Mass number and isotopes - 3.1.1.2

What is the symbol for mass number?

A

What is the symbol for atomic/proton number?

Z

What are the things that mass spectrometry is used for?

• To identify elements

• To determine relative molecular mass

What is relative atomic mass?

The average mass of one atom of an element compared with 1/12 of the mass of one carbon atom

How do you calculate Ar?

(Abundance x mass)1 + (Abundance x mass)2 / total abundance

What is relative molecular mass?

Sum of Ar of all atoms in a molecule

What happens during stage 1 of TOF mass spectrometry? (1)

• Electron impact ionisation

• Sample is vaporised and bombarded with high energy electrons

• An electron is knocked off each particle, forming 1+ ion

When is electron impact ionisation used?

For elements/substances with a low molecular mass

What can be a disadvantage to using electron impact ionisation?

Can cause molecular ion to become fragmented

What is the general equation for electron impact ionisation?

X(g) ——> X+(g) + e-

What happens during stage 1 of TOF mass spectrometry? (2)

• Electrospray ionisation

• Sample is dissolved in a volatile solvent

• Then injected into a mass spectrometer using hypodermic needle, creating a fine mist

• Needle is attached to a high voltage power supply so as sample is injected, particles are ionised by gaining a proton (H+) from solvent

When is electrospray ionisation usually used?

For elements/substances with a higher Mr

What is an advantage of electrospray ionisation?

Fragmentation is unlikely

What happens during stage 2 of TOF mass spectrometry?

• 1+ ions are accelerated using an electric field (they’re attracted to a negatively charged plate)

• They’re accelerated to have the same kinetic energy

• All 1+ ions will have the same kinetic energy so their velocities will depend on their mass

• Lighter ions will move faster, heavier ions slower

What happens during stage 3 of TOF mass spectrometry?

• 1+ ions will move through the hole in negatively charged plate and move into flight tube

• Time of flight if the ions in the tube will depend on their velocity

• Lighter ions reach detector first

What happens during stage 4 of TOF mass spectrometry?

• 1+ ions will hit the negatively charged detector plate

• They gain an electron

• Which discharges the ion and causes a current to be produced

• Detector produces a mass spectrum

What is important about the size of the current produced?

The size of the current produced is proportional to the abundance of ions hitting the plate

What is important about the conditions of the apparatus?

Kept under a high vacuum to prevent the ions produced from colliding with molecules in the air

What does a mass spectrum show?

The mass to charge ratio on the x - axis (m/z)

Abundance on y - axis (%)

What’s the first step of calculating TOF?

Calculating mass of an ion using Avagadro’s constant:

Ar x 10-³ / 6.022 × 10²³

What’s the second step to calculating TOF?

Calculating energy/velocity:

v = √(2KE/m)

What’s the third step to calculating TOF?

Calculating time/distance: