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