Topic 7 - Modern Analytical Techniques 1

Question 1

What is Mass Spectrometry?

Answer 1

Mass Spectrometry is an analytical technique that is used to identify the m/z ratio of the molecules present in a sample.

Question 2

What is Mass Spectrometry used for when an element is out through it?

Answer 2

Mass Spectrometry is used for measuring the mass to charge ratio of atoms of the element. From your mass spectrum you can calculate the Relative Atomic Mass of your element/sample.

Question 3

What is Mass Spectrometry used for when a molecule is put through it?

Answer 3

Mass Spectrometry is used to measure the mass to charge ratio of the fragments of a Molecule. These Fragment ion peaks can help to deduce the structure of the molecule e.g its functional group.

Question 4

What is the formula for Relative Atomic Mass?

Answer 4

Relative Atomic Mass = Σ Isotopic Mass x Relative Abundance/ Total Abundance (not always 100)

Question 5

What happens in the ionisation chamber of the Mass Spectrometer?

Answer 5

The sample is bombarded with high energy electrons causing it to lose an electron and become an ion.

Question 6

What is used to deflect the ions moving through the mass spectrometer?

Answer 6

• Magnet
• Magnetic Field
• An Electromagnet

Question 7

Why is there a vacuum in a mass spectrometer?

Answer 7

• As otherwise the air particles would interfere with the movement of/ collide with / deflect the ions
• The presence of air would cause additional/incorrect peaks to be detected

Question 8

Write an equation for what happens when the ethane molecule undergoes ionisation in the mass spectrometer?

Answer 8

CH3CH3 + e- → CH3CH3+ + 2e-

OR

CH3CH3 → CH3CH3+ + e-

Question 9

Describe the process by which neutral species are produced in a mass spectrometer.

Answer 9

The Molecular ion is bombarded with high energy electrons which cause the molecule to lose an electron and become a molecular ion.
Then, this molecular ion may undergo fragmentation to form a positive fragment ion and a neutral species.
This neutral species cannot be detected by the Mass Spectrometer.

Question 10

What is the molecular ion peak/parent ion peak equivalent to in Mass Spectrometry?

Answer 10

The molecular mass/ molar mass

Question 11

Why is there often a very small peak one unit higher than the molecular ion peak?

Answer 11

Due to the presence of the carbon-13 isotope in the organic compounds

Question 12

How would you distinguish propanal and propanone from their mass spectra?

Answer 12

Both propanone and propanol have molecular ion peaks at m/z 58. However, propanone has fragment ion leaks at m/z 15 and 43, whereas propanal has its peaks at m/z 57 and 29

Question 13

Write equations showing the formation of the two fragments at m/z 15 and m/z 43 from the propanone molecular ion.

Answer 13

C2H5O+ → CH3CO+ + CH3

Question 14

What is Infrared Spectrscopy?

Answer 14

Infrared Spectroscopy is an analytical technique used to identify the functional groups present in a molecule due to the change in vibration of molecules when they absorb IR radiation.

Question 15

What happens when a substance is irradiated by Infrared Absorption?

Answer 15

The molecules present in the substance will absorb some of this infrared radiation causing the bonds of the molecule to vibrate. The different functional groups will absorb the infrared radiation at a characteristic frequency. When talking about absorption refer to the BOND absorbing the IR radiation.

Question 16

What are the 3 types of vibration?

Answer 16

• symmetric stretch
• asymmetric stretch
• bending

Question 17

What does the characteristic frequency that the bond vibrates at depend on?

Answer 17

The functional group present

Question 18

What are stretching vibrations?

Answer 18

This is when the atoms move closer together and further away from each other

Question 19

What are bending vibrations?

Answer 19

This is when the atoms remain the same distance apart from each other but move positions relative to each other

Question 20

What can Infra red Spectroscopy be used to determine?

Answer 20

The functional groups present in a sample

Question 21

What is responsible for Infra red Absorptions?

Answer 21

The bond e.g O-H, C-H etc.

Question 22

What is the Fingerprint region? What is it range?

Answer 22

The fingerprint region is the part of the spectrum which is completely unique to each molecule and can be used to help identify an unknown molecule. It’s range is from 0cm -1 → 1500 cm-1.

Question 23

When describing an Infrared Spectra, what must you do?

Answer 23

• Draw out the display formula of the molecule you have been given
  -Mark off where the fingerprint region is on your spectrum
• Mark the peaks present on the spectrum
• Identify the functional group that corresponds to the peak on the spectrum
• Use the exact wave numbers and describe the peak as sharp or broad

Question 24

Why can’t you just use Infrared Spectroscopy when analysing a compound?

Answer 24

As some absorption bands overlap each other so therefore other analytical techniques such as mass spectrometry must be used

Question 25

Why does O2 not produce an infrared spectroscopy?

Answer 25

Only molecules that undergo a net change in the dipole moment during vibrational and rotational motions can absorb IR radiation. Therefore diatomic molecules do not produce an Infrared Spectroscopy as there is no change in the dipole moment so no IR radiation is absorbed.