CH_318N_Lecture2-08_Spring_08

CH_318N_Lecture2-08_Spring_08 - Lecture 2 Mass Spectroscopy...

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1 Chemistry 318N Lecture 2 Mass Spectroscopy January 17, 2007 Chemistry 318N Mass Spectrum of 1-Butene Molecular ion Base Peak
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2 Chemistry 318N Chemistry 318N The Nitrogen Rule c Nitrogen rule: if a compound has – zero or an even number of nitrogen atoms, its molecular ion will have an even m/z value – an odd number of nitrogen atoms, the molecular ion will have an odd m/z value
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3 Chemistry 318N Resolution C 2 H 4 O 2 and C 3 H 8 O both have a nominal mass of 60. However, we can still distinguish between them by high- resolution MS Molecular Formula 60.02112 60.05754 60 60 Precise Mass Nominal Mass C 2 H 4 O 2 C 3 H 8 O C 3 H 6 O and C 3 H 8 O have nominal masses of 58 and 60 respectively, and can be readily distinguished by low-resolution MS Chemistry 318N
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4 Chemistry 318N M M+1 Octane Chemistry 318N 98.0 80.9163 100 78.9183 32.5 36.9659 100 34.9689 4.40 33.9679 0.78 32.9715 100 31.9721 0.20 17.9992 0.04 16.9991 100 15.9949 35 Cl 81 Br 37 Cl 79 Br 33 S 32 S 34 S 16 O 17 O 18 O 79.904 35.453 32.066 15.999 bromine chlorine sulfur oxygen Isotopes
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5 Chemistry 318N Calculating M+1 c M+1 = [(abundance of heavier isotope) x (number of atoms in the empirical formula)] c Thus, for octane, C 8 H 18 M M of % 17 . 9 288 . 0 88 . 8 )) 18 016 . 0 ( ) 8 11 . 1 (( 1 = + = × + × = + Chemistry 318N Calculated Spectrum Scheffield ChemPuter http://www.chem.shef.ac.uk/WebElements.cgi$isot Formula: C 8 H 18 mass % 114 100.0 __________________________________________________ 115 8.8 ____ 116 0.3 117 0.0
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6 Chemistry 318N M+2 Peaks c The most common elements that give rise to M + 2 peaks are chlorine and bromine c Chlorine in nature is 75.77% 35 Cl and 24.23% 37 Cl – a ratio of M to M + 2 of approximately 3:1 indicates the presence of a single chlorine in a compound c Bromine in nature is 50.7% 79 Br and 49.3% 81 Br – a ratio of M to M + 2 of approximately 1:1 indicates the presence of a single bromine in a compound Chemistry 318N M+2 Peaks c Sulfur is the only other element common to organic compounds that gives a significant M + 2 peak and it is small 32 S = 95.02% and 34 S = 4.21% Result of isotope pattern calculation Formula: C1H4S1 mass % 48 100.0 ________________ ~~ _____________________ 49 1.9 _ 50 4.5 __
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7 Chemistry 318N M+2 and Statistics-Cl 2 c Possible ways of combining two Chlorines – 35-35 (70) , 35-37 (72) and 37-37 (74) – Three peaks of what relative intensity? – assume that the probability of 35 is 0.75 and of
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CH_318N_Lecture2-08_Spring_08 - Lecture 2 Mass Spectroscopy...

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