Topic 7 - Modern analytical techniques

Question 1

what does the largest m/z mean in a mass spectrum?

Answer 1

the peak would be caused by the complete molecule, and will be equal to the molar mass of the molecule. This peak is called the molecular ion.

Question 2

what type of substance is read by the mass spectrometer?

Answer 2

positive ions

Question 3

How does a molecular ion form and how do you draw it?

Answer 3

• bombarded with electrons

- square brackets with positive charge and dot.

Question 4

what is fragmentation? What is formed by fragmentation?

Answer 4

• when the molecular ion is broken into fragments by breaking covalent bonds.
• positive ion (responsible for the peak).
• free radical.

Question 5

what does a molecule become?

Answer 5

a molecular ion.

Question 6

what does a molecular ion become?

Answer 6

• positive ion (read by spectrometer)

- free radical.

Question 7

what do peaks indicate on the mass spectrum?

Answer 7

the mass/charge (m/z) of every fragment.

Question 8

what sort of modern analytical technique is used in breath alcohol analysers?

Answer 8

infrared spectroscopy.

Question 9

What is infrared spectroscopy used for?

Answer 9

• predicting the spectrum of an unknown compound
• deducting the functional groups from a list of wave numbers
• deducing the structure from wavenumbers and molecular formula.

Question 10

what does the amount of infrared radiation energy absorbed by a molecule depend on (stretching vibration)?

Answer 10

• length of the bond
• strength of the bond
• mass of each atom involved in the bond.

Question 11

why don’t simple non-polar molecules e.g H2 and Cl2 absorb infrared radiation?

Answer 11

• they do not have a dipole
• when the molecule is stretches due to vibration, there is no change in the dipole, so there is no absorption of infrared radiation.

Question 12

How does bond stretching occur?

Answer 12

Bond absorbs IR and uses it to alter the length of the bond.

Question 13

what is the fingerprint region?

Answer 13

• between 1500 and 500 cm^-1.
• caused by the vibration of the molecule as a whole (and not the individual bonds).
• the fingerprint region is unique for every compound.

Question 14

what does the region above 1500cm^-1 show? (above fingerprint region)

Answer 14

the points of strong absorption (above 1500cm^-1) can show what bonds and functional groups are present in a compound.

Question 15

Do we use the fingerprint region (1500 - 500cm^-1) when identifying bonds from wave numbers?

Answer 15

• no

- use above 1500cm^-1.