A gas mass spectrometer detects the concentration of elements in known and unknown samples by sensing the deflection of charged ions in a magnetic field. Inorganic analysis produces a characteristic spectrum, while organic analysis is more complex. In a gas chromatography mass spectrometer, compounds are separated by gas chromatography before being sent to the gas mass spectrometer. Optimization is required to determine the correct settings, but these instruments produce cost-effective and reliable results.
A gas mass spectrometer is an analytical instrument used to determine the concentration of elements in known samples and as a tool to infer the composition of unknown samples. It works by sensing the deflection of charged ions derived from the atom or molecule in a magnetic field. In inorganic analysis, each elementary atom produces a characteristic spectrum. Less massive atoms are deflected more, as are atoms with a higher charge. Several improvements to this basic setup make the gas mass spectrometer useful in organic analysis and 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 which detaches one or more electrons from the atom. The now positively charged ion passes through a magnetic field at right angles, which exerts a lateral force on the ion. The degree of deflection is directly proportional to the charge-to-mass ratio of the ions.
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 needed, otherwise the newly created ion would soon collide with an air molecule. In the ionization chamber, an electrically heated metal coil radiates electrons laterally, knocking electrons out of the atoms that form ions, which are then collected in an electron trap. The ion chamber operates on 10,000 positive volts.
The positive ions are accelerated out of the ion chamber by an ion repelling plate held at a slightly higher positive voltage. The stream of highly energized particles is concentrated into a narrow 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 focus the ion flux of interest on the sensing plate. The detector compares the electric current produced by each ion stream to determine its relative abundance.
Each element has a characteristic spectrum. A spectrum is a graph of the relative abundance of each charge-to-mass ratio. Each line on the graph correlates to the relative concentration of ions produced by the elimination of the first electron, followed by the second electron, the third, and so on. By comparing a spectrum to the elemental mass spectra in the references, the element that produces the spectrum can be determined.
Using the gas mass spectrometer in organics analysis is a little more complicated. Organic compounds will create a variety of ionized chips in the ion chamber. The mass spectra of even simple organic compounds are much more complex and are often subject to multiple interpretations. The gas mass spectrometer can be used to confirm the identity of an organic compound if the spectrum is very clean, but correlation results from other techniques are often needed.
In a gas chromatography mass spectrometer (GC/MS), a mixture of compounds is first separated by gas chromatography and then sent to a gas mass spectrometer. In the gas chromatography portion of this combined instrument, vaporized molecules separate due to 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 provide separate, clean samples of each compound. Optimization is required to determine the correct GC 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 cost-effective and reliable results.
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