Atomic Structure & Time Of Flight Spectrometry

what condition must the whole mass spectrometer apparatus be kept under, and why?

under a high vacuum

• to prevent the ions that are produced from colliding with molecules present in the air

Definition of relative atomic mass

The average mass of an atom of an element, compared to 1/12th the mass of an atom of carbon-12

Definition of relative molecular mass

The average mass of a molecule compared to the 1/12th the mass of an atom of carbon-12

Stages of TOF

1. Ionisation

2. Acceleration

3. Ion drift

4. Detection

Electrospray ionisation

typically used with large, organic molecules (so that fragmentation does not occur)

• the sample is dissolved in a volatile and polar solvent

• the sample is injected at high pressure through a fine, hollow needle connected to the positive terminal of a high voltage supply

• this causes the sample molecule to gain a proton from the solvent

• X(g) + H+ ———> XH+ (g)

electron impact ionisation

• Electron gun fires high energy electrons

• Vapourised atoms are bombarded by high energy electrons, which knocks out an electron

• X(g) ——→ X+ (g) + e-

why must the sample be ionised?

• so that it can accelerate towards the detector

• so that it can be detected

acceleration

• the positive ions formed from either of the ionisation methods are then accelerated to a constant kinetic energy by an electric field, towards a negatively charged plate

• all ions have the same kinetic energy, but the lighter ions travel faster

Ion drift and detection

• the ions travel along the flight tube, where they reach a detector

• the positive ions hit the detector, causing the electrons to flow, producing a current

• the amount of current produced is proportional to the abundance of the species

• the greater the current, the greater the abundance

Identify which one of the isotopes will be deflected the most

the one with the lowest m/z value

suggest what might causes the RAM of a sample to be different from the RAM given in the periodic table

Other isotopes are present

• Similarity: the peaks would be at the same m/z values

• Reason: because the same element is being detected

• Difference: there would be an additional peak at ½ m/z valye

• Reason: because 2 electrons are knocked out

The mass spectrum of tellurium also has a small peak at m/z = 64. Explain the existence of this small peak.

2 electrons are removed from tellurium, forming ¹²⁸Te²⁺