Mass Spec

Question 1

What is a TOD and what is it used for?

Answer 1

A TOF mass spectrometer is used to determine information about elements and compounds.

It can determine the relative isotopic masses of isotopes of elements and their relative abundance allowing us to determine its Ar.
It can find the Mr of substances made of molecules.

Question 2

Electron impact

Answer 2

• A vaporised sample is injected at low pressure.
• High energy electrons from an electron gun are fired at the sample.
• One electron from each particle is usually knocked off forming a positive ion.

Question 3

Electro spray ionisation

Answer 3

• Sample dissolved in a volatile polar solvent and injected through a needle
• The tip of the needle has a high voltage.
• The particles are ionised by gaining a proton from the solvent as it leaves the needle.
• The solvent evaporates leaving a gas made up of positive ions.

Question 4

Describe stage 2: acceleration

Answer 4

• The positive ions are accelerated by an electric field.
• The electric field gives the same KE to all the ions.

Kinetic energy = 1⁄2 mv2

• Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass.
• Lighter particles have a faster velocity, and heavier particles have a slower velocity

Question 5

Describe stage 3: ion drift

Answer 5

• The positive ions travel through a hole in the negatively charged plate into a tube with no electric field.
• They drift through the tube at the same speed as they left the electric field.
• The ions are distinguished by different flight times – lighter ions will drift at higher speeds.

Question 6

How are the ions detected?

Answer 6

• The positive ions hit a negatively charged electric plate.

• When they hit the detector plate, the positive ions are
discharged by gaining electrons from the plate.

• This generates an electric current that is measured.
• The detector records:
• time taken for the ion to reach it
• current

• Greater abundance of an isotope generates a larger current

Question 7

How is the data analysed?

Answer 7

• The data collected is used to calculate the mass to charge (m/z) ratio needed to produce a mass spectrum.
• If the sample is an element, each line represents a different isotope of the element.

Question 8

How does the data look like depending on the method used?

Answer 8

• As all ions produced by electron impact have a 1+ charge, the m/z of each peak is the same as the relative mass of that isotope
• If electrospray ionisation is used, a H+ ion would have been added to each particle so the m/z of each peak would be one unit greater than the relative mass of each isotope.