3.2.11 Analytical Techniques - Mass spectrometry

Specification

The mass spectrometer

Mass spectrometers have several uses in the fields of detection and analysis.

• Isotopic measurements
• Organic structural determination
• Accurate mass determinations

The mass spectrometer

Operating principles

The mass spectrometer is an instrument used for two main purposes:

• 1. Measuring the exact masses of atoms.
• 2. Measuring the masses of the breakdown products from molecules when they are smashed to pieces by high energy electrons. This is also called the fragmentation pattern and may be useful in elucidation of the structure of a molecule.

The operating principles are the same in both cases. A sample is injected into the instrument and it is vaporised before meeting a stream of high energy electrons that turn the atoms into ions (by dislodging electrons) or, if we are dealing with molecules, causes the molecules to break apart (fragment). The ions that are produced in each case are separated by magnetic fields and detected with a high degree of accuracy.

Mass spectrometer stages of operation:

• injection
• vaporisation
• ionisation
• acceleration
• deflection
• detection
  

The final read-out may be graphical or digital and gives information about the relative abundance of all of the particles produced by the stream of electrons as well as their exact masses. A typical graphical read-out for the analysis of an element looks like this:

Mass spectrum of chloropropane

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Fragmentation

In organic chemistry the mass spectra of molecules are rather more complex due to the breakup (fragmentation) of the molecule in the electron beam and immediately afterwards.

Once the molecular ion has been formed in the high energy beam of electrons, it is likely that this ion wlll break apart into fragments. Each fragment is either an ion itself, or a neutral species. The ions formed by fragmentation can also be detected in the mass spectrometer trace.

The likelihood of a specific ion forming depends on the bond energy of the bond that must be broken and the stability of the fragment formed.

Typically, alkyl groups, acyl groups and allyl groups are most easily formed and often appear in mass spectra.

As each of these fragments has a specific m/e value, an experienced analyst recognises the fragments as they are formed and uses this information to help build a structure of the fragmenting molecule.

The spectrum showing several different peaks due to fragmentation, gives rise to a fragmentation pattern, which is the suggested way that a specific molecule has broken apart

There are two approaches to fragmentation.

1 Start from the molecular ion and calculate the mass of the particles that have been subtracted from the m/e value of the molecular ion to give the peaks seen.

The second approach involves looking at the fragments at the low m/e end of the spectrum.

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Rearrangement

One complication that arises during molecule fragmentation is that the bonds don't simply break giving fragments, but they can also reform and fragments themselves can rearrange to give more stable structures.

Full treatment of this is beyond the course, but it should be noted, as fragments often appear that cannot be explained by simply breaking bonds.

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