Carbon black is commonly found as an additive in rubber, and used as supports for catalysts. To the untrained analyst these materials can provide some unexpected surprises when using ATR spectroscopy. Read on to discover how to avoid the common pitfalls…

ATR spectroscopy relies upon total internal reflection within the ATR crystal at the interface between the crystal and your sample. For this to occur the refractive index of the sample must be significantly below the refractive index of the crystal. When the refractive index of the sample approaches that of the crystal light is transmitted into the sample and spectral artefacts are observed in the spectrum.

Most organic materials have a refractive index ~1.5-1.6 and can be studied by an ATR equipped with diamond or ZnSe which have refractive indexes of 2.4 and 2.41, respectively. Penetration depth of light into a sample also decreases with increasing crystal refractive index so for optimal signal-to-noise ratios higher refractive index materials (such as Si or Ge, with refractive indexes of 3.41 and 4.00, respectively) are best avoided unless the analyst wishes to study the surface of their sample.

Carbon Black has a refractive index significantly higher than organic materials, and a direct consequence of this is that if a Diamond ATR is used severe band distortions are observed. This is due to anomalous dispersion and presents as asymmetric peaks. The stronger peaks in the spectrum also exhibit negative dips to higher wavenumbers. These artefacts can be easily eliminated by simply choosing an ATR crystal with a higher refractive index such as Ge, which then ensures that total internal reflection occurs and a good spectrum is obtained.