12章MASS SPEC-1-3.ppt

Mass Spectrometry Molecular weight can be obtained from a very small sample. It does not involve the absorption or emission of light. A beam of high-energy electrons breaks the molecule apart. The masses of the fragments and their relative abundance reveal information about the structure of the molecule. Mass Spectrometry HOW DO WE ACHIEVE THIS? IONIZING METHODS ELECTRON IMPACT MS of METHANE Typical Mass Spectrum Electron Impact Ionization A high-energy electron can dislodge [disl?d?] an electron from a bond, creating a radical cation (a positive ion with an unpaired e-). Separation of Ions Only the cations are deflected [diflekt] by the magnetic field. Amount of deflection depends on m/z. The detector signal is proportional to the number of ions hitting it. By varying the magnetic field, ions of all masses are collected and counted. = Mass Spectrometer The Mass Spectrum Masses are graphed or tabulated according to their relative abundance. The GC-MS High Resolution MS Masses measured to 1 part in 20,000. A molecule with mass of 44 could be C3H8, C2H4O, CO2, or CN2H4. If a more exact mass is 44.029, pick the correct structure from the table: Molecules with Heteroatoms Isotopes: present in their usual abundance. Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak. If Br is present, M+2 is equal to M+. If Cl is present, M+2 is one-third of M+. If iodine [ai?dai:n] is present, peak at 127, large gap. If N is present, M+ will be an odd number. If S is present, M+2 will be 4% of M+. = Isotopic Abundance Mass Spectrum with Sulfur Mass Spectrum with Chlorine Mass Spectrum with Bromine Mass Spectra of Alkanes More stable carbocations [kɑ:b?kei??n] will be more abundant. Mass Spectra of Alkenes Resonance-stabilized cations favored. 质谱 Mass Spectrometry Spectrometry NOT Spectroscopy Spectroscopy n. 光谱学, 波谱学, 光谱仪 Spectrometry n. 度谱术，质谱计 Spectrometry : Spectrometer Abbreviated to: Mass spec. or MS 光波与光谱的关系 范围 光区 光谱类型 跃迁类型 10-4～10-2nm