Mass spectrometry

• Mass spectra can be used to identify the molecular mass of an organic compound and to gain further info about its structure
• When an organic compound is placed in a mass spectrometer, it loses an electron and forms a positive ion = the molecular ion.

  The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound
• For propan-1-ol, the following equation shows the formation of the molecular ion:

  $CH_3CH_2CH_2OH -> CH_3CH_2CH_2OH^+$$+$$e^-$ molecular ion

  m/z = 60
• The molecular ion $M^+$ is the positive ion formed when a molecule loses an electron
• To find the molecular mass, the molecular ion peak (M+ peak) has to be located. The M+ peak is the clear peak at the highest m/z value on the right hand side of the mass spectrum
• You will usually also see a very small peak one unit after the M+ peak. This is the M+1 peak.

  The M+1 peak exists because 1.1% of carbon is present as the carbon-13 isotope.

  E.g. propan-1-ol has a molecular mass of 60, but a small proportion of the alcohol molecules will contain an atom of C13 and thus have a molecular mass of 61. This gives the small M+1 peak
• In the mass spectrometer, some molecular ions break down into smaller pieces known as fragments in a process called fragmentation.

  The other peaks in a mass spectrum are caused by fragment ions, formed by the breakdown of the molecular ion
• The simplest fragmentation breaks a molecular ion into two species - a positively charged fragment ion and a radical.

  Any positive ions formed will be detected by the mass spectrometer, but the uncharged radicals are not detected.
• The mass spectrum of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures. Mass spectra can therefore be used to help identify molecules
• Don't really need to know this but:

  Number of C atoms = height of M+1 peak /height of M peak X100