Lecture14CHM233(10)final

Lecture14CHM233(10)final - Organic Chemistry, Third Edition...

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1 Organic Chemistry, Third Edition Janice Gorzynski Smith University of Hawai’i Chapter 13 MS IR
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2 Mass spectrometry is a technique used for determining the molecular weight and the molecular formula of an organic compound. A molecule is vaporized and ionized by bombardment with a beam of high-energy electrons. The energy of the electrons is ~ 1600 kcal (or 70 eV). It takes ~100 kcal of energy to cleave a typical ! bond, so 1600 kcal is an enormous amount of energy. The electron beam ionizes the molecule by causing it to eject an electron. Overview of Mass Spectrometry
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3 Schematic of a Mass Spectrometer Figure 13.1
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4 The electron beam ionizes the molecule, it forms a radical cation (M +• ). M +• is called the molecular ion or parent ion . The mass of M +• represents the molecular weight of the compound. Because M +• is unstable, it decomposes to form fragments of radicals and cations that have a lower molecular weight than M +• . Function of a Mass Spectrometer
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6 The mass spectrometer analyzes the masses of cations . A mass spectrum is a plot of the amount of each cation vs. its mass-to-charge ratio ( m/z , where m is mass, and z is charge). Since z is almost always +1, m/z actually measures the mass ( m ) of the individual ions. Function of a Mass Spectrometer
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7 The tallest peak in the mass spectrum is called the base peak . For methane the base peak is also the M +• peak (molecular ion) ; this is usually not the case. Most C atoms have an atomic mass of 12 ; 1.1% have a mass of 13. 13 CH 4 is responsible for the peak at m/z = 17. It is called: M + 1 peak.
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This note was uploaded on 01/03/2012 for the course CHEM 233 taught by Professor Anamoore during the Fall '10 term at ASU.

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Lecture14CHM233(10)final - Organic Chemistry, Third Edition...

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