Chem 2131, Section 5: Mass Spectroscopy

Section 5: Mass Spectroscopy

Text sections:	Supplementary Material (available through the Bookstore)
Questions:	Pick up at my office (on the counter across from my office door)

Topics covered:
(note: figure numbers are from the supplementary material, not the textbook)

1. Basic Instrument - source, mass analyser, detector
     - mass analyser serves same purpose as column in chromatography, separates be mass, sensitivity +/- 1 amu
     - actually separates by mass: charge ratio (m/z), ions positively charged
Figure: 11-1

2. Isotopes - sensitivity of +/- 1 amu means isotopes are differentiated
    - recall isotopes: same element (number of protons, electrons), different number of neutrons
               - nominal mass: proton = 1 amu, neutron = 1 amu, electron = 0 amu
                         - in amu of isotope is sum of protons and neutrons
     - low resolution MS = differentiate molecules/ions by nominal mass only
     - high resolution MS = to higher number of sf, there is a difference in proton and neutron mass, and can see effect of binding energy, therefore can determine molecular formula exactly
     - natural abundance: some elements contain several naturally occuring isotopes, MS will detect characteristic isotope pattern
               - a statistical distribution of isotopes will exist if multiple atoms of such an element

3. Atomic Mass Spectroscopy - ICP-MS
     - use inductively-coupled plasma source (ICP)
     - plasma breaks everything into component elements and ionises the atoms
     - skim plasma and run through mass analyser to separate atoms by mass
Figure: 11-12

4. Molecular Mass Spectroscopy Sources
     (a) Hard sources - large amount of fragmentation
               (i) electron impact - gas-phase analyte molecules pass through an electron stream
               (ii) chemical ionisation - carrier gas added, ionised by electrons; carrier gas ions then ionise analyte
     (b) Soft sources - little fragmentation
               (i) fast-atom bombardment (FAB) - analyte molecules bombarded by high-speed atoms
                         - analyte is in solid form - atoms bombard solid
               (ii) matrix-assisted laser desorption/ionisation (MALDI) - analyte mixed with absorbing species (matrix)
                         - laser hits matrix, excites absorbing species, energy transfer to analyte to ionise into MS
               (iii) electrospray - analyte in solution is aspirated into MS, passes through voltage to ionise
               (iv) field ionisation - gas-phase analyte molecules pass through high voltage to ionise
Figure: 20-3

5. Mass analysers
     (a) quadrupole - two positive and 2 negative poles, all hooked up to voltage that can cause polarity change
               - positive poles are low mass filters - all ions repelled
                         - when polarity changed to negative, ions attracted, light ions crash and are eliminated
               - negative poles high mass filter - all ions attracted
                         - when polarity changed, ions repelled, only light ions repelled enough to avoid crashing
     (b) magnetic sector analysers - ions accelerated through voltage, then sent through curved magnetic field
               - ions that are too light curve too fast and crash into inside wall
               - ions that are too heavy curve too slow and crash into far wall
               - by changing accelerating voltage, can control speed and therefore mass of ion that makes it through
               - double focussing = machine with two such curves for higher resolution
where B = magnetic field of magnet
r = radius of curve
e = charge on electron (constant)
V = accelerating voltage      (c) time-of-flight (ToF) - ions accelerated through voltage, then allowed to drift through free fall
               - the lighter the atom, the faster it's moving, the faster it reaches the detector
     - resolution = m/Dm (~10² for low res, ~10⁴ for high res)
     - tandem mass spec - two MS linked (usually with different sources, i.e., one hard, one soft)
Figures: 11-4, 11-6, 11-8, 11-9, 20-12, 20-13

6. Quantitative analysis - usually involves some sort of standard
     - can normally assume that detector has linear response over entire mass range (peak height quantitative)
               (a) calibration line with series of known concentrations of analyte
               (b) "spiking" with a non-interfering element (In or Rh, which have low natural abundancies)
               (c) addition of isotopically-labelled standard of analyte
     - ¹⁴C dating = quantitative ratio of ¹⁴C/¹²C

7. Qualitative analysis - identification of unknowns and use in synthesis
     (a) following reaction or identifying product of a reaction
               - with high res and isotopic analysis, able to determine molecular formula
     (b) fragmentation pattern - used to identify chemical units (e.g., amine, phenyl, carbonyl, etc.)
               - molecules break up through their weakest bonds, fragments indicate original connectivity of parts
               - complications - fragments could re-arrange (at low m/z), secondary fragmentations less predictable
               - limitations - difficult to determine certain types of isomers (optical; cis / trans; o-, m-, p-benzene)

Last updated, April 2, 2004
Page maintained by Craig MacKinnon

1.	Basic Instrument - source, mass analyser, detector - mass analyser serves same purpose as column in chromatography, separates be mass, sensitivity +/- 1 amu - actually separates by mass: charge ratio (m/z), ions positively charged Figure: 11-1

2.	Isotopes - sensitivity of +/- 1 amu means isotopes are differentiated - recall isotopes: same element (number of protons, electrons), different number of neutrons - nominal mass: proton = 1 amu, neutron = 1 amu, electron = 0 amu - in amu of isotope is sum of protons and neutrons - low resolution MS = differentiate molecules/ions by nominal mass only - high resolution MS = to higher number of sf, there is a difference in proton and neutron mass, and can see effect of binding energy, therefore can determine molecular formula exactly - natural abundance: some elements contain several naturally occuring isotopes, MS will detect characteristic isotope pattern - a statistical distribution of isotopes will exist if multiple atoms of such an element

3.	Atomic Mass Spectroscopy - ICP-MS - use inductively-coupled plasma source (ICP) - plasma breaks everything into component elements and ionises the atoms - skim plasma and run through mass analyser to separate atoms by mass Figure: 11-12

4.	Molecular Mass Spectroscopy Sources (a) Hard sources - large amount of fragmentation (i) electron impact - gas-phase analyte molecules pass through an electron stream (ii) chemical ionisation - carrier gas added, ionised by electrons; carrier gas ions then ionise analyte (b) Soft sources - little fragmentation (i) fast-atom bombardment (FAB) - analyte molecules bombarded by high-speed atoms - analyte is in solid form - atoms bombard solid (ii) matrix-assisted laser desorption/ionisation (MALDI) - analyte mixed with absorbing species (matrix) - laser hits matrix, excites absorbing species, energy transfer to analyte to ionise into MS (iii) electrospray - analyte in solution is aspirated into MS, passes through voltage to ionise (iv) field ionisation - gas-phase analyte molecules pass through high voltage to ionise Figure: 20-3

5.	Mass analysers (a) quadrupole - two positive and 2 negative poles, all hooked up to voltage that can cause polarity change - positive poles are low mass filters - all ions repelled - when polarity changed to negative, ions attracted, light ions crash and are eliminated - negative poles high mass filter - all ions attracted - when polarity changed, ions repelled, only light ions repelled enough to avoid crashing (b) magnetic sector analysers - ions accelerated through voltage, then sent through curved magnetic field - ions that are too light curve too fast and crash into inside wall - ions that are too heavy curve too slow and crash into far wall - by changing accelerating voltage, can control speed and therefore mass of ion that makes it through - double focussing = machine with two such curves for higher resolution where B = magnetic field of magnet r = radius of curve e = charge on electron (constant) V = accelerating voltage (c) time-of-flight (ToF) - ions accelerated through voltage, then allowed to drift through free fall - the lighter the atom, the faster it's moving, the faster it reaches the detector - resolution = m/Dm (~10² for low res, ~10⁴ for high res) - tandem mass spec - two MS linked (usually with different sources, i.e., one hard, one soft) Figures: 11-4, 11-6, 11-8, 11-9, 20-12, 20-13

6.	Quantitative analysis - usually involves some sort of standard - can normally assume that detector has linear response over entire mass range (peak height quantitative) (a) calibration line with series of known concentrations of analyte (b) "spiking" with a non-interfering element (In or Rh, which have low natural abundancies) (c) addition of isotopically-labelled standard of analyte - ¹⁴C dating = quantitative ratio of ¹⁴C/¹²C

7.	Qualitative analysis - identification of unknowns and use in synthesis (a) following reaction or identifying product of a reaction - with high res and isotopic analysis, able to determine molecular formula (b) fragmentation pattern - used to identify chemical units (e.g., amine, phenyl, carbonyl, etc.) - molecules break up through their weakest bonds, fragments indicate original connectivity of parts - complications - fragments could re-arrange (at low m/z), secondary fragmentations less predictable - limitations - difficult to determine certain types of isomers (optical; cis / trans; o-, m-, p-benzene)