Chem 550, Spring, 2012 Part I: OVERVIEW OF MASS SPECTROMETRY: I. BASIC ELEMENTS OF A MASS SPECTROMETER Inlet System or Chromatograph Ion Source Mass Analyzer Detector Computer II. ION SOURCES A. Electron Ionization(EI) positive ions 1
The Electron Ionization (EI) Source (EMS, vol 4, chap 2, and vol 6, chap 3) 2
EI source generates ions and forms them into a beam. An EI source-another view 3
B. Chemical Ionization Positive and Negative Ions 1. Positive ions exemplified with methane CI 4
2. Negative ions 3. Electron Capture 4. Atmospheric pressure CI (APCI) 5. Role of Thermochemistry: proton affinities, gasphase basicities, electron affinities, ionization energies 5
C. Fast Atom Bombardment (FAB) and Liquid Secondary Ionization MS (LSIMS) 1. Cs Ion Gun 2. Xe Atom Gun Xe Slow Atoms Xe + Xe + Ionization Acceleration Neutralization Slow Ions Fast Ions Xe Fast Atoms 6
FAB at the molecular level... 7
D. Matrix-Assisted Laser Desorption Ionization (MALDI) Hillenkamp and Karas, Tanaka (Nobel Prize, 2002) 8
Wolfgang Paul (1913-1993 Franz Hillenkamp and Michael Karas Hans Dehmelt 1922-
E. Electrospray Ionization (ESI) John Fenn 10
Two mechanisms of ESI: 11
G. Ion Sources for Inorganic Analysis 1. Inductively coupled plasma (ICP) RS Houk, Iowa State The Torch 12
2. Glow-discharge mass spectrometry 13
Ionization in GDMS: EI A + e A + + 2e Penning Ar m + X Ar + X + + e Associative Ar m + X ArX + + e Symmetric CE A + + A A + A + Asymmetric CE A + + B A + B + A glow discharge is a partially ionized gas consisting of ~ equal concentrations of + and charges and neutral species. Cathode and anode immersed in a low-pressure ( 0.1 10 Torr) gas medium. Application of an electric field across the electrodes causes breakdown of the gas (normally argon) and the acceleration of e - and + ions towards oppositely charged electrodes. Cathodic sputtering gives utility in analysis: atomization of a solids for excitation and ionization. Sputtering involves collisions of energetic particles onto a surface where, after collision, they transfer KE in a series of lattice collisions. Betti, Maria. Isotope ratio measurements by SIMS and glow discharge mass spectrometry (GDMS). IJMS (2005), 242, 169-182. 14
H. Historically important ion sources 1. Field ionization/field desorption To mass spectrometer 15
2. Californium-252 Plasma Desorption Mass Spectrometry (Cf-252 PDMS), R. Macfarlane, 1976 16
Examples of PDMS spectra 17
3. Laser Desorption Mass Spectrometry (predecessor to MALDI) 4. Multiphoton ionization mass spectrometry (REMPI) 18
5. Thermospray mass spectrometry (predecessor to ESI) 19
III. MASS ANALYSIS A. Magnet Sector and Double Focusing Instruments m/ z 2 Br 2V 2 20
Commercial Magnet Sector Mass Spectrometer (dbl focusing) 21
B. Tandem Sector mass spectrometer 22
C. Time-of-Flight Mass Spectrometer mv 2 2 qv and t l v TOF with Reflectron 23
D. Tandem Time-of-flight mass spectrometer 24
Commercial TOF Instrument June 21, 2010 25
E. Quadrupole mass filter 26
F. Triple Quadrupole 27
G. Quadrupole/Time-of-flight mass spectrometer 28
H. Ion Trap Mass Spectrometer 29
The quadrupole ion trap used in ESI and LC/MS/MS Mode (commercial instruments from Agilent, Bruker, Thermo, Varian 30
Ion cloud in quadrupole ion trap on alternate cycles of the AC Mechanical Analogy for the storage feature of the quadrupole ion trap 31
3D trap and the linear ion trap. Latter used in Thermo Instruments 32
I. Linear Ion Trap 33
Thermo LTQ (linear ion trap) interfaced to LC 34
J. Fourier transform mass spectrometer (ion cyclotron resonance mass spectrometer) f qb 2 m 35
R. Smalley, Rice Univ, Nobel Prize, 1996 36
Bruker 12 Tesla FT ICR mass spectrometer Gross Lab WU 37
K. Quadrupole Ion Trap/FTMS 38
L. Orbitrap The ions are injected with velocity perpendicular to long axis of the Orbitrap (z-axis). Injection at a point displaced from z = 0 gives ions pot energy in z. Injection at this pt on the z-potential analogous to pulling back a pendulum bob and then releasing it to oscillate. Ion motion along z is an harmonic oscillator and is completely independent of radial (r) and angular (ϕ) motion. Ion mass/charge ratio m/z is simply related to the frequency of ion oscillation along the z-axis. ω = [(z/m) k] 0.5 Features: high mass resolution (up to 150,000), large space charge capacity, high mass accuracy (2-5 ppm), a mass/charge range of at least 6000, and dynamic range greater than 10. Used for LC/ESI/MS and MS/MS. Accurate m/z used for routine structure confirmation and for metabolomics. 39
Ions produced by ESI (on left), pass thru collision quadrupole, selection quadrupole, and into linear quadrupole ion trap, where they are stored and bunched. This permits using pulsed mass analyzer (Orbitrap) to work with continuous ESI. After accumulation and bunching, the exit lens ( Lens 1 ) is pulsed low, the ion bunches traverse the ion transfer lens system and are injected into the Orbitrap (shown end-on). Ion detection is of the frequency of the z-axis motion: ω = [(z/m) k] 0.5 40
Transient Decay and mass spectrum of bovine insulin: Pks at 100 m/z spacing are from mass calibrant 41
M. Ion Mobility 42
Waters Synapt G2 in Blankenship lab at WU 43
IV. INLET SYSTEMS A. Liquid/Gas Inlet System Molecular Flow (Knudsen): Low P such that collisions with walls are more frequent than with other molecules. Vel is same across whole cross section of tube, and flow det d by P gradient. Viscous Flow: P is high such that there is velocity gradient across tube. Quantity of gas det d by square of P gradient and coefficient of viscosity. Intermediate Flow: Mean-free path comparable to tube width. Avoid this! 44
B. Solids Probe Inlet or Direct Inlet System ln 0 P H 2 vap 1 1 P1 R T1 T2 Clausius-Clapeyron Equation 45
Diagram of Probe and Interface for Solid Introduction into EI, CI, and FAB sources 46
To estimate temperature of vaporization... ln H 0 vap P 1 1 2 P R T T 1 1 2 S vap H T b vap Trouton s Rule: the std entropies of vaporization are close to 85 J K -1 mol -1 for non polar liquids. 47
C. GC/MS A. B. GC/MS of glucocerebrosides from the spleen of a patient with Gaucher's disease. Note that the instrument gives (A) the chromatogram (total ion) and hundreds of mass spectra. That of C-18 is shown (B). 48
D. LC/MS (also Capillary Electrophoresis/MS) E. Some Ionization Methods serve as their own inlet: (e.g., FAB, ESI, DESI, DART, ICP, MALDI). 49
IV. DETECTOR SYSTEMS FOR MS A. Faraday Cup B. Electron Multiplier 50
C. Daly Detector D. Post-Acceleration Detector 51
E. Channeltron Detector 52
F. Multichannel Plate MCP is a specially fabricated plate that amplifies electron signal similar to secondary electron multiplier (SEM). Unlike SEM, MCP has several million independent channels; each works as independent electron multiplier. MCP as an assembly of millions miniature SEMs. MCP consists of a two-dimensional periodic array of very-small diameter glass capillaries (channels) fused together and sliced in a thin plate. A single incident particle (ion, electron, photon etc.) enters a channel and emits an electron from the channel wall. Secondary electrons are accelerated by an electric field developed by a voltage applied across the both ends of the MCP. They travel along their parabolic trajectories until they in turn strike the channel surface, thus producing more secondary electrons. This process is repeated many times along the channel, producing several thousand electons, which emerge at rear of plate. If two or more MCPs are operated in series, a single input event will generate a pulse of 10 8 or more electrons at the output. MCPs can be position sensitive; i.e., produce an image of the incoming ion beam. Used in TOF and multichannel sectors instruments. G. Image Current Detection (in FTMS and Orbitrap) 53
V. THE OUTPUT A. Mass Spectrum B. MS/MS: Product-Ion Spectrum, Precursor-Ion Spectrum, Constant-Neutral-Loss Spectrum C. Higher Order MS/MS MS n D. Ion Chromatograms 1. Total Ion Current (TIC) 2. Selected Ion (Reaction) Chromatogram 54
The literature of mass spectrometry 1. Encyclopedia of Mass Spectrometry 2. J. Am. Soc. For Mass Spectrometry 3. J. Mass Spectrometry 4. Int. J. Mass Spectrometry 5. European Journal of Mass Spectrometry 6. J. Mass Spectrom Soc of Japan 7. Mass Spectrometry Reviews 55