4.2.4 (f,g) Mass Spectrometry

Syllabus

(f) use of a mass spectrum of an organic compound to identify the molecular ion peak and hence to determine molecular mass

{Limited to ions with single charges.}

{Learners will not be expected to interpret mass spectra of organic halogen compounds.}

{Limited to organic compounds encountered in this specification (see also 6.3.2 e).}

{Learners should be aware that mass spectra may contain a small M+1 peak from the small proportion of Carbon-13.}

(g) analysis of fragmentation peaks in a mass spectrum to identify parts of structures.

{Learners should be able to suggest the structures of fragment ions.}

Powerpoint

What does this mean?

How it works

Again, it's not necessary to understand how a Mass Spectrometer works for the exam but a simple explanation may help understand the topic in general.

The sample is put in as a gas and then ionised by firing electrons at it.

Depending on what you want to believe this either deflects away electrons in the molecules making them positive ions, or gets absorbed by the molecule making it unstable and causing it to break into a positive ion and another electron.

X + e^- → X⁺ + 2e^-

X⁺ is the whole molecule except for one electron of almost no mass.

So it is called the Molecular Ion and has the same mass as the substance we are studying.

Mass Spectrometers can't make molecules larger so the Molecular Ion will be shown as the heaviest ion.

And its mass is the substance's M_r.

The may be a very small peak a touch heavier than the molecular ion.

This is the M + 1 peak and is still the whole molecule but containing a ¹³C atom rather than the usual ¹²C isotope.

M+1 peaks are very small because the % of ¹³C is so small in any sample.

The ions are then accelerated and then deflected around a curve by an electromagnet.

This sorts them by mass and charge.

Light ions are more easily deflected than heavy ones.

2+ ions are more easily deflected than 1+ ions and can form when an ion is hit by two electrons before acceleration.

Ions are then detected electrically - the more ions that arrive the bigger the current they will induce.

Videos

Fragmentation

Pentane has a mass of 72 - we can read this from the peak on the right - the molecular ion.

Notice that the x-axis is m/z -which stands for mass-charge ratio - the two factors that affect deflection.

The other peaks are formed when a molecule of Pentane splits into an ion and a radical when hit by the electron.

CH₃CH₂CH₂CH₂CH₃+ e^- → CH₃CH₂CH₂CH₂⁺ + •CH₃ + 2e^-

This will produce a peak at 57 - the M_r of CH₃CH₂CH₂CH₂

The radical would not be detected, although it is likely that the same fragmentation might happen as:

CH₃CH₂CH₂CH₂CH₃+ e^- → •CH₃CH₂CH₂CH₂ + CH₃⁺ + 2e^-

Which would produce a peak at 15 (not shown).

Fragmentation allows us to tell the difference between molecules with the same M_r.

For instance, both Ethanol and Methoxymethane (CH₃OCH₃) have the formula C₂H₆O.

So they both have a molecular ion at 46.

But only Ethanol can produce a fragment of CH₃CH₂⁺ at 29.

So it is possible to distinguish between them in this way.

Intersetingly, the peak at 29 mis more common than the molecular ion - we call it the base peak.

You won't have to know why it's more common.

You need to be able to identify what fragments are responsible for each peak.

This is a matter of adding up.