Chapter13

Chapter13 - Mass Spectrometry Introduction Mass spectrometry is a technique used for measuring the molecular weight In a mass spectrometer a

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1 Mass Spectrometry Mass spectrometry is a technique used for measuring the molecular weight. In a mass spectrometer, a molecule is vaporized and then ionized by bombardment with a beam of high- energy electrons. The energy of the electrons is ~ 1600 kcal/mol (or 70 eV). The electron beam ionizes the molecule by causing it to eject an electron. Introduction
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2 Experimental Setup
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3 Mass Spectrometry When the electron beam ionizes the molecule, the species that is formed is called a radical cation , and symbolized as M + . The radical cation M + is called the molecular ion or parent ion . The mass of M + represents the molecular weight of M. Because M is unstable, it decomposes to form fragments of radicals and cations that have a lower molecular weight than M + . The mass spectrometer analyzes the masses of cations. A mass spectrum is a plot of the amount of each cation (its relative abundance) versus 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.
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4 CH 4 spectrum The tallest peak in the mass spectrum is called the base peak . The base peak is typically the M peak. Though most C atoms have an atomic mass of 12, 1.1% have a mass of 13. Thus, 13 CH 4 is responsible for the peak at m/z = 17. This is called the M + 1 peak.
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5 Fragmentation Pattern Why is there no H radical peak or 12 CH 3 D peak?
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6
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Compounds with Nitrogen Hydrocarbons that contain only C, H, and O atoms always have a even mass.
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This note was uploaded on 03/08/2012 for the course CHEM 351 taught by Professor Staff during the Fall '08 term at BYU.

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Chapter13 - Mass Spectrometry Introduction Mass spectrometry is a technique used for measuring the molecular weight In a mass spectrometer a

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