Chapter 8 - Mass Spectroscopy

Chapter Eight Mass spectroscopy It used to quantify known materials, to identify unknown compounds within a sample, and to elucidate the structure and chemical properties of different molecules. The complete process involves the conversion of the sample into gaseous ions, with or without fragmentation, which are then characterized by their mass to charge ratios (m/z) and relative abundances. Basic Principle A mass spectrometer generates multiple ions from the sample under investigation, it then separates them according to their specific mass-tocharge ratio (m/z), and then records the relative abundance of each ion type. The first step in the mass spectrometric analysis of compounds is the production of gas phase ions of the compound, basically by electron ionization. This molecular ion undergoes fragmentation. Each primary product ion derived from the molecular ion, in turn, undergoes fragmentation, and so on. 1 The ions are separated in the mass spectrometer according to their mass-tocharge ratio, and are detected in proportion to their abundance. A mass spectrum of the molecule is thus produced. It displays the result in the form of a plot of ion abundance versus mass-to-charge ratio. Ions provide information concerning the nature and the structure of their precursor molecule. In the spectrum of a pure compound, the molecular ion, if present, appears at the highest value of m/z (followed by ions containing heavier isotopes) and gives the molecular mass of the compound. Components and Instrumentation The instrument consists of three major components: Ion Source: producing gaseous ions from the substance being studied. Analyzer: resolving the ions into their characteristics mass components according to their mass-to-charge ratio. Detector System: detecting the ions and recording the relative abundance of each of the resolved ionic species. 2 A mass spectrometer should always perform the following processes: Produce ions from the sample in the ionization source. Separate these ions according to their mass-to-charge ratio in the mass analyzer. Eventually, fragment the selected ions and analyze the fragments in a second analyzer. Detect the ions emerging from the last analyzer and measure their abundance with the detector that converts the ions into electrical signals. Process the signals from the detector that are transmitted to the computer and control the instrument using feedback. Example: mass spectra of pentan-3-one CH3CH2COCH2CH3 CH3CH2CO-CO- CH3CH2COCH2CH3 3