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CU-Boulder CHEM 5181 - Chemical Ionization & Related Atmospheric Pressure Methods

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9/29/20091Chemical Ionization &&Related Atmospheric Pressure MethodsIonization Lecture IICHEM 5181, CU- BoulderFall 2009J. Kimmel /J. L. Jimenez“The development of mass spectrometry can be seen, from one perspective, to be based on the invention and utilizationbe based on the invention and utilization of ion sources of ever greater power and more general applicability.”- R. G. Cooks et al., J. Mass Spec, 2005, 1261•Early stages of MS: Precise determination of atomic masses and isotope abundances.•Last 50 years: Shift towards analytical applications involving molecules of greater complexity•Last 15 years: Explosion of biological applications•Last ~5 years: Ambient Ionization methods Recommend: Vestal, Chem. Rev., 101, 361, 2001.9/29/20092Creating the gas-phase ion ….In theory, a mass spectrometer is capable of measuring any gas-phase molecule that carries a charge.Need an energetic process. Must convert to ion. Prefer to preserve properties of sample that are of interest.Physical states of sampleChromatographic couplingSample/matrix complexityRelevant considerations (brainstorm)?ppyDestruction of matrixSelectivityIonization efficiency Coupling / transmission to MS Molecular mass and/or structural elucidationDegree of FragmentationSources to (possibly) be discussedMolecular Analysis1. Chemical Ionization (CI)2.Atmospheric Pressure CI (APCI)p()3. DART (and such)4. Electrospray (ESI)5. Nanospray 6. Secondary Ion (SIMS) / Fast Atom Bombardment (FAB)7. Matrix-Assisted Laser Desorption/Ionization (MALDI)8. Desorption Electrospray Ionization (DESI)Elemental Analysis1. Thermal Ionization2. Spark Source3. Glow Discharge4. Inductively-Coupled Plasma (ICP-MS)9/29/20093Chemical Ionization (CI)Introduced in 1966 by Munson and Field1, it was a direct outgrowth of fundamental studies of ion/molecule interactions.Where other techniques rely on interaction of molecule and electron, photon, or electric fi ldi i ti f th l t l l M i hi d th h tifield, ionization of the analyte molecule, M, is achieved through reaction with a reagent ion, R+GENERAL 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)1. Munson and Field, JACS, 2621, 1966.2.Collision of reagent ion with gasphase 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 ±+ N4CI Ion SourceFrom BarkerSimilar to EI source. •Higher P•Simultaneous introduction of M and R9/29/20094CI ReactionsIn step 1, maintaining a large excess of R compared to M ensures preferential ionizedStep 2 requires collision, therefore source is held at higher pressure than typical EIL = 4.95/pmTorrMany types of reactions can account for ionization in Step 2; Proton transfer is the most common.Proton transfer: M +RH+ (M+H)++ R0.01 cm= 4.95/ pmTorrp = 495 mTorrCharge Transfer: M + R·+ M·++ RElectron capture: R + e- R-Adduct formation (slow): M + RH+ (M-RH)+M + MH+ (M-MH)+Where EI (for instance) is a method with a standard configuration and a single mechanism, CI describes a broad lfthdclass of methods•Many configurations (# increasing rapidly)•Many mechanisms (i.e., reagent ions and reactions) Textbook emphasizes most common implementation9/29/20095CI: Reduced Fragmentation• EI produces an assembly of molecular ions with internal energies between ~0 and 10 eV. As a result, spectra are often dominated by fragment ions.• For the proton transfer reaction RH++ M → MH++ RThe degree of fragmentation of MH+will depend on the internal energy of the products, which in turn depends on ΔH of the reaction. • ΔH depends on the relative proton affinities (PA) of the reactants (Recall that PA equals the negative of ΔH for the protonation reaction).ΔH = PA(R) - PA(M)• Observation of MH+implies that PA(M) > PA(R)• Choice of reagent gas systems can be tailored to the problem to be solved.• Degree of fragmentation also depends on source pressure and temperture – collisional relaxation can compete with fragmentationΔH = PA(R) - PA(M)If ΔH is POSITIVE: ____________If ΔH is NEGATIVE: ____________ IfΔHisVERY VERYNEGATIVE:If ΔH is VERY, VERYNEGATIVE: ____________R = ReagantM = Analyte Molecule9/29/20096Example: Methane as Reagent SpeciesCH4+ e- CH4++ 2e-CH4 ++ CH4 CH5++ CH3CH5+C2H5+CH4+ CH3++ HCH4+ CH2++ H2CH3 ++ CH4 C2H5++ H2CH2++ CH4 C2H3++ H2+ HC2H3++ CH4 C3H5++ H2C3H5+Relevant reaction:CH4 + H+ → CH5+ PA(CH4) = -ΔH = 131 kcal mol-1Relevant reaction:C2H4 + H+ → C2H5+ PA(C2H4) = -ΔH = 162.6 kcal mol-1Question?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?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 know9/29/20097Question?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?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) ΔH = PA(R) - PA(M)To be spontaneous, ΔH must be negative. Therefore, PA(M) must be greater than PA(isobutane) and PA(methane)Question?From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eVWhich is reagant gas is more likely to yield fragmenation of M?(A) Isobutane(B) Methane(C) Depends on structure of M9/29/20098Question?From the text: PA(methane) = 5.7 eV; PA(isobutane) = 8.5 eVWhich is reagant gas is more likely to yield fragmenation of M?(A) Isobutane(B) Methane(C) Depends on structure of MAnswer: (B) ΔH = PA(R) - PA(M)ΔH will be more negative when methane is used FragmentationEINOTECI, R= Methane (PA=5.7 eV)CI, R= Isobutane •Many instruments include dual sources: CI for molecular weight; EI for ID by fragmenation•The unpredictable nature of CI fragmentation between sources preventsFrom de Hoffmann(PA=8.5 eV)sources prevents development of spectral libraries.9/29/20099Selective DetectionEICI: CH4CI: IsobutaneHydrocarbons have lower proton affinity than Butyl methacrylate From HoffmannPA(methane) = 5.7 eV; PA(isobutane) = 8.5


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CU-Boulder CHEM 5181 - Chemical Ionization & Related Atmospheric Pressure Methods

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