Section 1 : Atomic structure - Relative Mass and The Mass Spectrometer

Question 1

Which piece of equipment is use to determined the relative atomic mass of an element?

Answer 1

A time of flight Mass Spectrometer

Question 2

Why is the time of flight spectrometer set up in a vacuum?

Answer 2

The time of flight spectrometer is set up in a vacuum so the particles can pass through without any hinderance.

Question 3

What are the 4 steps of using a mass spectrometer?

Answer 3

Ionisation
Acceleration
Ion drift
Detection

Question 4

There are two ways to ionise a sample what are these two methods?

Answer 4

Electro spray ionisation

Electron impact ionisation

Question 5

Explain electrospray ionisation:

Answer 5

In electrospray ionisation the sample being ionised is dissolved in a solvent. The dissolved sample is pushed through a small nozzle at high pressures. A high voltage is applied to it which causes each particle to gain a H+ ion . the solvent is then removed leaving a gas made up of positive ions.

Question 6

Explain electron impact ionisation:

Answer 6

In electron impact ionisation the sample being anyalsised is vaporised and an electron gun is used to fire high energy electrons at it. This knocks 1 electron off each particle so the particles become +1 positive ions.

Question 7

Regardless of which method of ionisation is the same type of ion is produced in the ionisation stage , which charge does this ion have?

Answer 7

Positive , the ions created are positive +1 ions.

Question 8

After the ionisation stage in a mass spectrometer what happens?

Answer 8

Acceleration

Question 9

How does the acceleration procedure in a mass spectrometer take place?

Answer 9

Ions are accelerate by a magnetic field. The magnetic field gives the same kinetic energy to all ions. Lighter ions experience more acceleration as they are given the same kinetic energy as heavier ions but are lighter so accelerate more.

Question 10

Which stage occurs after acceleration in the mass spectrometer?

Answer 10

Ion drift

Question 11

What happens in the ion drift stage of the mass spectrometer?

Answer 11

Ions entre a region with no magnetic field . They drift through it at the same speed that they left the magnetic field in the acceleration stage so lighter ions continue to move through the ion drift chamber more quickly than heavier ions.

Question 12

What happens in the detection state of the mass spectrometer process?

Answer 12

Lighter ions travel through the ion drift chamber more quickly so reach the detectors more quickly . As the ins hit the detector they cause a current to flow. The more ions that hit the detector the greater the current produced . By looking at the sizes of the currents produced you can calculate abundance.

Question 13

How do you work out relative atomic mass using a mass spectrum?

Answer 13

(mass of isotope 1 x abundance isotope 1 ) + (mass isotope 2 x abundance isotope 2 ) + etc ….. all divided by total abundance (abundance 1 + abundance 2 + etc…