B4 - Mass Spectrometry

Question 1

What is mass spectrometry used for?

Answer 1

To identify the molecular mass of an organic compound and to gain further information about its structure.

Question 2

What happens when an organic compound is placed in the mass spectrometer?

Answer 2

It loses an electron and forms a positive ion, the molecular ion.

Question 3

What does the mass spectrometer detect after the organic compound has been placed in there?

Answer 3

The mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound.

Question 4

How can you find the molecular mass from a mass spectrum?

Answer 4

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

Question 5

What is the M + 1 peak? What causes it?

Answer 5

It is a very small peak one unit after the M+ peak. It exists because 1.1% of carbon is present as the carbon-13 isotope.

Question 6

What is fragmentation?

Answer 6

The process in which some molecular ions break down into smaller pieces known as fragments, when in the mass spectrometer.

Question 7

What causes the other peaks in a mass spectrum?

Answer 7

Fragment ions.

Question 8

What does the simplest fragmentation involve?

Answer 8

Breaking a molecular ion into two species - a positively charged fragment ion and a radical.

Question 9

Are radicals, created from fragmentation, detected by the mass spectrometer?

Answer 9

No, as they are uncharged.