4 mass spectrometry mass spectrometry ms an analytical

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Unformatted text preview: l carbons in acids and esters appear at δ 160-180 δ= 13.4 Mass Spectrometry 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 Mass Spectrometry (MS) A Mass Spectrometer A mass spectrometer is designed to do three things 1. convert neutral atoms or molecules into a beam of positive (or negative) ions 2. separate the ions on the basis of their mass-to-charge (m/z) ratio 3. 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 H H C H+e H + H HC H + 2e H Molecular ion (a radical cation) • Molecular ion (M): a radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer • It does not matter which electron is lost; radical cation character is delocalized throughout the molecule; therefore, 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 – at times, however, it is useful to depict the radical cation at a certain position in order to better understand its reactions CH3 CH2 OCH(CH3 ) 2 . . CH3 CH2 OCH(CH3 ) 2 Mass Spectrum • Mass spectrum: a plot of the relative abundance of ions versus their mass-tocharge ratio • Base peak: the most abundant peak – assigned an arbitrary intensity of 100 • The relative abundance of all other ions relative is reported as a % of abundance of the base peak MS of dopamine – a partial MS of dopamine showing all peaks with intensity equal to or greater than 0.5% of base peak M-29...
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