Mass Spectrometer: Sample is placed, vaporised and ionised to form positive ions. Ions are
accelerated. Heavier ions move slowly and more difficult to deflect than lighter ions, so ions of
each isotope separated. In vacuum so no interference with other molecules.
• Molecular Ion: It is the positive ion formed when molecule loses an electron. So formation of
molecular ion is CH3CH2 -> CH3CH2
+ + e-
. The molecular ion peak is CH3CH2
+ with m/z is 29. The
molecular ion peak gives molecular mass- the peak at the highest m/z value on the right hand side.
• m/z : The mass spectrometer detects mass to charge ratio m divided by z of the molecular ion. So
m/1.
• Mass Spectrum: On x- axis of mass spectrum is the mass to charge ratio. Y axis is percentage
abundance. The greater the abundance, the taller the peak.
• Will be smaller peaks after M+ peak, so ignore these tiny dots.
• M + 1 Peak: Will also see a small peak one unit after the M+ peak, called M + 1 peak. It exists
because 1.1% of carbon is carbon- 13 isotope. So if Mr of 60, a small proportion of organic
compound will contain 13C so have molecular mass of 61.
• Fragmentation: Positive ion splits. The fragmentation breaks a molecular ion into two – a positively
charged fragment ion and a radical. The positive ions will be detected, but the uncharged radicals
will not.
Unique: The mass spectrum of each compound is unique, as molecules will fragment in different
ways, even if same Mr.
• Identifying Fragment Ion: Can identify fragment ions by taking ‘chunks’ of the molecular ion which
would add up to m/z value of fragment peak e.g. would take CH3 (15), CH2 (14), O etc. Can also
look at difference of Mr, to work out which ‘chunk’ is missing.
• Remember to include + for fragment peak/ ion, so would be CH3
+
.
• Will sometimes have more than one possible structure for fragment ion.
• Even the little lines on the m/z value are fragment peaks.
• m/z values around 56 as Mr of iron is 55.8. Number of m/z values around 56, gives how many
isotopes.