Calculating Relative Atomic Mass

• The relative atomic mass quoted on the periodic table is a weighted average of all the isotopes
• If asked to give the species for a peak in a mass spectrum then give charge and mass number e.g ²⁴Mg2+
• R.A.M = the sum of (isotopic mass × % abundance) ÷ 100
• R.A.M = the sum of (isotopic mass × relative abundance) ÷ total relative abundance

  If relative abundance is used instead of percentage abundance use this equation
• Mass Spectrometer have been included in planetary space probes so that elements on other planets can be identified. Elements on other planets can have a different composition of isotopes
• Cl has two isotopes Cl³⁵ (75%) and Cl³⁷ (25%)
• Br has two isotopes Br⁷⁹ (50%) and Br⁸¹ (50%)
• If a molecule is put through a mass Spectrometer with an electronic impact ionisation stage it will often break up and give a series of peaks caused by fragments. The peak with the largest m/z however will be due to the complete molecule and will be equal to the relative molecular mass Mr of the molecule. This peak is called the parent ion or molecular ion
• If a molecule is put through a mass Spectrometer with electrosoray ionisation then fragmentation will not occur. There will be one peak that will equal the mass of the MH+ ion. It will therefore be necessary to subtract 1 to get the Mr of the molecule. So if a peak at 522.1 is for MH+ , the relative molecular mass of the molecule is 520.1
• This is the mass spectra for Cl2

  
• This is the mass spectra for Br2

  The 160 peak has double the abundance of the other two peaks because there is double the probability of 160 Br⁷⁹-Br⁸¹+ as can be Br⁷⁹-Br⁸¹ and Br⁸¹-79

  
• This is the mass spectra for butane (C4H10)

  Here 58 is the molecular ion C4H10+