Mass spectrometry

Mass spectrometry AS level AQA flashcards

What is mass spectrometry used for?

It is used to determine the masses of isotopes of element and their relative abundances. It is also used to determine the masses of compounds.

What are the 2 ionisation techniques?

Electron impact and electro spray ionisation

What is electron impact ionisation?

The sample is vaporised. High energy electrons are fired at sample from an electron gun. One electron is knocked off the atom/compound. This creates a positively charged particle.

Give an equation for electron impact ionisation.

X(g) + e- —> X(g) + 2e- (The 1+ ions are then attracted towards a negative electric plate where they are accelerated)

What is fragmentation and how do they occur?

The molecular ions are formed by knocking off an electron in ionisation and the molecules are likely to break up into fragments when they are subjected to electron impact ionisation. There are many peaks seen on a spectrum due to the different fragments.

What is electrospray ionisation?

Sample is dissolved in a volatile solvent. It is injected through a fine hypodermic needle to give a fine mist. The tip of the needle carries a high voltage. Each atom/compound picks up a proton as it leaves the needle becoming positively charged. (This technique is largely used for larger molecules like protein. As it is a soft ionisation process, fragmentation rarely occurs).

What charge does the atom gain in electrospray ionisation?

The mass of the atom is +1 greater than the actual mass as it gains a proton H⁺

What happens when an electric field is applied to ions

An electric field is applied to give all ions with the same charge a constant kinetic energy

Formula for kinetic energy

KE = ½mv². (KE = kinetic energy in J, m = mass of particle in kg, v = velocity of the particle in ms^-1)

Formula for the velocity of each particle

V = square root (2KE/m) heavier particles move slower than lighter particles

What is the flight tube

The flight tube is the region with no electric field which ions enter. It separates the particles based on their different velocities.

Formula for time

T = d/v (d = distance, v = volume)

Formula for the time along the flight tube

T = d(square root of m/2KE)

Which ions hit the negatively charged plate?

The positively charged ions hit the negatively charged plate and generate a small electric current as they gain an electron. The size of the current is proportional to the abundance of the ion.

How do you identify unknown elements/compounds

Take a mass spectrum to find m/z. The compound would give a signature fragmentation pattern, which acts like a fingerprint for that particular molecule. Compare with mass spectrum of known compounds to identify the compound. A pure sample is required for identification.