MASS SPECTROMETRY

Nota:

• - pg for molecules of molecular mass ~ 1000 - ppb levels for atomic mass spec

Nota:

• both organic and inorganic

Nota:

• - wide mass range possible, from CO2 to proteins and polymers fragmentation patterns - specific molecular formula for molecules, atoms, fragments

Nota:

• A + 2 elements: Si, Cl, S, Br

1. Compound containing only C, H, O, X (X = halogen) will have even molecular weight
2. uneven number of N will have an odd molecular weight
3. even number of N will have an even molecular weight

1. Take care when considering M+H or other pseudomolecular ions rules above are opposite

Nota:

• x = # of C and Si y = # of H, halogen z = # N and P

1. provide different information
2. used for different substances/sample delivery systems

1. can have more than one mass analyser in the system

Nota:

• sample vaporised and then ionised sample thermally stable mass generally limited < 1000

1. EI
   1. Bombard molecules w electron- mol vapour
   2. ~ 1:10^6 molecules actually ionised
   3. Relaxation of the highly excited radical M+· occurs via fragmentation

      Nota:

      • Amount fragmentation depends on ability of molecule to absorb/ redistribute excess energy
   4. standard = 70eV)
   5. collision peaks

      Nota:

      • normally only one need to worry about is collision with H gives enhanced (M + 1)+ peak often written as (M+H)+
2. CI
   1. +ve
      1. ionize molecule using small gaseous ions

         Nota:

         • CH4, NH3, isobutane that ionised by EI large excess gas CH4:M ~ 103:1 so EI only hits gas not sample
      2. Most common molecular ions observed

         Nota:

         • CH4 MH+, [M+CH5]+,[M+C2H5]+ NH3 MH+, [M+NH4]+ i-C4H10 MH+
   2. -ve
      1. electron capture M + e- + B --> M- + B

         Nota:

         • neutral molecule B required to absorb excess energy from system
      2. proton abstraction A- + M --> [M-H]- + AH

         Nota:

         • A- (e.g. OH-) used to abstract the proton from M
   3. less fragmentation than EI - likely to see mol' ion
3. ICP (Inductively Coupled Plasma)

   Nota:

   • x get molecular fragment laser hits sample - carrier gas come across the sample, blow to ICP - ionize the sample
   1. Atomic MS
   2. interface with MS detector modified for low pressures required
   3. atomizer & ionizer with AAS & AES
   4. Can analyse solid samples by using laser ablation technique

Nota:

• sample converted directly into gaseous ions can be used for mass up to 100,000+

1. MALDI

   Nota:

   • Matrix Assisted Laser Desorption Ionisation
   1. thermally labile species
   2. ionise molecules up to 100,000s Da
   3. ‘soft’ ionisation technique [M+H] [M+Na]
   4. Mix analyte with matrix (organic acid) and dry on metal plate

      Nota:

      • Mix analyte with matrix (organic acid) and dry on metal plate
   5. Aim laser at mixture and get both analyte and matrix desorbed and ionized

      Nota:

      • analyte molecules ionized by proton transfer from matrix
   6. Matrix
      1. high light absorption at wavelength of laser
      2. low sublimation temperature
      3. miscible with your sample
2. FAB

   Nota:

   •  - Fast Atom Bombardment
   1. Particle beam used for desorption - neutral Xe/Ar
   2. Matrix is small organic molecule - glycerol/3-nitrobenzylalcohol
   3. Sample/matrix mix often viscous liquid rather than dry sample
   4. Mostly [M+H]+ or [M+Na]+

      Nota:

      • often have low mass region dominated by matrix ions
3. ESI

   Nota:

   • Electro-Spray Ionisation
   1. Allows direct production of ions from liquid samples

      Nota:

      • means easy to connect to LC instruments flow rate ~ 1μL/min
   2. (droplets of analyte become charged) polarity of droplet = polarity of charge on the needle
   3. solvent evaporates between needle and cone
   4. Droplet size reduced to point where surface tension cannot sustain the charge - Rayleigh point
   5. Analyte ions often multiply charged

      Nota:

      • means can actually analyse quite large proteins without digesting them or having to use mass analyser with large range
   6. Very soft ionisation

      Nota:

      • - can use technique to look at protein-protein interactions  - x provide much structural information - less fragmentation
   7. for small molecules

      Nota:

      • useful for thermally labile species finding molecular masses can attach to high resolution mass analyser for exact mass determinations most efficient if already have ions in solution
      1. +ve ion mode pH < 7 -ve ion mode pH > 7
   8. APCI (another version of ESI)

      Nota:

      • Atmospheric Pressure Chemical Ionisation
      1. = CI -at atmospheric pressure
      2. gaseous ionisation source
      3. orthogonal design (i.e. drying gas at right angles)

         Nota:

         • reduces noise by removing incompletely dry molecules/fragments
      4. - corona discharge used in place of filament in traditional CI - repeated collisions with analyte that forms ions – mostly [M+H]+ / [M+gas]+
   9. APPI

      Nota:

      • Atmospheric Pressure Photo Ionisation
      1. add a chemical dopant that acts like reagent gas use UV lamp to generate photons that ionise analyte and dopant