Time of flight (TOF) mass spectrometry

What is a common form of mass spectrometry?

Time of flight (TOF) spectrometry

What are the uses of mass spectrometry?

Carbon 14 dating

Detecting illegal drugs

Forensic science

Space exploration

What is the first stage of TOF and what happens in this stage?

The first stage is known as ionisation and there are two different types:

Electron impact (also known as electron ionisation) and electrospray ionisation

What happens in electron impact?

Only used for elements with low Mr compounds such as hydrogen for example

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

This knocks off one electron from each atom/ molecule to form 1+ ions

X(g) → X+(g) + e- ( g would be the sample)

What happens in electrospray ionisation?

Only used for high Mr compounds (e.g. proteins)

Sample is dissolved in a volatile solvent (e.g. methanol. water) and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber

A very high voltage is applied to the end of the needle where the spray emerges (the needle is positively charged)

The particles gain a proton and become ions as a fine mist

X(g) + H+(g) → XH+(g) (g is the sample)

The solvent evaporates leaving 1+ ions

What si the second stage of TOF and what happens?

Acceleration of ions

The ions are accelerated using an electric field so that all the ions have the same kinetic energy

What is the 3rd stage of TOF and what happens?

Separation of charged ions

Ion drift- the ions then enter the flight tube

Ions with different masses (m) have a different time of flight

The lighter ions travel faster and take less time to reach the detector

What is the 4th stage of TOF and what happens?

Detection

The detector is an negatively charged plate- a current is produce when the ions hit the plate- the more ions that hit the detector the bigger the current

The mass of the ions hitting the detector cab be calculated from the time of flight

The mass spectrum shows the number of particles (abundance) of each mass that hit the detector

The horizontal axis is actually the mass to charge ratio of the particles that hit the detector, but as the charge is usually +1 the m/z ratio is effectively the mass

In an electroscopy ionisation mass spectrum, the main peak is usually at Mr +1, and so the Mr is 1 less than the molecular ion peak (e..g if peak is 505, the Mr is 504