Two workers were sent into a large tank to weld a plate onto the inside wall. They were found dead two hours later by a security guard. There was nothing suspicious as the guard reported he saw them happily welding the plate into position. The forensic experts decided to take a sample of the air inside the tank before they entered. They ran the sample of air through a mass spectrometer resulting in the spectrum below.
Using the evidence below try to explain what may have happened.

Next problem to solve.