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Mass Spectrometry As Analytical Tool in Catalysis Research

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Mass Spectrometry As Analytical Tool in Catalysis Research MassMass spectrometryspectrometry asas analyticalanalytical tooltool inin catalysiscatalysis researchresearch Katrin Pelzer Functional Characterization AC, FHI-MPG Berlin 17/11/2006 Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 1/66 ContentsContents • Fundamentals of mass spectrometry • Instrument set up – Ionization – Mass separation – Ion detection • Signal evaluation • Application example in catalysis Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 2/66 MassMass spectrometry spectrometry Introduction • Diagnostic aid in – Process engineering – Technology and product development – Medicine and basic scientific research • Gas mixture separation – Mass/charge ratio of the ions – Generation of vacuum Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 3/66 MassMass spectrometry spectrometry General application • Analytical technique used to measure the mass-to-charge ratio of ions – Analysis of catalytic reactions on the surface of solids • Composition of a physical sample – Generation a mass spectrum representing the masses of sample components • Identification of compounds by the mass of molecules or their fragments • Determination of the isotopic composition of elements in a compound or its structure by observing its fragmentation • Quantification using carefully designed methods – Composition and purity of process gases • mass spectrometry is not inherently quantitative • Studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in vacuum) • Determining physical, chemical or biological properties of compounds with a variety of other approaches – Biochemical substance transformations Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 4/66 MassMass spectrometryspectrometry Background • Substance is bombarded (with an electron beam having) sufficient energy to fragment a molecule • Positive fragments (cations and radical cations) are accelerated in a vacuum through a magnetic field and are sorted on the basis of mass-to-charge ratio • Ions carry a unit positive charge • m/e value is equivalent to the molecular weight of the fragment – −1e= −1,602 176 53(14) · 10−19 C • Analysis: re-assembling of fragments backward to generate the original molecule Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 5/66 MassMass spectrometryspectrometry Composition • All mass spectrometers consist of three distinct regions: 1) Ionizer 2) Ion Analyzer 3) Detector • Charged particles are formed by ionization – Impart of enough energy to remove an electron from molecules - +. - CH3OH + 1 e → CH3OH + 2 e • Ionization produces singly charged ions containing one unpaired electron • A charged molecule which remains intact is called the molecular ion • Energy imparted by the electron impact and instability in a molecular ion can cause that ion to break into smaller pieces (fragments) +. + . CH3OH (molecular ion) → CH2OH (fragment ion) + H +. + . CH3OH (molecular ion) → CH3 (fragment ion) + OH Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 6/66 MassMass spectrometerspectrometer Instrumental set up Acceleration through Ionization of a low a charged array concentration of sample molecules Ions having follow the path of the analyzer, exit through the slit and collide with the Collector Varying the strength of the magnetic field, varies the mass-to- Generation of an electric current, charge ratio amplification and detection. Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 7/66 ComponentsComponents forfor analysisanalysis Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 8/66 IonizationIonization Ionisation methods Gas-Phase Ionization Thermal Ion sources (Electron Impact Ionization = EI) (Inductively Coupled Plasma = ICP) (Chemical Ionization = CI) (Spark Source = SS) (Desorption Chemical Ionization = DCI) (Negative-ion chemical ionization = NCI) Atmospheric Pressure Ionization Field Desorption and Ionization (Electrospray Ionization = ESI) (Field Desorption = FD) (Atmospheric Pressure Chemical Ionization = APCI) (Field Ionization = FI) Particle Bombardment Laser Desorption (Fast Atom Bombardment = FAB) Matrix-Assisted Laser Desorption Ionization = (MALDI) Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 9/66 IonizationIonization methodsmethods Ionization Typical Method Highlights method Analytes Relatively •Impact of accelerated electrons with sample form Electron Impact small ions by collision (EI) volatile •Hard method versatile provides structure info + Relatively •Ion formation by H transfer to sample Chemical small MH + C H + --> MH + + C H no fragmentation Ionization (CI) 2 5 2 2 4 volatile •Soft method, molecular ion peak [M+H]+, Peptides •Formation of charged liquid droplets from which ions Electrospray Proteins are desolvated or desorbed (ESI) nonvolatile •Soft method ions often multiply charged Carbohydrates Fast Atom •Impact of high velocity atoms on a sample dissolved Organometallics Bombardment in a liquid matrix Peptides (FAB) •Soft method but harder than ESI or MALDI nonvolatile Matrix Assisted Peptides •Impact of high energy photons on a sample Laser Desorption Proteins embedded in a solid organic matrix (MALDI) Nucleotides •Soft method very high mass Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 10/66 IonIon SourcesSources Electron Impact Ionization • Axial ion source • Grid ion source – High sensitivity – Open construction: low desorption rates – Good injection conditions – Easy degassing by electron bombardment – Residual gas analysis – Partial pressure determination in UHV – W, Re filaments – Desorption measurements – W filament Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 11/66 IonIon SourcesSources Electron Impact Ionization • Cross-beam ion source • Prisma ion source – Directed gas jet without wall interaction – Dual filament: W, Y2O3 – Prevention of vapor condensation – Two cathodes available – Two filaments W, Re, Y2O3 – Similar grid ion source – Outside to suppress surface reactions • Applications: – Residual gas analysis – Molecular beam measurements – Isotope ratios – Material flows Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 12/66 OperationOperation ofof CrossCross beambeam IonIon SourceSource Electrode configuration and potential characteristics • Filament emits electrons thermally: W, Re, Y2O3 coated Ir cathodes – Focused by electrical extraction fields to reach ionization area • Crucial process: Neutral particles are ionized by electrons • Positive ions are accelerated (short dwell time) – Reduces undesired ion-neutrals- reactions – Faster penetration of transition fields Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 13/66 IonIon SourcesSources Contaminants • Adsorbents on the ion source surface: EID ions – UHV conditions – 16 (O+), 19 (F+), 23 (Na+), 35/37 (Cl+), 39/41 (K+) – Reduced by degassing • Strongly reduced field axis voltage – EID ions less effected, formed on the highest potential – Detection of normal ions: higher “Field Axis” Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 14/66 MassMass spectrometerspectrometer Ionizations process • Small fraction of sample is converted in ionized state – Singly and multiply charged positive ions • Collision energy influences the number and type of ions – 50-150 eV Ionic current: 60-100 eV Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 15/66 IonizationsIonizations processprocess Cracking pattern of CO2 • Fragment distribution at 70 eV • Library for compound of frequent interest – E.g. Quad-Star • Distribution depends on ionization energy, temperature, transmission characteristics of the mass analyzer Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 16/66 IonizationsIonizations processprocess Multiply charged ions • Production suppressed by lower electron energies • < 43 eV for Ar/Ne mixtures – Minimize 40Ar++ at 20 amu for 20Ne Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 17/66 IonizationsIonizations processprocess Overlapping of ion currents • Encountered frequently with gas mixtures Intensity amu • Total intensity made by superimposition – Less suitable for quantitative determination (e. g. oxygen, CO) – Requires calibration factors and matrix calculations Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 18/66 MassMass AnalyzersAnalyzers Analyzer System Highlights Quadrupole Unit mass resolution, fast scan, low cost Sector (Magnetic and/or Electrostatic) High resolution, exact mass Theoretically, no limitation for m/z Time-of-Flight (TOF) maximum, high throughput Very high resolution, exact mass, perform Ion Cyclotron Resonance (ICR) ion chemistry Mass Spectrometry Modern Methods in Heterogeneous Catalysis Research K. Pelzer 17/11/2006 19/66 Time-of-flightTime-of-flight massmass analyzersanalyzers • Ion separation by virtue of their different flight times over a known distance • Ions of like charge have equal kinetic energy and then are directed into a flight tube 2 –Ekin = 1/2 mv , m = mass;
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