1.1 Atomic structure + Mass Spectrometry

What is the purpose of Mass spectrometry?

What are the steps involved?

• To determine all isotopes present in a sample of an element, to hence identify the element.

VIADD:

• Vaporisation

• Ionisation (2)

• Acceleration

• Detection

• Data Analysis

Describe the steps involved in TOF Mass spectrometry

1 - Vaporisation of the sample

2 - Ionisation: Electron impact vs Electrospray ionisation:

• Sample must be ionised so that 1) Positive ions can be attracted by the negative plate + move towards it and 2) So that the ions can be detected

3 - Acceleration:

• Positive ions are accelerated

• By attraction to an electric field / negatively charged plate

• To all gain the same KE

4 - Detection:

• Positive ions travel through flight tube + reach detector

• They generate a current as they gain electrons

• Current is produced by transfer of electrons from detector » positive ions (gain of electrons)

• Size of the current = Proportional to abundance of isotopes

5 - Data Anlaysis:

• Current = read by a computer to produce a mass spectrum » determines identity of element

Describe Electron impact ionisation

-Used for elements with smaller RAM

-Can cause fragmentation if used on larger organic compounds

• Hot electron gun fires HIGH ENERGY electrons at sample

• This knocks of 1 e- per isotope

• Forming +1 positive ions

E.g: Mg_(g) → Mg_(g)⁺ + e-

Describe Electrospray ionisation

-Used for larger organic compounds

• Sample is dissolved in a volatile liquid

• Then injected into a fine hypodermic needle

• Tip of needle = has high voltage

• Each sample gains a proton / H⁺ ion.

State why the inside of a TOF Mass spectrometer is operated under vacuum (2)

• To stop ions produced from colliding with air / other particles

• So that only the ions produced from the sample are present