the mass spectrometer

Question 1

what is the mass spectrometer?

Answer 1

• a machine used to analyse elements or compounds

Question 2

what does a mass spectrometer give information about?

Answer 2

• the relative atomic mass of an element and the relative abundance of its isotopes or the relative molecular mass of a molecule if you use it to analyse a compound

Question 3

how many things are there that happen when a sample is squirted into a time of flight mass spectrometer (TOF)? and name them

Answer 3

4 things:
- ionisation
- acceleration
- ion drift
- detection

Question 4

describe ionisation

Answer 4

• the sample needs to be ionised before it enters the mass spectrometer. two ways of doing this are:
  -> electronspray ionisation - in this method, the sample is dissolved in a solvent and pushed through a small nozzle at high pressure, a high voltage is applied to it, causing each particle to gain a H+ ion, the solvent is then removed, leaving a gas made up of positive ions
  -> electron impact ionisation - in this method, the sample is vaporised and an electron gun is used to fire high energy electrons at it, this knocks one electron off each particle, so they become 1+ ions

Question 5

describe acceleration

Answer 5

• the positive ions are accelerated by an electric field, the electric field gives the same kinetic energy to all the ions, the lighter ions experience a greater acceleration - they’re given as much energy as the heavier ones, but they’re lighter so accelerate more

Question 6

describe ion drift

Answer 6

• the ions enter a region with no electric field, they drift through it at the same speed as they left the electric field, so the lighter ions will be drifting at higher speeds

Question 7

describe detection

Answer 7

• because lighter ions travel through the drift region at higher speeds, they reach the detector in less time than heavier ones, the detector detects the current created when the ions hit it and records how long they took to pass through the spectrometer, this data is then used to calculate the mass / charge ratio values needed to produce a mass spectrum

Question 8

What is a mass spectrum?

Answer 8

• a type of chart produced by a mass spectrometer

Question 9

What does a mass spectrum show?

Answer 9

• information about the sample that was passed through the mass spectrometer

Question 10

What is mass spectrometry used to identify?

Answer 10

• elements

Question 11

What do elements with different isotopes produce?

Answer 11

• more than one line in a mass spectrum because the isotopes have different masses
• this produces characteristic patterns which can be used as fingerprints to identify certain elements