Surface contamination: – The obvious contamination/alteration of surfaces that can be the result of less-than careful sample preparation – Solid surfaces can adsorb gases: At 10-6 torr, a complete monolayer of a gas (. CO) takes just 3 seconds to form. At 10-8 torr, monolayer formation takes 1 hour. – Most studies are conducted under vaccuum – although there are newer methods that don’t require this.