Ioniza7on fd sample is applied

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Unformatted text preview: n (API) –  sample (in solu7on) is fed to the ion source via a capillary –  at the capillary 7p (kept at a high poten7al), a nebulizing gas creates an aerosol (fine droplets) –  in the spray, the droplet size decreases as the solvent rapidly evaporates; concentra7on of the ions increases –  Coulombic explosion generates isolated ions CHM3122 Mass Spectrometry 18 Let’s compare: ES and EI of lactose CHM3122 Mass Spectrometry 19 Summary of Ioniza7on Methods CHM3122 Mass Spectrometry 20 Ion Separa7on Methods MASS ANALYZERS •  The analyzer separates the mixture of ions generated by the ioniza7on step according to m/z, crea7ng a spectrum –  magne7c sector mass spectrometer –  quadrupole mass spectrometer –  ion trap mass spectrometer –  7me of flight mass spectrometer –  Fourier Transform mass spectrometer CHM3122 (see pages 9 ­13 of Silverstein) Mass Spectrometry 21 Magne7c sector mass spectrometer The ion is subjected to a deflec7ng force Bvz that curves its trajectory CHM3122 Mass Spectrometry 22 Quadrupole mass spectrometer CHM3122 Mass Spectrometry 23 Summary of Analyzers CHM3122 Mass Spectrometry 24 Determining the Molecular Formula Using MS, we can find the exact mass of the sample → molecular formula Example : CO, N2, C2H4 have approximately equal masses of 28 Da. CHM3122 Mass Spectrometry 25 Determining the Molecular Formula Using the exact masses for the most abundant isotopes: 12C16O : 12.0000 + 15.9949 = 27.9949 Da 14N : 2 x 14.0067 2 = 28.0062 Da 12C 1H : 2(12.0000) + 4(1.00794) 24 = 28.0312 Da CHM3122 Mass Spectrometry 26 Determining the Molecular Formula The molecular species that contain less abundant isotopes will give peaks at M+1, M+2, etc. Example : CH4 –  12C1H4 : –  13C1H4 : –  12C2H1H3 : –  13C2H1H3 : CHM3122 16 Da 17 Da 17 Da 18 Da M+ M+1 M+1 M+2 Mass Spectrometry 27 Determining the Molecular Formula Some general (but approximate) equa7ons to calculate rela7ve peak intensi7es: %(M+1) = 100 (M+1)/M = 1.11 x (# of C) + 0.016 x (# of H) + 0.38 x (# of N) + … %(M+2) = 100 (M+2)/M = (1.11 x (# of C))2/200 + (0.016 x (# of H))2/200 + 0.20 x (# of O) Example : For CO, N2, and C2H4, the rela7ve peak intensi7es will be: compound M M+1 M+2 CO 100 1.15 0.21 N2 100 0.76 0.0 C2H4 100 2.28 0.02 CHM3122 Mass Spectrometry 28 Halogenated compounds •  the isotopes of Cl and Br give dis7nc7ve spectra •  Rela7ve abundances: –  35Cl 100% –  37Cl 32.5 % –  79Br 100% –  81Br 98.0 % CHM3122 Mass Spectrometry 29...
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