Spectroscopic Methods and more week 6 Flashcards
MASS SPECTROMETRY (MS)
Molecule resolution data
FAST (up to 500 spectra/sec)
Works for mixtures and pure compounds
Single molecule MS is possible – routinely ng/pg per ml concentrations
ELEMENTS
unique proton number
isotope
different neutron number
The effect of isotopes is
additive
average mass
The mass of the molecule that accounts for the average isotopic distribution.
accurate mass
monoisotopic
The mass of the molecule that consists only of the principle isotopes.
Nuclear binding energy
Nuclear binding energy holds protons and neutrons together in a nucleus.
As different elements/isotopes have different numbers of protons and neutrons the energy needed to hold them together is different
Mass spectrometers can
ONLY detect ions.
cation
Loss of an electron (mostly pi or inorganic systems)
[M]+
an anion
Loss of an acidic proton (eg carboxylic acids)
[M-H]-
it cannot lose inner proton
Ion separation in mass spectrometers depends on the
ion’s mass to charge ratio
m/z
HARD
High energy ionisation processes
Con: Too much energy can cause molecules to break apart (fragment)
Pro : Ionisation of ‘difficult’ molecules
SOFT
Low energy ionisation processes
Con: Too little energy – might be insufficient to ionise certain molecules
Pro : Little/No fragmentation
Electron ionisation (EI) is
hard ionisation
Electron ionisation (EI) steps
- sample is gaseous
- filament wire has a current. Electrons collide with typically lone pairs of electrons or electrons in multiple bonds.
- the analyte loses an electron.
- the high energy electron goes to trap electrode.
- this creates an cation.
electron ionisation description
Beam of electrons of wavelength close to bond length destructively interferes with bonding electrons causing an electron to be lost (mostly pi or inorganic systems)
chemical ionisation description
Gaseous analyte reacted with a high energy source of protons… CH5+
Generates protonated molecular ions [M+H]+
chemical ionisation is
hard but not too hard.
chemical ionisation steps
- gaseous sample molecules collide with positive reagent ions.
they react.
electrospray ionisation
Solvent is evaporated from charged droplets of analyte in solution leaving the molecule as either …
cationised eg. [M+H]+ or anionised eg [M-H]-
electrospray ionisation steps
- capillary is attach to high voltage.
- solution is dissolved in ionic compound.
- it is dissolved.
- it evaporates as voltage is applied.
- solvents evaporates
- columbic explosion as it near like ions
electrospray ionisation is
soft for polar molecules
magnetic
The radius that an ion path is curve is dependent on the m/z ratio of the ion and the magnetic field strength
rods
4 rods , 2+, 2-.
there is a voltage.
only ions with desire m/z can travel.
others are neutralised.
