Advanced Higher Unit 3 Mass Spectrometry

Mass spectrometry can be used to determine the accurate molecular mass and structural features of an organic compound. It involves the formation and detection of ions and measures the mass and relative abundance of these ions. It is classed as a non-destructive technique because of the extremely small sample size (~ g)

Mass Spectrometer

The Process The process is carried out in a high vacuum to avoid collisions. The process is carried out in a high vacuum to avoid collisions. Ionisation - The sample is vaporised and bombarded with an electron beam, which has enough energy to ‘knock off’ some of the outer electrons of the sample molecules resulting in the formation of positive ions. Acceleration - The positive ions are accelerated and focussed into an ion beam by an electric field.

Deflection - The ion beam then passes through an electromagnet that deflects the ions. The extent of the deflection will depend on the mass/charge ratio of an ion (heavy ions will be deflected the least) Detection - The strength of the magnetic field can be altered to deflect ions of varying mass/charge ratios onto the detector. Results - A mass spectrum is a plot of relative abundance (%) against mass/charge (m/z) ratio. The highest peak on the spectrum is defined as the base peak and is assigned a value of 100 %.

One Compound - Why so Many Peaks? Ionisation of an organic molecule (M) M  M + + e - M + is an ion radical and is referred to as the parent ion or the molecular ion. Due to the high energy of the electron beam the molecular ions are broken up into fragments.

e.g. butyl ethanoate (see mass spectra data on page 6 of your Unit 3(d) notes) CH 3 COOCH 2 CH 2 CH 2 CH 3 Peak at m/z 116 Peak at m/z 43 (an ion fragment) Peak at m/z 56 (an ion fragment) no peak NB Only positive ion fragments will be detected. CH 3 COOCH 2 CH 2 CH 2 CH e - CH 3 CO + + C 4 H 9 O C 4 H CH 3 COOH

It is possible for more than one electron to be knocked off to form an M 2+ parent ion. e.g. C 10 H 8 Peak at m/z 128 (i.e. 128/1 = 128) Peak at m/z 64 (i.e. 128/2 = 128) C 10 H e - C 10 H e -

Points to Note The base peak (most intense peak) is not always the molecular ion peak. The intensity of a peak represents the stability of the ion formed. The M + peak will give you the molecular mass of the compound. A mass spectrum of organic compound is also known as the fragmentation pattern. Generally small peaks are ignored. Mass spectra were also used to analyse elements and their isotopes.

The Difference Between Peaks The difference between peaks can also give you information about fragments e.g. if the molecular ion peak is at 58 and there is another peak at 43, a difference of 15, this would suggest that a –CH 3 fragment has been broken off Common Mass Differences C6H5C6H5C6H5C6H577 COOH45 CH 3 O 31 C2H5C2H5C2H5C2H529 C=O or C 2 H 4 28 OH17 CH 3 15 Suggested Group Mass Difference

High Resolution Mass Spectrometry This can be used to distinguish between molecules with similar molecular mass. This can be used to distinguish between molecules with similar molecular mass. FormulaMolecular Mass Exact Mass C 4 H 4 N 5 C 4 H 10 O 4 C 6 H 4 NO 2 C 6 H 6 N 2 O

Exercise Try the exercise on page 7 of your Unit 3(d) notes. Work through the mass spectra questions on the Philip Harris revision sheets.