BME 210 Lecture 17 Mass Spectrometry

BME 210 Lecture 17 Mass Spectrometry - 17. Mass...

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17. Mass Spectrometry
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Mass Spectrometry Overview Mass Spectrometry (or Mass Spectroscopy) – a method based on measurements of the mass/charge ratio of ions Mass spectrum represents masses of individual components in a sampled mixture Mass Spectrometry is used: 1. To identify molecules by their masses or mass spectra of their constituent components 2. To sequence amino acids in a peptide or a protein
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Operation principle of MS MS works by: 1. Ionizing molecules into preferably monovalent ions in gas form 2. Accelerating ions in electric field 3. Separating ions using electric or magnetic fields 4. Measuring deflections or other parameters of ion motion that depend on the ion mass/charge ratio
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The motion of an ion is governed by the Lorentz force : F L = q( E + v x B ) = m a q - charge E - electric field (vector) B - magnetic field (vector) v - velocity (vector) m – mass a – acceleration (vector) Acceleration and, therefore, deflection depend on m/q deflection measurements can be used to determine the ion mass Ion movement in electric and magnetic fields deflection v ion + beam E = 0 F L = q v x B = m a
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Main components of a mass spectrometer 1. Ion Source 2. Mass Analyzer 3. Detecto r
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Simple mass spectrometer 1. Sample injector (sample is often liquid) 2. Heater to vaporize the sample molecules in gas form 3. Ionizer 4. Ion accelerator (in a vacuum) 5. Analyzer (magnetic deflector; in a vacuum) Lighter ions are deflected stronger than heavier ions 6. Detector
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Ionizers: Electron beam ionization Electron impact ionizer : molecules are exposed to a high energy beam of electrons • Beam electrons do two things to sampled molecules: 1.They displace electrons from the molecules to produce cationic molecular ions 2.They typically fragment the molecular ions into smaller ions • This method is used for relatively small, stable organic molecules • Large molecules such as proteins are fragmented too much
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Mass spectrum Mass spectrum of a benzyl alcohol with MW = 108: • Peak at m/z = 108 - the molecular ion • Peak at m/z = 91 – molecular ion without (-OH) group (loss of 17 D) • Peak at m/z = 79 – molecular ion without (-CH 2 OH) group (loss of 31 D) • Small peaks at 109 and 110 correspond to the presence of small amounts of C-13 in the sample (which has about 1% natural abundance)
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Ionization of biomolecules Biomolecules (proteins, peptides and sugars) and long, organic polymers, are too fragile and quickly fragment
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BME 210 Lecture 17 Mass Spectrometry - 17. Mass...

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