Time of Flight Mass Spectrometry

Mass Spectrometry

Instrumental technique used to find the relative mass of elements and compounds

Stage 1 - Ionisation: Electron Impact

Used for elements and low Mr compounds

• high energy electrons fired at sample by electron gun

• this knocks off one electrons from each atom/molecule to form a 1+ ion

Stage 1 - Ionisation: Electrospray Ionisation

Used for high Mr compounds

• the sample is dissolved in a volatile solvent and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber

• a very high voltage is applied to the end of the needle where the spray emerges (the needle is positively charged)

• the particles gain a proton and become ions as a fine mist

• the solvent evaporates leaving 1+ ions

Stage 2 - Acceleration

• the ions are accelerated using an electric field so that all the ions have the same kinetic energy

Stage 3 - Deflection

• ion drift - the ions enter the flight tube

• charged particles will be deflected by a magnetic or electric field

• ions with different masses have a different time of flight

Stage 4 - Detection

• the detector is a negatively charged plate

• a current is produced when the ions hit the plate - the more ions that hit the detector, the larger the current

• the mass spectrum shows the number of particles of each mass that hit the detector

Interpreting the relative abundance graph

• horizontal axis is the mass to charge ratio (m/z) of the particles that hit the detector but charge is usually 1+ so m/z = Mr

• in an electrospray ionisation, the main peak is usually at Mr + 1 (because the molecule gains a proton)

Average mass of atoms (Ar) =

total mass of all atoms / total number of all atoms

Mass =

Mr / Avogadro’s constant (6.022 × 10²³)

Kinetic Energy (KE) =

mv² / 2

Distance² (d²) =

2KEt² / m

t² (x) / m (x) =

t² (y) / m (y)

Units for mass

kg

Units for velocity

ms^-1

Units for length/distance

m

Units for time

s