Mass spectrum

Question 1

How a mass spectrum works

Answer 1

• inject sample
• heater VAPORISES the sample
• Electron beam IONISES sample atoms
• POSITIVELY CHARGED X+ ions are ACCELERATED into magnetic field by positively charge plates
• Magnetic field DEFLECTS and SEPARATES ions based on their mass/charge ratio
• The HEAVIER the ion, the LESS it is deflected .
• Assume ONLY POSITIVELY CHARGED IONS are produced.

Question 2

Mass spectrum of an element

Answer 2

• Produced by COMPUTER SOFTWARE when the ions are detected
• Displayed on SCREEN or PRINTED

X axis ~ m/z
Y axis ~ % abundance

Question 3

Finding the MOLECULAR MASS of an organic compound from a mass spectrum

Answer 3

• The PEAK with HIGHEST m/z value
• With a SIGNIFICANT ABUNDANCE
• It is called the MOLECULAR ION ,M +

Question 4

The molecular ion

Answer 4

• Formed when an organic compound is IONISED
• GENERAL equation for forming the molecular ion from an organic compound M is:

M + e- ———– M+ + 2e-

Mr = M+ peak found at m/z

• The molecular ion has the SAME mass as the molecule as the ELECTRON has NEGLIBLE MASS.
• It appears as the peak with the highest m/z value with the most significant abundance in the mass spectrum

Question 5

Fragmentation

Answer 5

• EXCESS ENERGY from the IONISATION process can cause the M+ ion to vibrate more VIGOROUSLY.
• This causes them to FRAGMENT into a positively charged ion and NEUTRAL RADICAL SPECIES
• ONLY the positively charged fragment ions will be:
• accelerated
• deflected
• detected
• The FRAGMENT ION can further fragment in the spectrometer.
• The peak with the highest m/z value (M+) indicates the Mr
• The other peaks provide STRUCTURAL information about the compound

Question 6

COMMON FRAGMENTS IONS
Due to breakage of C-C bonds in particular, fragmentation peaks can occur at a range of m/z values:

Answer 6

15 ~ CH3+

17 ~ OH+

29 ~ CH3CH2+ / CHO+

31 ~ CH+OH+

43 ~ CH3CH2CH2+ / CH3+CHCH3 / CH3CO+

45 ~ COOH+

57 ~ CH3CH2CH2CH2+ / (CH3)2CHCH2+ /
(CH3)3C+CH3CH2CO+

71 ~ CH3CH2CH2CH2CH2+ / (CH3)2CHCH2CH2+ / CH3CH2CH2CO+

Question 7

ISOTOPES in a mass spectrum using CARBON

Answer 7

After the SIGNIFICANT peak representing the Mr , a SMALL peak can be seen

• C-13 atom in a molecule gives a tiny M+1 peak in the spectrum
• An EVEN SMALLER peak can arise when two C-12 atoms are replaced with C-13 atoms , called the M+2 peak.