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What is a Gas Mass Spectrometer?

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  • Written By: M.J. Casey
  • Edited By: Daniel Lindley
  • Last Modified Date: 03 November 2016
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A gas mass spectrometer is an analytical instrument used to determine the concentration of elements in known samples and as a tool to deduce the composition of unknown samples. It works by detecting the deflection of charged ions derived from the atom or molecule in a magnetic field. In inorganic analysis, each elemental atom produces a characteristic spectrum. Less massive atoms are deflected more, as are atoms with a greater charge. Several enhancements to this basic configuration make the gas mass spectrometer useful in organic analysis as well as elemental determination.

In basic gas mass spectrometers used for elemental analysis, a liquid sample is first prepared by extracting or otherwise isolating the element of interest from the original sample. The liquid is then vaporized and ionized by bombardment with an electron stream that knocks off one or more electrons from the atom. The now positively charged ion passes through a magnetic field at right angles, which exerts a sideways force on the ion. The degree of deflection is directly proportional to the charge to mass ratio of the ions.

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While the principle of the gas mass spectrometer is easily understood, the instrument is a careful combination of components. The vaporized sample is introduced into an evacuated ionization chamber. A vacuum is required, or the newly created ion would soon collide with an air molecule. In the ionization chamber, an electrically heated metal coil radiates electrons sideways, knocking off electrons from the atoms forming ions, which are then collected at an electron trap. The ionization chamber is operated at a positive 10,000 volts.

The positive ions are accelerated out of the ionization chamber by an ion repellant plate held at a slightly higher positive voltage. The stream of highly energized particles is concentrated into a tight beam and then passed through a magnetic field induced by an electromagnet. Depending on the mass to charge ratio, the ions will be deflected to a lesser or greater extent. The charge on the electromagnet can be varied to bring into focus the ion stream of interest on the detection plate. The detector compares the electric current produced by each ion stream to determine the relative abundance.

Each element has a characteristic spectrum. A spectrum is a chart of the relative abundance of each charge/mass ratio. Each line on the chart is related to the relative concentration of the ions produced by knocking off the first electron, followed by the second electron, the third, and so on. By comparing a spectrum to elemental mass spectra in references, the element producing the spectrum can be determined.

The use of the gas mass spectrometer in organic analysis is a bit more complicated. Organic compounds will create a large variety of ionized fragments in the ionization chamber. The mass spectra of even simple organic compounds are much more complex and are often subject to more interpretation. The gas mass spectrometer may be used to confirm identity of an organic compound if the spectrum is very clean, but often correlating results from other techniques are required.

In a gas chromatography mass spectrometer (GC/MS), a mixture of compounds is first separated by gas chromatography and then fed to a gas mass spectrometer. In the gas chromatography portion of this combination instrument, vaporized molecules separate by their ability to diffuse through a carrier gas. By varying the type, temperature, and flow rate of the carrier gas, different mixtures can be separated to give clean, separate samples of each compound. Optimization is necessary to determine the correct gas chromatograph and subsequent mass spectrometer settings. Once the sample source is characterized, such as in a manufacturing plant or a natural source such as an oil well, these instruments produce economical, reliable results.

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