QMS spectra collected at 20 K showing (A) the composition of the background rest gases and (B) the composition of the gases in the chamber while depositing CO gas onto the window. As shown in (A), the primary background gases prior to deposition are H₂ (2 m/z), H₂O (18 m/z), and a combination of ¹²CO and N₂ (28 m/z). During deposition (B), the partial pressure of H₂O is more than two orders of magnitude lower than CO, indicating that water would not have any effect on CO ice properties.