Chapter 8 – Mass Spectrometry MelsSchoolSite home8章–质谱 melsschoolsite回家.pptVIP

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Chapter 8 – Mass Spectrometry MelsSchoolSite home8章–质谱 melsschoolsite回家.ppt

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Chapter 8 – Mass Spectrometry MelsSchoolSite home8章–质谱 melsschoolsite回家

Chapter 8 – Mass Spectrometry Mass Spectrometry The mass spectrometer can be used for: Quantitative analysis – as a sophisticated and very sensitive detector that can measure how much of a substance is present. Qualitative analysis – to provide a unique fingerprint of a substance; this can be used to identify the substance from an online database or to give information about the structure of a new or unknown compound. Instrumentation The key principle of mass spectrometry is that a charged particle passing through a magnetic field is deflected along a circular path of radius proportional to the mass-to-charge ratio, m/e. Instrumentation cont… The sample as a gas, enters the evaluated tube. Positive ions are formed in the ionisation chamber when an electron beam dislodges electrons from the sample atoms or molecules. The positive ions are accelerated by an electric field. The ions enter a magnetic field perpendicular to their path. This cause the ions to move in a curved path with a radius that depends upon the mass-to-charge ratio of the ions. Only ions moving in a curved path of a particular radius, corresponding to a fixed m/e ratio, will reach the collector. Particles of different m/e ratio are able to reach the collector through adjustments to the accelerating voltage or the strength of the magnetic field. The collector measure the current due to the ions reaching the detector and the data is recorded as a mass spectrum. How the Spectrum is formed A molecular substance can give a range of peaks in the spectrum. Two factors can cause the many peaks in the spectrum: The fragmentation of the molecules into a large number of different positive ions. The occurrence of different isotopes of the atoms that make the molecules. Fragmentation The high energy electron beam can knock just one electron from the molecule, M, to form a positive ion. M+ is called the molecular ion or parent molecular ion. The parent molecular ion is a radical, with one unpaired electron. It