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

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Unformatted text preview: MW Determination and MF Hand Calculation MW&MF: the single most valuable information for unknown compound Molecular Ion Molecular ion is the ion of the highest mass in the spectrum (except for isotope peaks). Molecular Ion Molecular ion is the ion of the highest mass in the spectrum (except for isotope peaks). To Observed Molecular Ion or ESI 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. 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 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 Molecular Ion and Isotope Peaks of Methane m/z 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. The Nitrogen Rule (1) A molecule with even # of nitrogen, including zero, has even molecular mass....
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MS Lecture MW&MF - MW Determination and MF Hand...

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