Geoscientist’s Toolkit: Mass Spectrometer

Mass spectrometer schematic. Image credit: Wikimedia Commons, based on an image by USGS.

Mass spectrometers are incredibly important pieces of analytical equipment. They have been used on Mars, around Saturn, around Mercury (the planet), and many places in between. They are even found at airport security checkpoints.

Every mass spectrometer has three primary components: an ion source, an analyzer, and a detector.

In the ion source, atoms or molecules are charged—usually by having an electron knocked off—and are focused into a beam within a vacuum chamber. Most ion sources ionize samples when they are already in high vacuum, but some ionize at ambient pressure and then pump the ions into the vacuum.

Next, the ions move into the analyzer. This region separates the different ions in time or space based on the ion’s mass/charge ratio (in many cases, especially in geochemistry, the charge is +1). Although there are several different analyzer designs, the one used in isotope geochemistry is generally the magnetic sector. Here, the ions are passed through a strong magnetic field. When a charged particle moves through a magnetic field, the field exerts a force on the ion, causing its path to be deflected. Less massive ions will be deflected more sharply than more massive ions (equal force gives greater acceleration to smaller masses). This is shown in the picture above.
By changing the strength of the magnetic field, the mass(es) that reach the detector can be selected.

Finally, the ions enter the detection region. Here the current from the ion beam is amplified, and that signal is then recorded. More abundant ions will lead to higher current. Some mass spectrometers, such as the one in the schematic above, are equipped with multiple detectors to measure relative isotopic abundances of several ion masses simultaneously.

For isotope geochemistry, there are two general classes of isotope measurement: stable isotopes, and radio-isotopes.

Stable isotopes are often 1H, 2H (D), 12,13C, 14,15N, and 16,17,18O, though of course many other systems are used. These measurements can provide isotopic “fingerprints”, which can track where things are moving around, and how much mass is flowing.

Radio-isotope systems include 238U/206Pb, 235U/207Pb, 14C, 40K/40Ar, 87Rb/87Sr, and 147Sm/143Nd. These systems are generally used for geochronology, or tracking mixing between distinct sources with different chemistries and histories.

Mass spectrometry as a field is diverse in aims and equipment, but the general principles are the same: an ion source, an analyzer, and a detector. These instruments are a versatile part of the geoscientist’s toolkit.

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