Mass Spectrometry: How to Weigh an Atom
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Race to the Finish
From the accelerator, the daredevil ions pass into the part of the machine 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. But precisely how much each of the particles is deflected depends upon its mass. (Technically, it is the particle’s mass-to-charge ratio that accounts for its reaction to the magnetic field. It is possible for atoms to lose more than one electron during ionization, but most lose just one electron, and therefore have a charge of 1+. So we’ll dispense with “mass-to-charge ratios” and just call it the mass, for the purposes of this explanation.)
The greater the particle’s mass, the more it will resist the pull of the magnet. Think of how one-and-a-half tons of metal on wheels is able to hug a curve at 150 miles per hour, whereas you in your hatchback would be toast. Take a look at the Java applet below.
In this applet, the ions of different colors signify ions of different weights. The blue are the lightest, the green are heavier, and the red are heaviest. Start the applet by clicking on the Turn On button, then adjust the Magnetic Field Strength and see the effect. 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, like an ill-fated race car driver, veer into the inner or outer wall of the deflector tunnel, never crossing the finish line into the attached ion detector, never hearing the crowds roar. 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 and the checkered flags will wave. The arrival of each ion creates a pulse of electrons (kind of like the arrival of a race car creates a roar of applause) and this pulse is recorded.
PHYSICS FACTOID: The way ions behave in a mass analyzer (the lighter ions deflecting farther than the heavy ones) is explained by the second of Newton’s laws of motion: “The rate of change of the momentum of a body is directly proportional to the net force acting on it, and the direction of the change in momentum takes place in the direction of the net force.”
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: That’s their trophy. Take a look at this tutorial for an example of mass spectra of different elements.
After that mental work-out, you need a rest. Let’s make a quick pit stop before moving on from our by now tired NASCAR analogy. Now that you grasp the MS basics, you’re ready to move to the next level – a bit of a twist on the run-of-the-mill MS we’ve just described.
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