An unknown sample gives the following simplified mass spectrum for singly charged ions. The detection system is insensitive to isotopes present at natural abundances below 5%.
Peak label
m/z (amu)
Relative intensity (arbitrary units)
A
68.9257
60
B
70.9247
40
A reference table lists the following information for two neighboring elements:
• Gallium: naturally occurring isotopes at masses 68.9256 amu (60.11%) and 70.9247 amu (39.89%); average atomic mass 69.723 amu
• Germanium: multiple naturally occurring isotopes at masses 69.9243, 71.9219, 72.9235, 73.9212, and 75.9214 amu, all above 5% abundance; average atomic mass 72.63 amu
(a) Using the spectral data, compute the average atomic mass of the unknown element.
(b) Decide whether the unknown is Gallium or Germanium. Provide two independent pieces of evidence from the spectrum that support your choice.
(c) Suppose a very low-abundance third isotope at m/z=69.7 were actually present at 0.20% relative abundance, below the instrument’s detection limit. Re-calculate the average atomic mass that would result if this isotope were detectable and comment on whether its inclusion would change your identification in part (b).