1.4 The mass spectrometer

Define the term relative atomic mass

Mean mass of 1 atom of an element relative to 1/12 mass of one atom carbon-12

What is mass spectrometry used to find?

• relative atomic mass

• relative molecular mass

What are the 4 key stages in time of flight mass spectrometry?

1. Ionisation

2. Acceleration

3. Flight Tube

4. Detection

What are the two main ways a sample can be ionised?

• electron impact

• electrospray ionisation

When is electron impact used to ionise?

for elements and low Mr compounds

When is electrospray ionisation used?

for high Mr compounds

Describe the method of ionisation by electron impact

1. High energy electrons are fired at the sample from an electron gun

2. This knocks off one electron from each atom forming 1+ ions

3. These 1+ ions are then attracted to a negatively charged plate

What is the general half equation for electron impact?

X (g) → X+ (g) + e-  

Describe the method of ionisation by electrospray

1. Sample is dissolved in a volatile solution

2. This is injected through a hypodermic needle into a vacuum in the ionisation chamber

3. A very high voltage is applied to the end of the needle, making the needle positively charged

4. The particles gain a proton and become positively charged

5. The solvent evaporates leaving 1+ ions which are attracted to a negatively charged plate