17.1 - Mass Spectrometry

Question 1

What is mass spectra used for?

Answer 1

To identify the molar mass of an organic compound and its structure

Question 2

What is the molecular ion?

Answer 2

• When an organic compound is placed in a mass spectrometer, it loses an electron & forms a +ve ion known as the molecular ion.

Question 3

What is m/z?

Answer 3

A mass spectrometer detects the m/z (mass:charge ratio) of the molecular ion which gives the molecular mass of the compound.

Question 4

How do you identify the Mr from a mass spectra?

Answer 4

• M+ peak (molecular ion) must be identified

It is the peak furthest to the RHS of the graph.

Question 5

What is the M+1 peak?

Answer 5

The M+1 peak is the peak that may appear just after the M+ peak, this is due to the presence of a carbon-13 isotope that increases that compounds Mr by 1. This should not be considered as the Mr of the compound.

Question 6

What is fragmentation?

Answer 6

• Some molecular ions break down into smaller fragments
• Other peaks are known as fragment ion peaks
• The simplest form of fragmentation breaks down a molecule into a +ve fragment ion and a radical
• The +ve fragment ion will be identified and the uncharged radical won’t be detected.

Question 7

What are common fragment ions?

Answer 7

CH3+: 15
C2H5+: 29
C3H7+: 43
C4H9: 57

Question 8

What is the formula for working out the number of carbon atoms?

Answer 8

(M+1 peak height)/(M+ peak height) * 100