In a mass spectrometer, positive ions are produced when a gaseous mixture is ionized by electron bombardment produced by an electric discharge. When the electric-discharge voltage is low, singly positive ions are produced and the following peaks are observed in the mass spectrum:

Mass (amu) Relative Intensity

32…....................................................            0.3743

34…....................................................            0.0015

40….....................................................           1.0000

When the electric discharge is increased, still only singly charged ions are produced, but now the peaks observed in the mass spectrum are

Mass (amu) Relative Intensity

16…....................................................            0.7500

18….....................................................           0.0015

40 …....................................................           1.0000

What does the gas mixture consist of, and what is the percent composition by isotope of the mixture?