Mass Spectrometry

2 uses of mass spectrometry

- finding the abundance and mass of each isotope of an element (and therefore relative atomic mass)
- identifying unknown compounds

ionising compounds (1)

- always forms a 1+ ion
- 2 methods: electrospray and electron bombardment

electrospray

- used for high Mr compounds as other method too aggressive
- sample dissolved in volatile substance (eg water) and injected through very small needle
- particles gain a proton as they leave the needle
- the Mr therefore increases by 1

X (g) + H+ -> XH+ (g)

electron bombardment

- used for elements and low Mr compounds (method is more common)
- an electron is knocked off each particle by the high-energy electrons (fired from electron gun) to form 1+ ions
- sample must be gaseous

X (g) -> X+ (g) + e -

acceleration (2)

- electric field (created by plates) accelerates ions
- all have same kinetic energy

ion drift (flight tube) (3)

- vaccuum
- particles put in tube and left to drift
- the speed they travel is based on their mass (lighter particles move faster and have a shorter time of flight)

ion detection (4)

- +ve ions hit -ve plate, electrons flow to +ve ions (gain electrons back) and this produces an electric current
- size of current is proportional to the number of ions = abundance
- number of currents produced = number of isotopes

data analysis (5)

- computer measures size and amounts of currents and produces a graph
- x-axis is m/z (mass/charge which is often m/1 as they are 1+ ions)

high resolution mass spectroscopy

- give results to 4 decimal places
- easier to distinguish between molecules that have similar Mr

ratios of chlorine molecules on graph

3:1 for chlorine-35 and chlorine-37
9:6:1 for masses of 70, 72 and 74 (due to varying molecules)

ratios of bromine molecules on graph

1:1 for bromine-79 and bromine-81
1:2:1 for masses of 158, 160 and 162 (due to varying molecules)

kinetic energy equation

KE = 1/2.m.v^2

time of flight equation

t = d/v

explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer

only ions can be accelerated using charged plates

only ions create a current when they hit the negative detector plate