TN21-02 ATR Penetration Depth

This note compares the pathlengths of IR light within a sample in transmission or in contact with the internal reflection element (IRE) of an ATR.

ATR has become the dominant method for obtaining a spectrum of a solid sample. The ATR method exploits a phenomenon whereby an evanescent wave of infinite wavelength is generated at the interface of an IRE and a sample of interest, Figure 1.

The intensity of the wave is a function of the wavelength of light and decays exponentially with distance into the sample, falling to fractions of its starting intensity within a few microns.

Compared to the transmission method, ATR has a shorter effective pathlength which changes across the spectrum as a function of the wavelength of incident light. When a spectrum of a sample collected in ATR and transmission mode are scaled to match at the low wavenumber range (for instance at 1000 cm^‑1) the peaks to higher wavenumber will show a marked difference; the ATR spectrum will exhibit reduced intensity peaks relative to the transmission spectrum [1]. This is due to the decreasing depth of penetration of the incident light with increasing wavenumber. Several mathematical models are available to account for this effect and this note discusses the most common methods.

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