Lecture 3: Ionization Techniques – Part IIAnnouncementsScience (Journal) PresentationsElectrospray IonizationElectrospray MechanismESI Mech (con’t)ESI Mass SpectrumESI Source DesignControlled Current Electrolytic Flow CellESI: Concentration DependenceNanospraySlide 12Fast Atom Bombardment (FAB) and Secondary Ion Mass Spectrometry (SIMS)Producing Primary BeamStatic SIMSFAB and liquid-SIMSMatrix Assisted Laser Desorption Ionization (MALDI)Example of MALDI DataMALDI-TOF Mass SpectrometerAtmospheric Pressure Desorption IonizationDesorption Electrospray IonizationSlide 22Summary of DI TechniquesElemental Analysis NotesThermal IonizationThermal Ionization (cont’d)Spark Ionization SourceGlow Discharge SourceGD: Collisional Ion Formation ProcessesPenning IonizationInductively Coupled PlasmaICP SourceICP-MS vs ICP-OESComparison of Molecular Ionization TechniquesDesorption Ionization SummaryLecture 3:Ionization Techniques – Part IICU- Boulder CHEM 5181Mass Spectrometry & ChromatographyJ. KimmelFall 2006Announcements•HW 2 due Thursday•Journal Skim 3 due Thursday•Exam next Tuesday–Clicker review in class on Thursday•Please email 2 questions with answers to Joel by noon on Wednesday •We are keeping original presentation scheduleScience (Journal) PresentationsDate PresenterOct 9 CoburnOct 16 --------------Oct 23 ThalmanOct 30 Axson & CravenNov 27 RobinsonDec 4 TienesElectrospray IonizationAtmospheric pressure ionizationEnables MS detection of large, non-volatile molecules (e.g., proteins) with no fragmentation (→Nobel Prize 2002)Search “ESI-MS” = 13,000 articlesFenn’s 1985 A Chem paper cited 845 timesLiquid elutes through a high voltage tip Coulombic explosions yield a continuous mist of bare, gas-phase ions (positive or negative)Conveniently coupled to liquid separations Characterized by multiply charged ionsNewobjective.comElectrospray Mechanism•An electrolytic analyte solution is pushed through the conductive end of capillary (id 10-100 um) at very low flow rate (0.1-10 uL/min) held a few mm from the entrance of the MS•High potential (2-4 kV) induces a strong electric field (106 – 107 Vm-1) •For positive field, cations will move towards the liquid surface and anions will move towards the conductive tip. •Repulsions between adjacent cations combined with the pull of the cations towards the grounded MS inlet cause the surface to expand into a so-called ‘Taylor cone.’ Gomez & Tang, Phys Fluids, 1994, 6:404–414ESI Mech (con’t)Balance induced E field and surface tension of liquidTip of the cone elongates into a filament, which breaks up and emits a stream of charged droplets towards the inlet of the mass spectrometer. Evaporation of solvent from the droplets increases the charge density.At the ‘Rayleigh limit,’ repulsion between cations equal surface tension, causing ‘Coulombic explosions’ that produce even finer droplets. This process of evaporation and explosion repeats until fully desolvated ions are released.The release of ions occurs either by repeated fission events until total evaporation of the solvent (Charge Residue Model) or by direct ion emission from a charged droplet (Ion Evaporation Model). Gomez & Tang, Phys Fluids, 1994, 6:404–414From Fig 13-18 LambertESI Mass SpectrumESI-MS of Cytochrome C, ~12,360 DaHigh charge states make m/z practicle for most mass analyzer types.z can be determined by isotope distibution or sequence of peaks (see section 1.8.1 of De Hoffmann and HW #2)ESI Source DesignESI source must:1. Move ions from solution to the gas phase2. Transfer the gas-phase ions from atmospheric pressure to vacuum 3. Yield ion beam with maximum current and minimum kinetic energy distribution On 1.•Stable spray requires user optimization•High flow rates may require nebulizing gas to form dropletsOn 2.•Heated drying gas + capillary encourage desolvation, and limits solvent analyte-adduct formation during expansion•Pumping speed places practical limit on size of entrance aperature•Transfer of ions between stages of decreasing pressure can result in a total ion loss on the order of four to five orders of magnitudeOn 3.•Harnessing expansion•Constant Velocity = high E distribution For discussion, see: “ESI Source Design and Dynamic Range Considerations,” A. P. Bruins, in “Electrospray Ionization Mass Spectrometry,” R. B. Cole, 1997.From de HoffmannControlled Current Electrolytic Flow CellFrom: Cech and Enke, Mass Spec Rev, 20, 362, 2001.•Electrical circuit to sustain ESI current : (+) Terminal to tip, to counter electrode, to (-) Terminal.•Electrolysis at electrodes maintains the charge balance to allow continuous production of charged droplets.•In order to supply demanded current, potential at electrode/solution interface has value permitting the oxidation process characterized by lowest oxidation potential in solution.•This process determines the total # of ions that can be produced per unit timeESI: Concentration Dependence•It is the excess charge in final droplets that imparts charge to gas-phase ions. (See Fig 1.24 in De Hoffmann)•ESI is sensitive to concentration, not flow. Because limiting current, IM, is dependent on oxidation process at tip.•ESI response can vary significantly among different analytes that have identical concentrations•For a system with one analyte, spray current for will depend on its concentration and a analyte-specific rate constant. IA = kA[A]•For system of two analytes, A and B, IT = IM = IA + IB And, currents proportional to relative desorption rates and signal responses are coupled. Complicates quantification. (See section 1.8.4 of de Hoffmann)•For any system, dynamic range limited at high end (~1 mM) by:–Limited amount of excess charge–Limited space on droplet surface–Ion suppression •Consider separations prior to ESI to maximize sensitivityNanosprayNew Objective SilicaTips. Tip i.d. range from 5 to 30 um.Flow rate: 20 to 1000 nL/min•10 – 100 nl/min flow rate with fine spray tip•Flow rate and droplets 100-1000 times smaller than conventional ESI•Large proportion of analyte available for desorption from surface. 2-3 time higher ion current than ESI at a given concentration•Smaller tip close to orifice: narrow dispersion of droplets yields better transfer in MS•Orders of magnitude (2+) improvement in efficiency (analyte detected / analyte sprayed)•At these flow rates, ESI becomes “mass flux