Mass Spectrometry Flashcards
What does mass spectrometry measure?
The mass to charge ratio (M/Z), since Z is usually 1 it is usually equal to the weight.
What can mass spec show us?
Atomic mass Molecular weight Molecular formula Isotope ratio Chemical structure
What do you need to do to the sample before it can be analysed using mass spec?
Sample needs to be ionised
General process of mass spec:
ionisation
acceleration
deflection - by magnetic field according to mass
detection
Does mass spec weigh molecules?
Not technically, it measures the mass to charge ratio, since charge is normally one, we can usually take it as weight
What does mass spec measure?
The mass to charge ratio (m/z)
What is the general process of mass spec?
Sample ionised
accelerated magnetic field applied
deflection by field separates ions.
Detection
What species are detected by mass spec?
Only charged species
Can all species be detected by mass spec?
No, only charged species
What do little peaks next to bigger ones in mass spec mean?
isotopes present
Advantages of mass spec?
can be used on lots of substances (polar + non-polar)
Can be integrated with separation techniques (GC and LC)
Small sample size needed
Limitations of mass spec?
High purity sample needed
Destructive
Instruments can be expensive
Applications of mass spec?
Identification of unknown substance radiocarbon dating (abundance of C14 measured)
What does the sample need to be for mass spec?
Solid or liquid needs to be heated to vaporisation.
List the types of ionisation (4 of them)
Electron impact (hard)
Chemical (soft)
electrospray (soft)
MALDI (soft)
What are the soft ionisation techniques? (3)
Chemical
electrospray
MALDI
What type of molecules do you analyse with electron impact ionisation?
Small volatile ones
What type of molecules do you analyse with MALDI
Non-volatile peptides
Is there a lot of fragmentation with hard ionisation?
Yes
Is there a lot of fragmentation with soft ionisation?
No
Do hydrocarbons work well with soft ionisation?
No
What are the 3 types of mass analyser?
Magnetic sector
quadrupole
time of flight
What does varying the magnetic field (B) do in mass spec?
Lower B means lower m/z ions directed to detector
Higher B means higher m/z ions directed to detector
What are the disadvantages of a magnetic sector analyser?
Cost
size - it is huge!
General principle of quadrupole analyser
4 parallel electrodes. Ions attracted to -ve pole, trajectory is different depending on mass. Higher mass attracted less, machine cycles through for spectrum.
Advantages and Disadvantages of quadrupole analyser
- Low resolution
+ ‘low’ cost
+ reproducibility
Advantages of time of flight
Highest mass range
General principle of time of flight analyser
ion pulser accelerates all ions. Velocity depends on mass. Higher mass ions will take longer to reach detector.
Advantages and disadvantages of magnetic sector analyser
+ highest resolution
- expensive
- large
What is the most common detector in mass spec?
Electron multiplier (eg beryllium oxide) this amplification means we can have low detection limits
What is soft ionisation good for
when you don’t want many fragments, eg if you want molecular ion
Types of soft ionisation (3)
APCI
MALDI
Electrospray
What analyser usually goes with MALDI?
Time of flight analyser
What analyser usually goes with electrospray?
quadrupole
What kind of molecules are good for soft ionisation + example of what wouldn’t be good
Non-volatile, polar components
Not good for hydrocarbons
Good for big proteins
What is ACPI ionisation?
Firing electrons at atmospheric pressure with a reagent gas
What is ESI bad for?
Hydrocarbons and non-polar compounds
Does ESI cause a lot of fragmentation?
No
What is chromatography?
Separation due to affinity between two phases
What is normal phase liquid chromatography?
Non polar mobile phase
Polar silica phase
(polar phase retained)
What is reverse phase chromatography?
Polar mobile phase
Non-polar silica phase
(non-polar phase retained)
What is the retention time controlled by in gas chromatography
Heat
What is GC-MS suitable for?
small, volatile, not too polar compounds
What is GC-MS unsuitable for?
Big biomolecules will decompose with the heat
Advantages of mass spec
Can be used on lots of substances (polar + non-polar - high + low mw)
Doesn’t rely on functional groups
can be integrated with separation techniques
small sample size neeeded
Disadvantages of mass spec
High purity sample needed
Destructive technique
Instruments can be expensive
What atoms do things we use soft ionisation for contain?
O , N, S
What is soft ionisation not good for?
Hydrocarbons
What is electron impact ionisation good for?
Volatile organic molecules
What happens in electron impact ionisation?
Sample heated
electrons driven off a heated filament and attracted to an anode
collides with molecule, knocking off an electron and leaving a radical cation.
Is electron impact ionisation soft or hard?
Hard - can cause lots of fragmentation
Why is the molecular ion more stable in chemical ionisation than electron impact
The molecular ion isn’t a radical.
What happens in chemical ionisation?
Ionised reagent gas protonates the molecule (MH+) so m/z is one higher than molecular weight
What is the most common detector in mass spec?
an electron multiplier eg beryllium oxide releases electrons when hit - so amplifies number of electrons
why is an electron multiplier commonly used in mass spec for detection?
It amplifies the signal meaning we can have low detection limits
Soft ionisation methods:
APCI
MALDI
Electrospray (ESI)
Normal phase liquid chromatography
polar silica phase, non polar mobile phase (eg hexane) polar things retained
Reverse phase liquid chromatography
hydrophobic stationary phase that non polar compounds have high affinity to. Polar mobile phase
Polar things lowest retention time
What is retention time controlled by in gas chromatography?
Heat
What is GC-MS suitable for ?
Small, not too polar volatile compounds
What ionisation method is GC-MS usually paired with?
Electron impact ionisation
What does relative abundance show about the fragment in mass spec?
the relative stability of the fragment
What is heterolytic cleavage?
Pair of electrons moved onto one fragment (full arrow)
What is homolytic cleavage ?
One electron moves onto each fragment, forming a radical (single head arrow)
Do these cations increase or decrease in stability?
Primary - secondary - tertiary
Increase
Will noncharged species be seen in mass spec?
No
