Figure 5.

Band shifts in PTIR and ATR spectra: calculated band shifts for a Lorentzian band (10 cm⁻¹ half width) centered at 1650 cm⁻¹ (typical spectral position of the amide I band), for absorption coefficients ranging from a = 0.01 μm⁻¹ to 0.5 μm⁻¹ as a function of the sample thickness (up to 5 μm). The band position in the PTIR spectrum approaches the band position measured in ATR spectroscopy, as the sample height increases. Note that typical PTIR samples are significantly thinner than 5 μm.