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Mass Spectrometry

Mass Spectrometry. Relative atomic masses and the mass of individual isotopes can be determined using a mass spectrometer.

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Mass Spectrometry

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  1. Mass Spectrometry • Relative atomic masses and the mass of individual isotopes can be determined using a mass spectrometer. • The principle behind mass spectrometry is that a positively charge particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass-to-charge ratio, m/e.

  2. Mass Spectrometry • First a sample is vaporized and then injected into the instrument (or it must be vaporized immediately after being injected). • Atoms of the element (or molecule) are ionized by being bombarded with a stream of high energy electrons to generate positive ions. This is done so that the ions with a single positive charge are formed (a unipositive mixture of ions).

  3. Mass Spectrometry • These ions then pass through holes in parallel plates where they are accelerated. • The ions are then deflected by an external magnet. • The amount of deflection depends both on the mass of the ion and its charge. Heavier and less highly charged ions will be deflected less than lighter and more highly charged ions. • This way the ions can be separated by their masses.

  4. Mass Spectrometry • Ions with a particular mass-to-charge ration are then recorded on a detector, which measures the mass-to-charge ratio and the relative abundance of all the ions present. • The machine’s electron beam can be adjusted so that only positive ions with a single charge are detected, so that the mass-to-charge ratio is the same as the mass.

  5. Mass Spectrometry • A graph of the intensity of the detector signal versus particle atomic mass is called a mass spectrum. • From the mass spectrum of an element its relative mass can be calculated, as it is equal to the weighted mean mass of all the naturally occurring isotopes of that element relative to one twelfth of carbon-12.

  6. Mass Spectrometry • Mass spectrometers are used today to identify chemical compounds and analyze mixtures of substances. • A chemist might use this technique to determine the molecular structure of a newly synthesized compound or to identify a pollutant in the environment.

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