Mass Spectrometry

Accurately determines relative atomic masses.

Can be used to identify an element.

Drawing:

Mass Spectormeter Conditions

Kept under a high vacuum.

-So Ions do not collide wth air molecules.

The sample is is a gaseos state

Mass Spectrometry Stages

Ionisation

Acceleration

Deflection

Detection

Ionisation

A beam of Electrons form an 'Electron Gun' to knock of an electron to form a possitive Ion.

Nearly all Electrons lose 1 electron to form 1⁺ Charge.

Around 5% lose 2 electrons to for a 2⁺ Charge.

Acceleration

Positive Ions are attracted towards negatively charged plates and accelerated at high speed.

The speed they reach depends on mass.

Deflection

The beam of ions move into an electromagnetic field.

The heavier the ion, the less it is deflected.

The lighter the ion, the more it is deflected more.

Th strength of the electromagnet can be changed to direct the ions onto the detecter.

Deflection

The ions hit the detector, accept electrons (causeing them tio lose their charge) and create a current proportional to the abundence of each ion.

From the strength of the feild at which it hits the detector, a computer works out the mass to charge ratio (m/z).

This is normaly presented as a graph.

Calculating Relative Atomic Mass

(mass₁x Abundance₁)+(mass₂x Abundance₂)+...

Total Abundance

Abundance can be amounts or %.

Doubly Charged Ions

Durng ionisation a small number of ions with a 2⁺ Charge are formed. These are deflected more than singly charged ions.

They behave like ions with half the mass.

Therefore there is sometimes a small peak on a mass spectra graph.