DOC PREVIEW
CU-Boulder CHEM 5181 - Ionization Techniques

This preview shows page 1-2-19-20 out of 20 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

1Ionization Techniques – Part ICU- Boulder CHEM 5181Mass Spectrometry & ChromatographyProf. Jose-Luis JimenezFall 2009High VacuumSample InletIon SourceMassAnalyzerDetector RecorderMS Interpretation Lectures2Clicker Question• What did you think of HW #4?A. Easy in content and in programmingB. Hard in content, easy in programmingC. Easy in content, hard in programmingD. Hard in content, hard in programmingE. Neither of the above3Magnetic Forces• What is the direction of the magnetic force on this ion (at rest)A. Towards leftB. Towards rightC. UpwardsD. Outwards from boardE. There is no force• What is the direction of the magnetic force on this ion (moving)A.Towards leftB.Towards rightC.UpwardsD.Outwards from boardE.There is no force+Br+Brvr4Updated Schedule for Course5Ionization 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 / DART• Ionization for Elemental Analysis– Thermal Ionization Source – Spark Source– Glow Discharge– Inductively-Coupled Plasma (ICP)Q: why are so many ionization techniques used in MS?6Ionization Methods CharacteristicsFrom Lambert7Effect of Ionization TechniquesFrom Schewdt• Same molecule analyzed by 4 techniques • Q: which is preferable: A, B, C, D, E: it depends• Goal for today: understand why this happensABCD8Electron Ionization Source SchemeElectron CollectorElectronEmissionFilament-70 V+ExtractionPlate-300 VGround0 V9Electron Ionization SourceFromWatson10EI 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=IE11Electron Interaction Cross Sections (SF6)http://www.eeel.nist.gov/811/refdata/12Electron Interaction Cross Sections (CF4)http://physics.nist.gov/Divisions/Div842/Icamdata/PDF/1Databases/christo.pdfhttp://www.eeel.nist.gov/811/refdata/13Ionization Efficiency vs. Electron Energy 1FromWatson14Ionization Efficiency vs. Electron Energy 2FromHoffmann15The Concept of Cross Section• Electrons are coming perpendicular to pagePhysicalCross SectionScatteringCross Section70 eV IonizationCross Section15 eV IonizationCross SectionLow EnergyAttachment(e.g. SF6)16Time 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)10-16s>10-12 s10-5smolece+eefrag1frag2+frag1+“PostSourceDecay”17EI 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 energy18Energy Balance of EI (eV)• Q: is energy conserved?A. YesB. Nomoleculeeee70550.14.919Internal Energy Distribution after EIFromLambert20“Soft” and “Hard” Events 1From Lambert21“Soft” and “Hard” Events 2From LambertSee also: http://schwinger.caltech.edu/~carl/ionization.html22Fragmentation 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?23EI: Fragmentation vs. Electron Energy• Clicker Q:• Is operation at low electron energy desirable, in order to get better information at high m/z?A. YesB. NoC. Doesn’t matterD. Not sureFromHoffmann24EI Fragmentation vs. Molecular StructureFromLambertQ: why is there such a dramatic difference in the spectra?A. Different amount of energy absorbedB. Different molecular stabilityC. Different propensity to tunnelingD. Different oxidation state E. Dunno25Breakdown 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-CH2OH26Clicker Q on Breakdown Curve• Chemical ionization transfers a very well defined amount of energy to the analyte molecules. Which of the following spectra are possible with CI?1.2.3.4.60602929312931 60A. All of themB. None of themC. Only #2D. Only #3E. #1, #2, and #327Breakdown Curve &Internal Energy Distribution of Molecular Ions IFromLambertChemical Ionization28FromLambertBreakdown Curve &Internal Energy Distribution of Molecular Ions II29Breakdown Curve +Internal Energy Distributionof Molec. Ions IIIFromLambert30(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 NIST31Electron 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 fragmentation32Advantages +


View Full Document

CU-Boulder CHEM 5181 - Ionization Techniques

Download Ionization Techniques
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Ionization Techniques and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Ionization Techniques 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?