17 Spectroscopy

Question 1

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

Answer 1

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

Question 2

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

Answer 2

• To find the molecular mass, the molecular ion peak has to be located.
• It is the clear peak at the highest m/z value on the right hand side of mass spectrum.

Question 3

Why do we see the M+1 peak?

Answer 3

It exists because 1.1% of carbon is present as the carbon-13 isotope.

Question 4

What is fragmentation?

Answer 4

When some molecular ions break down into smaller pieces known as fragments.

Question 5

What are the other peaks in the mass spectrum caused by?

Answer 5

They are caused by fragment ions, formed from the breakdown of the molecular ion.

Question 6

What does the simplest fragmentation break a molecular ion into?

Answer 6

• It breaks the molecular ion into a positively charged fragment ion and a radical.
• Only the positive ions will be detected by the mass spectrometer,

Question 7

What is the m/z value for fragment ion CH3+?

Answer 7

15

Question 8

What is the m/z value for fragment ion C2H5+?

Answer 8

29

Question 9

What is the m/z value for fragment ion C3H7+?

Answer 9

43

Question 10

What is the m/z value for fragment ion C4H9+?

Answer 10

57

Question 11

What are the two type of vibrations in bonds?

Answer 11

1) A stretch- Rhythmic movement along the line between atoms so that the distance between the two atomic centres increases and decreases.
2) A bend- results in a change in bond angle

Question 12

The amount that a bond stretches or bends depends upon:

Answer 12

• The mass of the atoms in the bond: heavier atoms vibrate more slowly than lighter atoms.
• The strength of the bond: stronger bonds vibrate faster than weaker bonds.

Question 13

How does infrared spectroscopy work?

Answer 13

1) The sample under investigation is placed inside an IR spectrometer.
2) A beam of IR radiation in the range 200-4000 is passed through the sample.
3) The molecule absorbs some of the IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample.

Question 14

What are the applications of infrared spectroscopy in real life?

Answer 14

• Remote sensors analyse the IR spectra of vehicle emission to detect and measure carbon monoxide,dioxide and hydrocarbons in busy areas to monitor pollution.
• Also used in roadside breathalysers to detect ethanol.

Question 15

What is the difference between infrared spectra and mass spectra?

Answer 15

Mass spectra is used to find the molecular mass of unknown/known molecule whereas IR spectra is used to confirm the presence of specific types of bonds.