Year 11 Chemistry - Mass Spectrometry and Relative Mass

what is 1 amu

the mass of 1/12 of a carbon - 12 atom

what is relative atomic mass

the average mass of atoms compared to 1/12 of the mass of a carbon - 12 atom

what is relative formula or molecular mass

the mass of a molecule or formula unit compared to the mass of 1/12 of a carbon - 12 atom

what is the weighted average

it takes into account isotopes and their abundances by averaging the relative masses

when was mass spectrometry invented

1927 by Francis Aston

what are the 5 steps of mass spectrometry

1. vaporisation
2. ionisation
3. acceleration
4. deflection
5. detection

what occurs during vaporisation and why must the sample be vaporised

the sample is injected inot the chamber at high vacuum and at high temperature to vaporise it

it must be vaporised so the atoms or molecules are mobile

what happens during ionisation

in the next chamber a beam of electrons knocks electrons off of vaporised atoms to form positive ions

the electron beam is produced by a heated metal coil and these electrons are attracted to a positive electron trap

the positive ions are forced out of chamber by positive ion repeller

how many electrons are typically lost and why

95% of ions gain only a 1+ charge and 5% gain a 2+ charge because it is difficult to strip a 2nd electron off an already positive ion

why must the mass spectrometer be a vacuum

to prevent uneccessary collisions of ions with air particles

what occurs during acceleration

in the 3rd chamber the positive ions pass through slits which have differing voltages

the last slit is always 0V

ions accelerate into a fine beam

what occurs during deflection

an electromagnetic deflects ions depending on their mass and charge

lighter ions deflect more

larger charged ions deflect more

the mass and charge form the m/z ratio

to analyse all ions electromagnet can be adjusted

if an ion hits wall of deflection chamber it becomes neutralised

what occurs during detection

when the positive ions hit detection plate, they neutralise due to an electorn leaving the plate ot join it

the electron leaves a space in the plate, causing other electrons to shuffle and fill space

the shuffling of electrons is detected as a current, which is amplified and recorded

the more ions, the larger the current

what does mass spectrometry produce

a stick diagram which graphs relative abundance against the m/z ratio

how do you determine the relative abundance percentage

1. add up all relative abundances
2. determine each abundance as a percentage out of abundance total

how do you determine relative atomic mass using stick diagram

1. use the calculated relative abundance percentages
2. multiply each isotopes relative mass by their percentage abundance
3. add up these values and divide by 100