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Mass spec and molecular Weights

Unit 1-8 Chapter 3.3. Mass spec and molecular Weights. The mass of one 12 C atom = 12 amu or Da Most elements occur in nature as mixtures of isotopes (same proton#, different neutron#) Average atomic mass or atomic weight= ∑( masses of its various isotopes x relative abundances).

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Mass spec and molecular Weights

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  1. Unit 1-8 Chapter 3.3 Mass spec and molecular Weights

  2. The mass of one 12C atom = 12 amu or Da Most elements occur in nature as mixtures of isotopes (same proton#, different neutron#) Average atomic mass or atomic weight= ∑( masses of its various isotopes x relative abundances). Atomic weight

  3. Relative atomic masses and the mass of individual isotopes can be determined using a mass spectrometer. The principle behind mass spectrometry is that a positively charge particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass-to-charge ratio, m/e. Mass Spectrometry

  4. First a sample is vaporized and then injected into the instrument. • Atoms of the element (or molecule) are ionized by being bombarded with a stream of high energy electrons to generate positive ions. This is done so that the ions with a single positive charge are formed (a unipositive mixture of ions). Mass Spectrometry

  5. These ions then pass through holes in parallel plates where they are accelerated. The ions are then deflected by an external magnet. The machine’s electron beam can be adjusted so that only positive ions with a single charge are detected, so that the mass-to-charge ratio is the same as the mass. This way the ions can be separated by their masses. Mass Spectrometry

  6. Ions with a particular mass-to-charge ration are then recorded on a detector, which measures the mass-to-charge ratio and the relative abundance of all the ions present. The amount of deflection depends both on the mass of the ion and its charge. Heavier and less highly charged ions will be deflected less than lighter and more highly charged ions. Mass Spectrometry

  7. A graph of the intensity of the detector signal versus particle atomic mass is called a mass spectrum. From the mass spectrum of an element its relative mass can be calculated, as it is equal to the weighted mean mass of all the naturally occurring isotopes of that element relative to one twelfth of carbon-12. Mass Spectrometry

  8. Purpose of Mass Spectrometry • Produces spectra of masses from the molecules in a sample of material, and fragments of the molecules. • Used to determine • the elemental composition of a sample • the masses of particles and of molecules • potential chemical structures of molecules by analyzing the fragments • the identity of unknown compounds by determining mass and matching to known spectra • the isotopic composition of elements in a molecule

  9. Mass Spectrum of CO2 Molecular ion peak [CO2]+ = 44 Fragment Peaks [C]+ = 12 [O]+ = 16 [CO]+ = 28

  10. Mass Spectrum of Bromine, Br2 Bromine has two isotopes: 50.69% 79Br and 49.31% 81Br Molecular Ion Peaks [79Br81Br]+ [79Br79Br]+ Fragments [81Br81Br]+ 79Br+ 81Br+

  11. Practice: Methyl Bromide, CH3Br

  12. Answers: Methyl Bromide, CH3Br [CH381Br]+ [CH81Br]+and [CH379Br]+ [CH281Br]+ [C81Br]+and [CH279Br]+ [CH3]+ [CH79Br]+ [81Br]+ [C79Br]+ [79Br]+

  13. Spectra of Larger Molecules Spectra of large molecules have many fragments, and the interpretation of their spectra is beyond the scope of this course. Codeine, C18H21NO3

  14. Mass Spectrometry in Forensics • Mass spectrometry is used to confirm the identify of unknowns, such as illegal drugs • Unknowns are often not pure, and must be separated from a mixture • Gas chromatography is used to separate the components of the mixture • Mass spectrometry “fingerprints” the components, so that they can be matched to existing known spectra Gas Chromatograph Mass Spectrometer Mixture

  15. 3.25, 3.26 Homework

  16. Formula Weight (FW) • Sum of the atomic weights for the atoms in a chemical formula • So, the formula weight of calcium chloride, CaCl2, would be Ca: 1(40.1 amu) + Cl: 2(35.5 amu) 111.1 amu • These are generally reported for ionic compounds (empirical formula)

  17. C: 2(12.0 amu) + H: 6(1.0 amu) 30.0 amu Molecular Weight (MW) • Sum of the atomic weights of the atoms in a molecule (molecular formula) • molecular formula = (empirical formula)xn [n = integer] • For the molecule ethane, C2H6, the molecular weight would be

  18. (number of atoms)(atomic weight) x 100 % element = (FW or MW of the compound) Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation:

  19. (2)(12.0 amu) %C = (30.0 amu) x 100 24.0 amu = 30.0 amu = 80.0% Percent Composition So the percentage of carbon in ethane is…

  20. Page 96: 3.17, 3.19 (f) and (g), 3.21 (b) and (c), Homework

  21. Chapter 3.4 Moles

  22. Avogadro’s Number • 6.02 x 1023 • 1 mole of 12C has a mass of exactly12 g

  23. Molar Mass • By definition, these are the mass of 1 mol of a substance (i.e., g/mol) • The molar mass of an element is the mass number for the element that we find on the periodic table • The formula weight (in amu/molecule) = molar mass (in g/mol)

  24. Using Moles Moles provide a bridge from the molecular scale to the real-world scale

  25. Mole Relationships • One mole of atoms, ions, or molecules contains Avogadro’s number of those particles • One mole of molecules or formula units contains Avogadro’s number times the number of atoms or ions of each element in the compound

  26. Page 97: 3.33, 3.37, 3.41 Homework

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