lecture 14 - Mass Spectrometry (MS) An analytical technique...

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Mass Spectrometry Mass Spectrometry Chapter 14 Chapter 14 Mass Spectrometry (MS) Mass Spectrometry (MS) ± An analytical technique for measuring the mass- to-charge ratio ( m/z ) of ions in the gas phase mass spectrometry is our most valuable analytical tool for determining accurate molecular masses also can give information about structure proteins can now be sequenced by MS A Mass Spectrometer A Mass Spectrometer ± A mass spectrometer is designed to do three things convert neutral atoms or molecules into a beam of positive (or negative) ions separate the ions on the basis of their mass-to-charge ( m/z ) ratio measure the relative abundance of each ion A Mass Spectrometer ± Electron Ionization MS in the ionization chamber, the sample is bombarded with a beam of high-energy electrons collisions between these electrons and the sample result in loss of electrons from sample molecules and formation of positive ions Molecular ion (a radical cation) A Mass Spectrometer A Mass Spectrometer ± Electron Ionization MS These ions are accelerated by a series of negatively charged plates into an analyzing chamber inside a magnetic (or electric) field perpendicular to the ion beam. The ion beam curves and the radius of curvature of each ion depends on its charge, its mass, the accelerating voltage, and the strength of the magnetic field. A mass spectrum is a graph of relative ion abundance versus m/z ration.
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Mass Spectrometry (MS) Mass Spectrometry (MS) Molecular Ion Molecular Ion ± Molecular ion (M): a radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer Molecular Ion ± For our purposes, it does not matter which electron is lost ; radical cation character is delocalized throughout the molecule; therefore, we write the molecular formula of the parent molecule in brackets with a plus sign to show that it is a cation a dot to show that it has an odd number of electrons Molecular Ion Molecular Ion at times, however, we find it useful to depict the radical cation at a certain position in order to better understand its reactions CH 3 2 OCH(CH 3 ) 2 . . 3 2 OCH(CH 3 ) 2 Molecular Ion at times, however, we find it useful to depict the radical cation at a certain position in order to better understand its reactions a molecular ion can undergo fragmentation but only charged fragments are detected Mass Spectrum Mass Spectrum ± Mass spectrum: Mass spectrum: a plot of the relative abundance of ions versus their mass-to-charge ratio ± Base peak: Base peak: the most abundant peak assigned an arbitrary intensity of 100 ± The relative abundance of all other ions is reported as a % of abundance of the base peak
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MS of dopamine MS of dopamine a partial MS of dopamine showing all peaks with intensity equal to or greater than 0.5% of base peak MS of Dopamine MS of Dopamine the number of peaks in the MS spectrum of dopamine is given here as a function of detector sensitivity HO HO NH 2 Number of Peaks Recorded
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This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

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lecture 14 - Mass Spectrometry (MS) An analytical technique...

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