Mass Spec Flashcards
what is the use of MS?
it is used to identify the type and amount of chemicals present in a sample according to m/z ratio and abundance of gas-phase ions.
how do ‘average mass’ and ‘nominal mass’ differ?
average mass is the sum of average atomic masses of the elements involved.
nominal mass is the sum of the atomic mass of the most abundant naturally occurring stable isotopes ONLY.
define ‘accurate mass’.
it is the experimentally determined mass that allows the elemental composition to be determined
- 5ppm is often the threshold to uniquely determine composition.
- 4dp at least
whats the typical procedure of MS?
the sample is ionized - causing fragmentation
ions are then separated according to their m/z ratio
ions are then detected by detector
what ion sources are used for gaseous sample introduction?
sample is evaporated and ionized.
electron ionization - hard
chemical ionization - soft
what ion sources are used for liquid sample introduction?
sample is introduced in spray/aerosol form - microdroplets
electrospray ionization - ESI
atmospheric pressure chemical ionization - APCI
atmospheric pressure photo ionization - APPI
fast atom bombardment - FAB
matric assisted laser desorption ionization - MALDI
inductively coupled plasma - ICP
what analytes are suitable for EI/CI?
Up to 1000 Da
non-polar compounds
volatile
EI can take solid, liquid or gaseous samples - CI takes gaseous
what analytes are suitable for ESI?
100 < 200k Da
Not non-polar compounds
- broad range
what analytes are suitable FAB?
2k - 10k Da
volatile
what analytes are suitable MALDI?
600 - 500k Da
non-volatile
how are EI/CI introduced?
direct or via GC
how are FAB/MALDI introduced?
direct in the matrix
how is ESI introduced?
Direct or via HPLC
what limitation do chiral compounds have when analysing?
chiral compounds racemise under strong heating
are FAB/MALDI hard or soft techniques?
Both soft
- though FAB is harder than ESI & MALDI
how does EI ionize compounds?
sample is bombarded by an electron beam (70eV) which removes an electron from the molecule
- producing charged ions
what detects the separated ions in EI?
an electron or photomultiplier detects the separated ions
why are radicals not detected is MS?
because radicals possess no charge
whatre the advantages of EI?
- reproducible method with high ionization efficiency and good sensitivity
- ionization is non-selective - all vaporized molecules can be ionized, including non-polar & insoluble
- fragmentation produces searchable ‘fingerprint’
- libraries of EI spectra available
whatre the disadvantages of EI?
- only +e ions formed
- samples should be volatile & thermally stable
- can not be interfaced with LC
- limited to MW lower than 1000 Da
- molecular ion frequently not observed due to extensive fragmentation
how do odd-electron ions fragment in small non-functionalised molecules? (molecular ions or fragment ions)
odd-electron ions may eliminate either a radical or an even-electron neutral species.
how do even-electron ions fragment in small non-functionalised molecules? (such as protonated molecules or fragments formed by single bond cleavage)
even-electron ions simply fragment to give an even-electron ion and a neutral species
they CANNOT lose radical species
how do alkyl halides fragment in EI?
sigma-bond cleavage of halide
how do aliphatic ketones fragment?
alpha-cleavage (second bond from oxygen)
how do aliphatic amines fragment?
alpha-cleavage (second bond from nitrogen)
what is the nitrogen rule?
For a compound with an even number of Nitrogens (0,2,4), it will have an even numbered molecular weight.
what defines the McLafferty fragmentation pattern?
loss of an alkene
how is CI used to ionize a sample?
CI is an indirect process involving an intermediate chemical agent.
The ion source is filled with a reagent gas which is ionized to create reagent ions - these then react with the analyte, typically producing quasimolecular ion = [M+H]+
what are the most frequently used reagent gases for CI?
methane and ammonia
does the choice of reagent gas matter for CI?
YES
- the applied reagent gas determines the ionization and fragmentation behaviour of the analyte.
whatre the advantages of CI?
- soft ionization technique typically giving molecular ion
- can be used for direct injection or via GC
- suitable for insoluble sample
whatre the disadvantages of CI?
- no fragment library, hence no search possible
- samples should be volatile & thermally stable
- cannot be interfaced with LC
- CI is limited to compounds < 1000 Da
how are analytes ionized in ESI?
the solvent is sprayed (nebulized) into spray chamber at atm pressure in presence of strong electrostatic field & heated drying gas - N2.
- nebulizer is at ground whilst the capillary is at high voltage.
what solvents are suitable for ESI?
Protic solvents
- water
- acetonitrile
- methanol
what phenomena allows for analysis of large molecules in ESI?
MULTIPLE CHARGING
whats the importance of the nebulizer in ESI?
It produces droplets of uniform size.
These droplets are charged and are attracted to the dielectric capillary
The heated N2 causes the droplets to shrink - desolvation.
Droplets continue to shrink until the repulsive electrostatic forces exceed the droplet’s cohesive forces, causing droplet explosions.
This is repeated until analyte ions are desorbed into gas phase
- driven by strong electric fields on the surface of the microdroplets - ion evaporation.
