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ArrowSingle Sector Mass Spectrometer

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Mass spectrometers (MS for short) are machines that give scientists a look at the composition and origin of a material by analyzing and quantifying its atoms and molecules. They do this by vaporizing the sample (if it’s not already a gas) to make it easier to work with, ionizing it to give it a charge, then seeing how those ions react to a magnetic field. That reaction tells scientists what a particle's mass is. The machine then sorts and counts these particles, revealing the specimen’s chemical makeup. The machine is able to distinguish among isotopes (atoms of the same element that have differing numbers of neutrons and, consequently, slightly different masses). Scientists can learn a lot from knowing how many of which isotopes make up a certain sample.

The part of the MS called the mass analyzer or deflector (so named because the positively charged ions inside will be deflected from their straight paths by a sideways magnetic field) is featured in the applet below. Depicted is a single sector mass spectrometer. (Scientists also use dual sector mass spectrometers).


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In this tutorial, the ions of different masses are depicted by different colors. The blue are the lightest, the green are heavier, and the red are heaviest. Adjust the Magnetic Field Strength slider and see the effect. (A second slider allows you to adjust Tutorial Speed; it may be easiest to start at the slowest speed.) Read on for an explanation of what you see.

In order to make it through the curved tunnel of the mass analyzer, the ions need to have just the right mass, proportional to the pull of the magnetic field (which, in the tutorial, follows the same direction as your eyes as they read these words – straight into the computer screen). If the ions are too light, or too heavy, they will veer into the inner or outer wall of the tunnel, never making it into the attached ion Detector. But if they have just the right mass, proportionate to the magnetic field, to allow them to travel through the mass analyzer unscathed, the particles will reach the detector. The arrival of each ion creates a pulse of electrons and this pulse is recorded.

So, the MS both weighs and counts. The mass of the ions is deduced by the force of the magnetic field required to guide them into the detector. And the number of ions of that given mass is counted as the ions pass into the detector. To count ions of other masses, the electromagnet’s field is rapidly varied under computer control, so that ions of all masses can be sampled in a fraction of a second.

In this way the range of possible masses for the particles is tested, resulting in a spectrum – the mass spectrum – for the substance under study. That spectrum reveals how many isotopes of a given element are to be found in the material. This is known as the isotope’s relative abundance – relative, that is, to the other isotopes found in the sample. Researchers use this data to glean information on the sample’s history and chemistry. Take a look at this tutorial for an example of mass spectra of different elements.

For a more thorough explanation of mass spectrometers and their uses, please visit Weighing Atoms: A Race Odyssey.

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