Mass Spectroscopy PDF

In UV-visible spectroscopy, absorption in the UV and visible regions of the electromagnetic spectrum causes electrons in atoms and molecules to move higher energy level. Covalent bonds vibrate with frequencies which correspond to frequencies in the infrared range of the electromagneti...

In UV-visible spectroscopy, absorption in the UV and visible regions of the electromagnetic spectrum causes electrons in atoms and molecules to move higher energy level. Covalent bonds vibrate with frequencies which correspond to frequencies in the infrared range of the electromagnetic spectrum. When molecules are irradiated with IR light the absorb energy and the amplitude of the vibrations increases.  MASS SPECTROMETRY SPECIFIC OBJECTIVES: 7.1. explain the basic principles of mass spectroscopy 7.2. explain the significance of the (M+1) peak in mass spectra 7.3. use mass spectral data to: (i) determine relative isotopic masses; and relative isotopic abundance; (ii) distinguish between molecules of similar relative molecular mass (iii) predict possible identities of simple organic molecules based on their  Mass Spectrometry Mass Spectrometry differs from the other spectroscopic techniques discussed before since it does not employ the used of electromagnetic radiation. This is an extremely powerful analytical technique and can be used to 1.Determine molecular weights 2.Structurally characterize unknowns 3.Identify components in a mixture 4.Study gas phase reactions 3  Basic Principle A mass spectrum is produced by exciting an atom or molecule in the gas phase with enough energy to cause it to ionize. The gaseous ions formed are usually positively charge (+1). The mass: charge ratio (m/z) and relative abundance, expressed as a percentage, of each of the gaseous ions are displayed on the horizontal 4  Basic Principle The unit on the x-axis is atomic mass units divided by the charge on the ion (z), which is usually +1. the unit on the y-axis of the spectrum is percentage relative abundance. The strongest peak (the base peak) is assigned by convention the value of 100% relative abundance. The percentages of the other ions are quoted related to that base peak. Detention depends on the particle carrying and electric charge. Neutral particles are not detected. The mass spectrum of a pure sample of an element which has more than one stable isotope will show peaks of mass: charge ratio  How mass spectrum is obtained The first step in the mass spectrometric analysis of compounds is the production of gas phase ions of the compound, basically by electron ionization. This molecular ion undergoes fragmentation. Each primary product ion derived from the molecular ion, in turn, undergoes fragmentation, and so on. The ions are separated in the mass spectrometer according to their mass-to- charge ratio, and are detected in proportion to their abundance. A mass spectrum of the molecule is thus produced. It displays the result in the form of a plot of ion abundance versus mass-to-  DEFLECT MASS ION SPECTROMETER IONIZATI ON DETECTI ON ACCELERAT ION Diagram of a mass spectrometer 8  Analysis of a compound in the mass spectrometer The main stages are: Vaporization of the sample Ionization: high-energy electrons from a heated cathode collide with atoms or molecules of the sample and knock out one or more of the electrons from the sample. Positive ions are formed. Acceleration: the ions are accelerated by an electric field (through a negatively- charged plate). Deflection: the ions are deflected (bent) by a magnetic field.  ELECTRON IMPACT IONIZATION H H H-C:H + e H-C H + 2e CH4 H H RADICAL CATION H H H H-C H H-C + + H H BOND-BREAKING RADICAL CATION H H-C + H+ H ONLY CATIONS ARE CARRIED TO DETECTOR  ELECTRON IMPACT IONIZATION A high-energy electron can dislodge an electron from a bond, creating a radical cation (a positive ion with an unpaired e-). H H H C C H H H H H H H e- + H C C H H C C+ H H H H H H H H C+ C H => H H  ELECTRON IMPACT IONIZATION ionization fragmentation...+ +. M M M + M radical ion cation radical molecule molecular ion fragment ion 12  The fragment in greatest quantity produces the tallest peak and it is called BASE PEAK When one electron is lost from the complete molecule a heaviest ion (the one with the greatest m/z value) peak produced is called the MOLECULAR ION PEAK (M) A peak with a mass 1 unit heavier than the molecular ion is called the M+1 peak The M+1 peak may be due to the presence of 13 C or 2H isotopes in some of the molecules http://chemguide.co.uk/analysis/masspec/mplus1.html#top 13 14 ______base peak given 100% abundance Molecular ion peak 15 Methyl Bromide: An example of how isotopes can aid in peak identification.79 The ratio of peaks containing Br and its isotope 81 Br (100/98) confirms the presence of bromine in the compound. Mass Spectra of 2- methylpentane More stable carbocations will be more abundant. 18 Mass Spectrum with Chlorine => 19  APPLICATIONS OF MASS SPECTROSCOPY To determine relative isotopic masses and relative isotopic abundance To distinguish between molecules of similar relative molecular mass To identify compounds based on their fragmentation patterns In carbon and other radioactive dating processes By satellites and spacecraft to identify particles intercepted in space Forensic science Drug testing and drug discovery Process monitoring in the petroleum Chemical and pharmaceutical industry Chapter 12 21  Page 378 1. Define the following terms: a. Molecular ion b. M+1 peak c. Base peak 2. a. In the mass spectrum of 2-methyl-2-butanol, peaks appear at m/e values of 56, 59, 71 and 73. Suggest a molecular formula for each of the peaks shown. b. Explain why the molecular ion peak did not appear on the mass spectrum of 2-methylbutan-2-ol but it appeared on the mass spectrum of its isomer pentan-2-ol? 22 3. a. Briefly explain how a mass spectrometer works to obtain a mass spectrum of a molecule b. The mass spectrum of pentan-3-one is given below Identify the following peaks in the spectrum: i.Molecular ion peak ii.Base peak iii.Suggest a possible identity for the species corresponding to the mass 29 23 2010. Question 5 Chapter 12 25 26