FTIR absorption spectra collected during TPD experiments on ¹²CO (A) and ¹³CO (B). The spectra shown are vertically shifted in increments of 0.050 absorbance units (from the highest temperature to the lowest temperature) to show the evolution of the IR signal throughout TPD experiments. For example, in (A), at 20 K, the peak absorbance is shown to be 1.099, but, accounting for the 0.750 absorbance unit shift, the peak absorbance is 0.349.