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Mass Spectrometry

Mass Spectrometry. Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University. Structure Determination. We need to know The structures of the starting reagents The structures of the products How is this done? MS – molecular weight IR – functional groups

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Mass Spectrometry

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  1. Mass Spectrometry Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

  2. Structure Determination • We need to know • The structures of the starting reagents • The structures of the products • How is this done? • MS – molecular weight • IR – functional groups • UV-Vis – conjugated pi system • NMR – C and H framework

  3. Mass Spectrometry • Mass of molecule • Structural data from the fragmentation of the molecule • Experiment • Vaporize sample • Bombard sample with high energy electrons • 70 eV = 6700kJ/mole

  4. What Happens?

  5. Radical Cation • M+. • Very high energy species • Energy is lost • Molecules fragments into small positively charged and neutral species • Pass stream of particles through a magnet • Charged particles are deflected and sorted • Sorting is done by m/z • Z = +1 • Peaks = mass

  6. GC-MS

  7. LC-MS

  8. m/z(ratio of mass to charge) • Plot intensity of ions versus m/z Number of ions (intensity) m/z

  9. What Does the Data Look Like? Methane Base Peak Propane Parent Peak

  10. Data Interpretation • Parent Peak = Molecular Ion • M+ = MW • Base Peak is set arbitrarily to 100% • High resolution mass spectrometry • Exact mass of a compound

  11. Data Interpretation - Methane

  12. 2,2-Dimethylpropane 2,2-Dimethylpropane No molecular ion peak is observed (CH3)3C+

  13. Methane and Propane • Let’s look at the spectral data for methane and propane again.

  14. Interesting Point Methane What is this peak? Base Peak Propane Parent Peak What is this peak?

  15. Peaks at m/z = 17 and 45amu • M+1 peaks are observed • Indicates presence of C13 and D in sample

  16. Fragmentation Patterns • Each molecule fragments in a unique manner • Molecules can fragment many ways • Generally more stable species form • Example: • Hexane

  17. Biological Mass Spectrometry • Biological samples • Large • Somewhat difficult to volatize • Soft ionization techniques are used • Electrospray ionization • Matrix-assisted laser desorption ionization • Time-of-flight mass analyzer

  18. ESI • Sample is dissolved in polar solvent • Sprayed through a steel capillary • Subjected to high voltage • Sample is protonated • Solvent is evaporated

  19. MALDI • Sample is absorbed on a matrix compound • Ionization occurs through a blast of laser light • Energy is transferred from matrix to sample • Sample is protonated

  20. Time of Flight • Protonated sample molecules • Focused in a small packet and given a boost of energy by accelerator electrode • Each molecule receives the same energy • E = 1/2mv2 • Velocity related to square root of the mass. • Lighter particles move faster • Heavy particles move slower • Amount of time to complete movement in the analyzer varies • Very sensitive technique

  21. Now What? • Practice • Practice • Practice

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