1. Mass Spectroscopy (MS) Separation of ions in gas phase based on different mass to charge ratios (m/z) Differing movement in electric (E) and/or magnetic (B) fields Molecular ion gives molecular weight Fragmentation to identify molecule (fingerprint); or high-resolution Peaks for different isotopes are easily observed even at low-res

2. MS originated with cathode ray experiments in late 1800’s; electrical discharge (plasma) in a sealed tube ~1900 Wien showed cathode rays could be deflected by a magnetic field 1913 JJ Thomson (1897 proved existence of electron using cathode ray exps) did 1 st mass spec http://web.lemoyne.edu/~GIUNTA/canal.html http://web.lemoyne.edu/~GIUNTA/canal.html Ne—1 st proof of isotopes

3. ESA Electrostatic analyzer (KE filter) -2 curved metal plates w DC voltage (+) ion sees two forces, electrostatic force and centripetal force -separates by KE

4. FW Aston (student of Thomson) won Nobel Prize for compiling data of isotopes and abundances for elements Said “MS is dead” once he finished isotopes ~1919 double focussing MS

9. Electron impact vs chemical ionization

10. Ion Sources: 1. Electron Impact (EI) M + e -  M  + + 2 e - ~1 in a million ionized Lots of fragmentation—advantage in i.d. Sometimes no molecular ion 2. Chemical Ionization (CI) Methane in EI source CH 4 + e -  CH 4.+ + 2e - (same as EI); also CH 3 + formed Lots of ion-molecule rxns CH 4 + + CH 4  CH 5 + + CH 3 CH 3 + + CH 4  C 2 H 5 + + H 2 Lots of CH 5 + and C 2 H 5 + C 2 H 5 + + M  MH + + C 2 H 4 (proton transfer; get peak at (M+1) + ) C 2 H 5 + + M  (M-H) + + C 2 H 6 (hydride transfer; get peak at (M-1) + ) Soft ionization source, less fragmentation, molecular ions at (M+1) + and (M-1) +