2Lecture 2:Ionization Techniques – Part ICU- Boulder CHEM 5181Mass Spectrometry & ChromatographyProf. Jose-Luis JimenezFall 2006High VacuumSample InletIon SourceMassAnalyzerDetector RecorderMS Interpretation Lectures3Ionization Methods Brainstorming• What are the desirable characteristics of an ionization method?4Ionization Methods CharacteristicsFrom Lambert5Ionization Techniques (that we will discuss)• Electron Ionization (EI)• Chemical Ionization (CI)• Electrospray (ESI) / Nanospray• Desorption Techniques– Fast Atom Bombardment (FAB)– Matrix-Assisted Laser Desorption/Ionization (MALDI)–DESI• Ionization for Elemental Analysis– Thermal Ionization Source – Spark Source– Glow Discharge– Inductively-Coupled Plasma (ICP)6Effect of Ionization TechniquesFrom Schewdt• Same molecule analyzed by 4 techniques • Information is complementary, use >1 technique if possible• Goal for today: understand why this happens7Electron Ionization Source SchemeElectron CollectorElectronEmissionFilament-70 V+ExtractionPlate-300 VGround0 V8Electron Ionization SourceFromWatson9EI Notes 1• Hot filament giving off electrons– “Thermoionic effect”– W or Re filament• Accelerated by a potential difference towards and anode• Interact with the gaseous molecules in their path– Do not “impact” them• Ionization Efficiency• What characteristic of the electron can we change to try to improve the results of ionization?present moleculesformed ions=IE10Electron Interaction Cross Sections (SF6)http://www.eeel.nist.gov/811/refdata/11Electron Interaction Cross Sections (CF4)http://physics.nist.gov/Divisions/Div842/Icamdata/PDF/1Databases/christo.pdfhttp://www.eeel.nist.gov/811/refdata/12The Concept of Cross Section• Electrons are coming perpendicular to pagePhysicalCross SectionScatteringCross Section70 eV IonizationCross Section15 eV IonizationCross Section13Time Scales of Ionization• What happens to the molecule when an electron goes by?– 70 eV electron => 5 x 106m/s– Molecule = 10 A = 1 nm• Transit time = 2 x 10-16s• Molecular vibrations > 10-12s• Electronic time scale ~ 10-16s• Frank-Condon principle: nuclei remain frozen in positionLog(t)1016s>10-12 s10-5smolece+eefrag1frag2+frag1+“PSD”14EI Notes 3• What electron energy would be most interactive with the molecule?• Each electron is associated with a wave• λ = h / (mv)• 2.7 A for 20 eV, 1.4 A for 70 eV• Wave is “dispersed” into many frequencies. If one of them has an energy hv corresponding to an electronic transition in molecule, energy transfer leads to excited electronic state– 10 to 20 eV are transferred to the molecule. Only 10 eV are needed to ionize, so rest of the energy can lead to fragmentation• Ionization potential: energy it would take to eject the weakest bound electron from the molecule• At very high energies the wavelength becomes too small, and the molecules become “transparent” to the electron. In other words: not enough time to interact + transfer energy15Ionization Efficiency vs. Electron Energy 1FromWatson16Ionization Efficiency vs. Electron Energy 2FromHoffmann17“Soft” and “Hard” Events 1From Lambert18“Soft” and “Hard” Events 2From Lambert19Fragmentation notes• Fragmentation depends on:– Internal energy deposition on the ion• Shapes of the potential energy hypersurfaces• Energy of the interacting electrons– Molecular structure resists fragmentation• Chemical nature of the analyte• Is fragmentation good or bad?20EI: Fragmentation vs. Electron EnergyFromHoffmann21EI Fragmentation vs. Molecular StructureFromLambert22Breakdown Curve for 1-propanol• This information can be precisely determined using electrons of a single energy, and scanning the energy• This is what is different between molecules (prev. slide)FromLambertCH3-CH2-CH2OH23Breakdown Curve &Internal Energy Distribution of Molecular Ions IFromLambertChemical Ionization24FromLambertBreakdown Curve &Internal Energy Distribution of Molecular Ions II25Breakdown Curve +Internal Energy Distributionof Molec. Ions IIIFromLambert26Internal Energy Distribution after EIFromLambert27(mainlib) 1-Propanol10 20 30 40 50 60 70050100152931424559OHReproducibility: 4 Spectra of 1-Propanol in NISTAgain note that it doesn’t correspond to 70 eVsof internal energy(mainlib) 1-Propanol10 20 30 40 50 60 70050100152931424559OH(mainlib) 1-Propanol10 20 30 40 50 60 70050100152931424559OH(replib) 1-Propanol10 20 30 40 50 60 70050100272931334259OH(replib) 1-Propanol10 20 30 40 50 60 70050100273142455759OHContributor: From Japan AIST/NIMC DatabaseContributor: Chemical ConceptsContributor: NIST Mass Spectrometry Data Center, 1990.Contributor: also from NIST28Electron Ionization Notes• Big advantage: high reproducibility of the fragmentation because– Purely physical (not chemical) process– Fragmentation involves only gas-phase unimolecular reactions• However all MS are far less reproducible than those based in the interaction of electromagnetic radiation with matter (IR, NMR)– MS depend on distribution of electron energies, time allowed for fragmentation29Advantages + Disadvantages of EIFrom Lambert30Appendix: Filament Emission and Failure Characteristics31Filament Emission Characteristics IThe Mass SpecHandbook of Service.Published by Scientific Instrument Services, Inc.http://www.sisweb.com/32Filament Emission Characteristics IIMS Handbook Of Service33Filament Burnout PatternsMS Handbook Of