spectroscopy and proof of structure: mass spectrometry

basic scheme of mass spec

1. vaporise and ionise molecule - hard techniques for small/volatile molecules, soft techniques for delicate molecules

2. accelerate ions - use plates of opposite charge to ions to give ions kinetic energy

3. separate ions by mass - mass filters or time of flight (deflect ions using magnetic field onto detector)

4. detect ions

electron impact ionisation

• use for volatile molecules

• bombard sample with electrons in ionisation chamber - use beam of high energy electrons (70 eV electrons)

• cations formed

• leads to extensive fragmentation

• cations then accelerated through negatively charged plates and deflected by magnetic field onto a detector

• computer produces mass spectrum of abundance against m/z

matrix assisted laser desorption and ionisation

• prepare sample in a matrix that absorbs light

• flash high beam laser light onto the sample

• matrix absorbs laser energy and converts it to heat so that the matrix vaporises, bringing the molecules being analysed with it and ionising them

• ions are sorted and separated by an extraction grid according to their mass and charge

electrospray ionisation

• used for soft molecules

• uses a solution sample - compound under study dissolved in a volatile solvent

• the solution is then converted into a fine spray at atmospheric pressure using a high electrode potential - can be operated in positive/negative ion mode to give rise to positively/negatively charged droplets

• solvent evaporates whilst sample is transported to counter electrode to form gas phase ions

• gas phase ions pass through mass analyser at counter electrode

time of flight mass spectrometry

• generate ions at a discrete time using MALDI or ESI

• accelerate ions with an electric field so that they move towards the detector

• smaller ions = arrive faster

• larger ions = arrive slower

• record time - by recording time between ion generation and arrival time at the detector m/z can be deduced 

fragmentation patterns

• fragmentation - when molecular ion is broken up into smaller fragments

• looking at fragmentation peaks tells about molecular structure - can reconstruct molecule from the fragments

• larger molecule = more fragments generated

• fragmentation routes can involve complex mechanisms with formation of radicals and neutrals as well but ions are only detected

• fragmentation follows pathway that leads to the most stable ions - in turn these give strongest peaks in mass spectrum

base peak

maximum intensity peak

molecular ion peak

• highest possible mass of molecule gives the molecular mass of the molecule

• this peak may be weak or absent if the molecular ion is very unstable

nitrogen rule

• molecular weight is always even from molecules made up of the common elements unless they contain an odd number of nitrogen atoms

rings plus double bonds

• For a molecule C_xH_YN_zO_n the ‘ring plus double bonds’ is given by:

RDB = X - ½Y + ½Z +1

• if RDB is 4 or more it may mean molecule is aromatic

exact masses

• high resolution mass spec can be used to determine empirical formula uniquely for species with similar masses

effect of isotopes on mass spec

• gives cluster of peaks with intensities that reflect the natural abundances of the isotopes

• abundances of isotopes well known

• splitting pattern gives a clue as to what elements are present, use simple statistics to work out splitting patterns