Chapter 14 MS.ppt

Mass Spectrometry 质 谱 4.1 Introduction Mass spectrometry MS is one of the most versatile and powerful tools of chemistry analysis. It provides quantitative information of atoms or molecules and qualitative information about molecules such as the molecular weight and valuable information about the molecular formula , using few nanomoles of the sample as little as 10-12g, 10-15 moles for a compound of mass 1000 Daltons . It also provides the most structural information that can confirm a structure derived from NMR and IR spectroscopy. Mass spectrometry fundamentally differs from other forms of spectroscopy such as UV-vis, infrared, NMR, etc. because electromagnetic radiation is not involved. The mass spectrometer works by using magnetic and electric fields to exert forces on charged particles ions in a vacuum. Therefore, a compound must be charged or ionized to be analyzed by a mass spectrometer. Furthermore, the ions must be introduced in the gas phase into the vacuum system of the mass spectrometer. The gas phase ions are sorted in the mass analyzer according to their mass-to-charge m/z ratios and then collected by a detector. In the detector the ion flux is converted to a proportional electrical current. The data system records the magnitude of these electrical signals as a function of m/z and converts this information into a mass spectrum. 离子源 Magnetic sectors Table 4-1 Exact masses of some common isotopes and simple molecular species, with the atomic mass of 12C taken to be exactly 12 Detectors 检测器 4.3 Determination of the Molecular Formula by Mass Spectrometry Consider a molecular ion with a mass of 44. This approximate molecular weight might correspond to C3H8 propane , C2H4O acetaldehyde , CO2, or CN2H4. Each of these molecular formulas corresponds to a different exact mass. Use of Heavier isotope Peaks LRMS Most elements do not consist of a single isotope but contain heavier isotopes in varying amounts. These heavier isotopes give rise to sma