Lecture 40: Mass spectrometry

what is spectroscopy?

how matter interacts with electromagnetic radiation

• the theory

what is spectrometry?

application of spectroscopy to produce results

• practical

what is a spectrometer?

• the instrument used to measure

• expensive but useful

what is mass spec?

• an analytical technique used to measure the mass of a charged ion

what are the 2 components of mass spec?

• ionisation

• separation of charged ions

what is the main process of ionisation?

molecules need to be charged

• sample goes in, gets charged(hit with something) and is then sent through to the analyser

mass to charge ratio is:

m= mass of ion

z= charge of ion

• mass to charge ratio m/z but since z=1usually, we just consider mass of ion

what are the different ways of achieving ionisation?

1. electron ionisation(electron impact)

2. chemical ionisation(stream of protons)

what is electron ionisation(electron impact)?

• hard technique

• generates a radical cation M+ in the +ve mode, or a radical anion M- in -ve mode

• results in lots of fragmentation

what is chemical ionisation(stream of protons)?

• soft technique

• generates ions through protonation to give MH+ or deprotonation to give MH-

• less fragmentation

how are charged ions separated?

1. ionised: picks up or loses e-

2. starts to fragment and is charged in a vacuum

3. smaller particles are more displaced by magnet so are foiund on the left side, large particles are not as displaced so found on the right

how can high resolution mass spec be used?(HRMS)

• mass spec can differentiate between these 2 molecules

• since the exact masses will differ, high res mass spec is used to predict molecular formula but not arrangement

• powerful technique used to determine specific compounds at very low concentrations

• same parent ion, different molecules will fragment differently

what is exact mass used for?

what is molecular weight used for?

• for mass spec/analysis of single molecules

• for yeild calculations- its calculated using average of isotopes

what is the nitrogen rule?

• applies to neutral molecules

• compounds containing C,H,N,O,S and the halogens have even molecular weights

  • compounds containing an odd number of N atoms are odd