Mass spectrometry

What is mass spectra used to do?

Identify the molecular mass of an organic compound and to gain further information about its structure

What is a molecular ion, and how is it formed?

When an organic compound is placed in the mass spectrometer, it loses an electron to form a positive ion - the molecular ion (M+)

• Molecular ions are usually formed as a result of electron bombardment

How does a mass spectrometer work?

It detects the mass-to-charge ratio (m/z) of the molecular ions which gives the molecular mass of the compound

Show the formation of the molecular ion from ethanol

C₂H₅OH + e^- → CH₂H₅OH⁺ + 2e^-

How do we find the molecular mass from a mass spectrum?

The molecular ion peak (M+ peak) is the clear peak at the highest m/z value on the right-hand side of the mass spectrum

On the mass spectrum you will usually see a very small peak one unit after the M+ peak. What is this and why does this exist?

This is the M+1 peak, which should be ignored

• This peak exists because 1.11% of all 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 ¹³C, and thus will have a molecular mass of 61, giving the M+1 peak

What is fragmentation and how does this occur?

The process where some molecular ions break down into smaller pieces known as fragments

• Excess energy from the ionisation process causes bonds in molecular ions to vibrate, weaken and split into fragmentation ions

Apart from the M+ peak, there are also other peaks on the mass spectrum. Why is this?

The other peaks are caused by fragment ions, formed from the breakdown of the molecular ion

What are the products of fragmentation?

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

Show the products of fragmentation of this molecular ion:

CH₃CH₂CH₂OH⁺

CH₃CH₂CH₂OH⁺ → CH₂OH⁺ + CH₃CH₂⋅

Give a similarity and a difference between these two mass spectra

• Both have M+ peak at 86

• Different fragmentation patterns

Give the m/z value for CH₃+

15

Give the m/z value for C₂H₅⁺

29

Give the m/z value for C₃H₇⁺

43

Give the m/z value for C₄H₈⁺

57

Give the m/z value for C=O⁺

28

Give the m/z value for COCH₃⁺

43

Give the m/z value for OH⁺

17

Give the main features of the spectrum

• M+ peak at m/z = 46

• Small M+1 peak at m/z = 47

• A number of fragment ion peaks

Give the fragment ions of the spectrum

• Peak at m/z = 15 for CH₃⁺

• Peak at m/z = 29 for CH₃CH₂⁺

• Peak at m/z =31 for CH₂OH⁺

• Peak at m/z = 45 for CH₃CH₂O⁺