Mass Spectrometry Flashcards
What is mass spectroscopy used for?
it gives information about the weight of molecules and their composition
can be used to identify or confirm the structures of compounds - can be used after separation by chromatography to identify them and with NMR
What is the 2 step process of mass spectroscopy?
1 - ionisation = MS only picks up cations (positive charge) or anions (negative charge)
2 - measurement of mass spectrum - MS only measures mass of charged species
What are the methods of ionisation for MS?
1 - electron ionisation/impact
2 - chemical ionisation
3 - fast atom bombardment
What is the method for electron ionisation?
most common method
sample is vaporised and bombarded with high energy electrons
effect is the removal of an electron from each of the sample molecules to give a radical cation (positive charge)
What are the advantages and disadvantages of electron ionisation?
not suitable for involatile (cannot be vaporised) or unstable compounds
- unstable compound can break completely and the molecular ion will not be picked up by the MS
radical cation can break or fragment to give a selection of smaller ions = only compounds are recorded by MS
- electron beam can cause cleavage of the weak bonds in the molecule = splits into fragments
What is the method for chemical ionisation?
the sample is reacted with an ionising reagent to give an ionised product = means it joins with another reactant
What are the advantages and disadvantages of chemical ionisation?
MS can be run in the positive (cation) or negative (anion) mode - not simultaneously
the additional molecular weight due to the ionising reagent must be taken into account when interpreting the final spectra ( finding the molecular weight) = subtract the ionising reagent weight
What is the method for fast atom bombardment (FAB)?
sample is bombarded with xenon atoms to give positive and negative ions
1 - mix the sample with a matrix = example - glycerol
2 - apply to the probe in solid form
3 - bombard with xenon atoms
What are the advantages and disadvantages of fast atom bombardment?
MS can pick up either of the ions generated (positive or negative)
useful for high molecular weight compounds (large) and involatile compounds (polymers)
How does MS instrumentation work?
small sample is required - 1 mg or less
interior of spectrometer is kept under high vacuum
sample is heated to vaporise it
sample is ionised and the ions are accelerated to high velocity by charged plates
beam of ions will change directions if a magnet is introduced
What are the factors that affect the effect the magnet has on ions in the MS?
magnet strength - stronger magnet means a bigger change in direction
charge of the ion - higher charger means it is more attracted to the magnet so bigger change in direction
mass of the ion - heavier ions will be deflected less than light ions as they move slower
What is the base peak and M+ in MS?
M+ is known as the molecular ion = last line and represents the weight of the molecule
Base peak is the mosts stable fragment = the highest peak/greatest intensity
What is the effect of isotopes on MS spectra?
peaks in the spectrum are surrounded by smaller peaks with a few mass units difference
most cannot be seen as an element is mainly one isotope - others are in low abundance
In large molecules there will be a greater chance of seeing peaks arising from isotopes with lower abundance
some have isotopic ratios - Cl is 3:1
What is the isotopic ratio of chlorine and bromine?
35Cl and 37Cl are in the ratio 3:1 - 2 peaks one a third the height of the other 2 units apart (units is m/z)
79Br and 81Br are in the ratio 1:1 - 2 peaks almost equal heights 2 units apart (units is m/z)
How does fragmentation occur in MS?
radical ions can react and break up whilst travelling through the spectrometer - forms radicals
when a radical cation forms the charge will be localised on the electronegative element with non-bonding electrons if present = positive charge on the O
