Mass Spectrometry Introduction to Field Desorption and FAB.pdf

Mass Spectrometry Introduction to Field Desorption and FAB.pdf

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  Mass Spectrometry: Introductionto Field Desorption and FAB Dr. Anand S. Burange Assistant Professor Wilson College, Mumbai Fight against COVID-19: Stay at home, learn and enhance your skills Date: 03/04/2020
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  2 Nobel Prize inChemistry: 1922 Received Nobel Prize for his discovery, development of mass spectrograph, of isotopes, in a large number of non-radioactive elements. Sir J. J. Thomson is considered as the grand father of Mass of Spectrometry. J. J. Thomson: Discovery of electrons (1897) Work on canal rays (positive ion rays):Taken up by F. W. Aston No Francis William Aston, FRS English Chemist
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  3 Applications of Mass-Spectrometry MassSpectrometry is used to study formation, Separation, detection and interpretation of ions in the gas-phase. Applications of Mass Spectrometry Formation Separation Detection Interpretation No Electron Impact (El) Chemical Ionization (CI) Field Ionization (FI) Field Desorption (FD) Plasma Desorption (PD) Laser Desorption (LD) Static Secondary ion(SIMS) Thermospray (TSP) Fast Atom bombardment (FAB) Electrospray (ESP) Multiphoton ionization (MPI) Matrix-assisted LD (MALDI) Ion source: Ionization Most Common
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  4 Spectroscopy Vs. Spectrometry Spectroscopy:Interaction of light with matter In Mass Spectrometry: It is a spectrum or plot of the distribution of ion masses corresponding to the formula weight of a molecule, fragments derived form the molecules or both. Sample should get vaporized first
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  5 Field Desorption Can notbe analysed by EI MS Difficult to vaporize Solubility issues . Very Low Volatile High molecular weight macromolecules (10,000 Da) Dalton: Unit of molar mass; 1 Da = 1.0 gmol-1
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  6 Field Desorption 1. Infield desorption (FD), the sample is first deposited on a specially prepared emitter consisting of a tungsten wire on which microneedles of pyrolytic carbon have been grown. 2.The sample is deposited from solution by microsyringe or by dipping the probe into a solution of the sample. The sample is then introduced into a special ion source, where a very high electric field causes the sample to be ionized and desorbed. 3. Desorption is a process in which a compound is released from a surface and ionized. 4. The technique makes it possible to ionize molecules with molecular weights. Simple model of field desorption microneedle ionization source
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  7 Field Desorption/Field Ionization Fielddesorption emitter holder and 10-μιτι tungsten emitter with carbonaceous needles (arrow) Scanning electron micrograph (500X) of high-temperature emitter Tungsten wire (10 μαι) covered with carbon "microneedles" grown in benzonitrile vapor under high field conditions Microneedle Formation: The temperature of the fine tungsten wire (10 μτη), which is controlled by the dc current, is adjusted to a value sufficient to achieve a high temperature (800-1000 °C) on the emitter wire. The hot emitter wires thermolyze the benzonitrile (~10 torr) at various points on each wire. These points act as centers for further cat-ionic polymerization during the high-voltage operation (7-10 kV).
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  8 Field Desorption Basis ofFD: When a high field is applied to an absorbed organic layer on a metal surface, it experiences an electrostatic force similar to that on the plates of a charged condenser. If the metal surface (anode) has the proper geometry (sharp tip) and under high vacuum (10-6 torr), this force can be sufficient to eject particles as positive ions that can be analyzed via a mass spectrometer Tunneling Questions 1. Will you get M+ peak for ethanol? 2. What will be m/e value for quasi- molecular ion of ethanol? 3. Where do you expect a peak for cluster ion generated in case of ethanol? ?
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  9 Fast Atom Bombardment 1.“Soft ionization technique’ : Useful to analyze polar biopolymers like peptides (upto `10,000 Da). 2. It involves the stream of fast moving atoms (like Xe, Ar or Cs of 6-10 keV) which is bombarded on the sample contained in a viscous liquid matrix like glycerol on a metal probe. Viscous matrix is used to get continuous resupply of fresh sample to surface to get intense and long lasting spectra. 3. Non polar compounds do not yield good FAB specta. 4. Ionization is usually by proton attachment/extraction [MH+ or M - H] or cation attachment [MNa+, MK+). Adduct ions with Na and K can be formed by using salts NaCl and KCl respectively.
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  10 FAB Questions 1. Will youget M+ peak in FAB? 2. What are the types of quasi-molecular ion peaks possible? 3. Why the viscous matrix was used in FAB?
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