ATR-FTIR Spectroscopy: Ideal for Neat or Optically Dense Substances
Attenuated Total Reflectance (ATR) employs an optical sensor material that has a sufficiently high index of refraction to enable internal reflection within the sensor. Depending on the optical material selected and how the sensor is fabricated, the number of reflections or nodes can be carefully controlled and effective pathlengths are achieved that are well matched to optically dense samples. Any substance in direct contact with the sensor is interrogated by the energy. The intensity of the FTIR spectrum is fundamentally dependent on the number of reflections, the depth of penetration of the evanescent wave into the sample, the number of molecules of interest in the sample, and their respective absorptivity.
In contrast to the transmission technique where light must transverse the sample, in ATR-FTIR the sample thickness is not relevant. A liquid or solid sample can be 10 microns thick or 10 cm thick and in either case, a useful FTIR spectrum can be acquired. This makes ATR ideal for studying a broad range of chemical reactions since no sample preparation or dilution is required to obtain useful spectra. In addition, ATR sensors are available that are resistant to abrasion and can withstand the harsh reaction conditions associated with many chemistries.
In Contrast: Transmission IR Spectroscopy Typically Requires Sample Preparation to Obtain Useful Spectrum
To obtain a transmission spectrum of a liquid, the sample is enclosed in a cell that has infrared transparent windows and the energy must transverse the sample. The intensity of the spectrum is related to the path length of the cell and the solution being analyzed. If the sample path length is too long and/or the sample contains solvent or solute molecules that are highly infrared absorbing or highly scattering, little or no energy will reach the infrared detector. This makes the analysis of most chemical reactions difficult or impossible to perform in transmission mode. In the cases where the right path length is matched to the analyte, the transmission spectrum is acquired. Because long path length cells can be constructed, transmission IR spectroscopy can be useful to identify and track low-concentration organometallic species in homogeneous catalysis reactions. For acquiring the IR spectrum of most solids, it is necessary to dilute the sample by either mulling in nujol oil, pressing a potassium bromide pellet, or preparing an ultra-thin slice of material.