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CU-Boulder CHEM 5181 - Final Review of MS Instrumentation

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1CH5181: Final Review of MS Instrumentation J. Kimmel12-Dec-2006Instrument ThemesVaporizationIonization VacuumFragmentationIon trajectory manipulation m/zselection, dispersion, …Ion detection (Gain)Figures of merit (Efficiency, Resolution, accuracy)2Ionizationn List keywords relevant to choice of ionization techniques …n List sources of energy for ionization techniques ….Ionizationn List keywords relevant to choice of ionization techniques …Degree of fragmentation (soft vs hard), state of sample, chromatographic coupling, destructive, selective, efficient, molecular analysis,atomic analysis, quantitative, ambient analysis, sample preparation,n List sources of energy relevant to ionization techniques ….Electrons, chemical reaction, strong E fields, discharge, heat, photons, momentum (neutral or fast atom) ….3Ionization Sources Discussed in LecturesMolecular Analysis1. Chemical Ionization (CI)2. Atmospheric Pressure CI (APCI)3. Electrospray (ESI)4. Nanospray5. Secondary Ion (SIMS) / Fast Atom Bombardment (FAB)6. Matrix-Assisted Laser Desorption/Ionization (MALDI)7. Desorption Electrospray Ionization (DESI)Elemental Analysis1. Thermal Ionization2. Spark Source3. Glow Discharge4. Inductively-Coupled Plasma (ICP-MS)Chemical IonizationGENERAL STEPS1. Reagent species is ionized by high-pressure electron ionizatione + R ? R±2. Collision of reagent ion with gas -phase analyte (present at <1% abundance of reagent) yields analyte ionR±+ M ? M1 ±+ N13. Potential fragmentation of M ±by one or more pathwaysM1 ± ?M2 ±+ N2?M3 ±+ N3?M4 ±+ N4Why high P? Why is analyte abundance lower?4Question?From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eVThe analyte molecule M can is known to ionize by proton a transfer mechanism with either methane or isobutane. Which is true?(A) PA(M) > 8.5 eV(B) 5.7 eV < PA(M) < 8.5 eV(C) PA(M) < 5.7 eV(D) I don’t knowQuestion?From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eVThe analyte molecule M can is known to ionize by proton a transfer mechanism with either methane or isobutane. Which is true?(A) PA(M) > 8.5 eV(B) 5.7 eV < PA(M) < 8.5 eV(C) PA(M) < 5.7 eV(D) I don’t knowAnswer: (A) ?H = PA(R) - PA(M)To be spontaneous, ?H must be negative. Therefore, PA(M) must be greater than PA(isobutane) and PA(methane)5Electrospray Ionization§Atmospheric pressure ionization§Enables MS detection of large, non-volatile molecules (e.g., proteins) with no fragmentation (? Nobel Prize 2002)§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.comWhat is the “Rayleigh Limit?”ESI ?Tip VOrifice VQ: Electrospray of an analyte of molecular mass = 300 lead to a peak at m/z = +100. Which of these conditions represent probable combinations?dA. d = 3 cm, V = 400, V = 0B. d = 2 mm, V = 2000, V = 0C. d = 2 mm, V = 0, V = 2000D. d = 2 mm, V = 5000, V = 3000E. A or BF. B or D6ESI ?FFor positive ions, Vtip> VorificeThis rules out (C), which is a negative ion condition.(A) creates a field that could drive positive ions towards the orifice. But values are not large enough to create fields typical of ESI.The strength of the field depends on the difference between tip and orifice. Therefore, (B) and (D) are equivalent.Tip VOrifice VQ: Electrospray of an analyte of molecular mass = 300 lead to a peak at m/z = +100. Which of these conditions represent probable combinations.dA. d = 3 cm, V = 400, V = 0B. d = 2 mm, V = 2000, V = 0C. d = 2 mm, V = 0, V = 2000D. d = 2 mm, V = 5000, V = 3000E. A or BF. B or DFast Atom Bombardment (FAB) and Secondary Ion Mass Spectrometry (SIMS)From WatsonDesorption techniques. Analyze the ions emitted when a surface is irradiated with an energetic beam of neutrals (FAB) or ions (SIMS).7Static SIMSn Low current (10-10A cm2) of keV primary ions (Ar+, Cs+ ..) impact the solid analyte surfacen Low probability of area being struck by mulitiple ions; less than 1/10 of atomic monolayer consumedn Primary ion beam focused to less than 1 um enables high resolution mappingn Sensitive technique for the ID of organic moleculesn Spectra show high abundance of protonated or cationizedmolecular ions.FAB and liquid-SIMSn Sample is dissolved in non-volatile liquid matrix is bombarded with beam of neutrals (FAB) or ions n Shock wave ejects ions and molecules from solution. Generally eject ions that already exist in solution.n Presence of charge has little effect on the desorption process. Neutrals used out of convenience for coupling to some instruments.n Use of liquid allows high primary ion currents while maintaining molecular ions ¨ Solution presents mobile, constantly renewed surface to beamn Disadvantage: substantial background of matrix (e.g., Glycerol) ions and matrix adducts.8Matrix Assisted Laser Desorption Ionization (MALDI)n Analyte molecules are embedded in a crystalline matrix composed of a low molecular weight organic species. n Dried mixture is struck with a short, intense laser pulse having a wavelength that is strongly absorbed by the matrix (often UV). n Rapid heating of matrix causes sublimation and expansion into gas phase. Intact analytemolecules carried with little internal energy.n Efficient, “soft,” and relatively universal(wavelength independent of analyte). Matrix isolates analyte molecules, preventing clusters.n Allows analysis of large (100s of kDa) intact biopolymers (2002 Nobel Prize)Image from: Univ. of Bristolhttp://www.chm.bris.ac.uk/ms/theory/maldi-ionisation.htmlClicker Questions - MALDIWhich statement(s) are true.1. In MALDI momentum transfer between matrix and analyte is the driving force for ionization2. MALDI mass spectra are dominated by monocharged molecular species3. MALDI matrix molecules generally have molecular weights greater than or equal to that of the analyte molecules4. MALDI is a soft technique, so spectra are free of fragment peaksA. 1B. 2C. 2, 3D. 2, 3, 4E. 3, 49Clicker Questions - MALDIWhich statement(s) are true.1. In MALDI momentum transfer between matrix and analyte molecules is the driving force for ionization 2. MALDI mass spectra are dominated by monochargedmolecular species.3. MALDI matrix molecules generally have molecular weights greater than or equal to that of the analyte molecules4. MALDI is a soft technique, so spectra are free of fragment peaks.A. 1B. 2C. 2, 3D. 2, 3, 4E. 3,


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