data collection. To perform a quantitative analysis, known standards may need to be used for calibration.
