MS Lecture MW&MF - MW Determination and MF Hand...

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MW Determination and MF Hand Calculation MW&MF: the single most valuable information for unknown compound
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Molecular Ion Molecular ion is the ion of the highest mass in the spectrum (except for isotope peaks).
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To Observed Molecular Ion or ESI
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Determining MW by MS [M+H] + = 279.0914 N H S N N O O H 3 C CH 3 H 2 N = C 12 H 14 N 4 O 2 S MW = 278.33 [M+H] = 279.33 Close, but not the same….
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Averaged MW and Monoisotopic MW MW for methane (CH 4 ) CH 4 may contain isotopes of carbon and hydrogen according to their natural abundances. Since MS sorts out ions one by one, [ 12 CH 4 ] +• and [ 13 CH 4 ] +• would show up at different m/z values. 12.011 x 1 + 1.0079 x 4 = 16.0426 This is called “averaged” MW, and NOT what MS will give you. To determine MW, find the molecular ion and use m/z value. If z = 1, m/z = MW. 12 CH 4 = 16, 13 CH 4 = 17, 12 CH 3 D = 17 etc D = 2 H
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Monoisotopic MW of Methane mass(amu) abundance 12 C 12.0000 98.9% 13 C 13.0034 1.1% mass(amu) abundance 1 H 1.007825 99.99% 2 H(D) 2.014102 0.015% CH 4 = 12.0000 x 1 + 1.007825 x 4 = 16.0313 amu ( exact mass of CH 4 ) It rounds to 16 ( nominal mass ). Most MS instruments can determine only nominal mass, except for high-resolution MS (expensive). cf. averaged MW = 16.0426
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Molecular Ion and Isotope Peaks of Methane m/z
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Calculating Exact Mass Find most abundant isotopes, and use their exact masses for calculation. C 6 H 6 O = 12.0000 x 6 + 1.007826 x 6 + 15.9949 x 1 = 94.0419 C 3 H 7 NO 2 S = 12.0000 x 3 + 1.007826 x 7 + 14.0031 x 1 15.9949 x 2 + 31.9721 x 1 = 121.0198 * ChemDraw can calculate exact mass.
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The Nitrogen Rule (1) A molecule with even # of nitrogen, including zero, has even molecular mass.
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