TOF Mass Spectrometry


Electrospray Ionisation

1. Sample is dissolved in volatile solvent and injected through a fine hypodermic needle to give a fine mist.

2. Tip of needle is attached to positive terminal of a high - voltage power supply.

3. Particles are ionised :  they gain a proton from the solvent as they leave the needle producing +ve ions

X(g) + H+ = XH+

4. The solvent evaporates while the XH+ ions are attracted towards a negative plate. Here they're accelerated.

  • Useful for substances with a high formula mass
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Electron Impact

1. Sample being analysed is vaporised and high energy electrons are fired at it.

2. The high energy electrons are from an electron gun : a hot wire filament with a current running through it emitting electrons.

3. This usually knocks off 1 electron from each particle forming a 1+ ion.

X(g) + e- = X+(g) +2e-

 X(g) = X+(g) + e-

4. The 1+ ions are then attracted towards a negative electric plate where they're accelerated.

  • Used for substances with a low formula mass.
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1. The +ve ions are accelerated using an electric field so they have the same kinetic energy.

2. As V=¬2KE/m, the velocity of each marticle depends on it's mass.

3. Lighter particles have a faster velocity and heavier particles have a slower velocity.

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Flight tube / Ion drift

1. The positive ions travel through a hole in the negatively charged plate into a tube.

2. The TOF of each particle through this tube depends on it's velocity so it depends on it's mass.

3. TOF is directly proportional to ¬mass of ions.

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1. Positive ions hit a negatively charged plate.

2. When they hit the detector plate the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hense an electric current that is measured.

3. The size of the current gives a measure of the number of ions hitting the plate.

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Data Analysis

1. A computer uses the data to produce a mass spectrum.

2. This shows a m/z ratio and abundance of each ion that reaches the detector.

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