What’s in an Oil Drop?

Weighing In

When scientists want to figure out what something is made of, on the molecular level, they weigh it. Your bathroom scale won’t do: molecules just don’t seem to register on them. So they use mass spectrometers instead. There are lots of different types, each offering its own advantages (for a discussion on MS basics and details on a type of MS designed at the lab to weigh comet dust, please see our Magnet Academy article Mass Spectrometry: How to Weigh an Atom). But when it comes to sorting out substances that are exceptionally complex, scientists usually opt for FT-ICR.

Each atom has a distinct atomic mass (also called atomic weight), which is a function of the number of its protons and neutrons. A carbon atom, for example, weighs 12.011 Daltons (the unit of mass for atoms). A tin atom weighs 118.69.

Actually, that’s a lie. There’s not a single tin atom in the universe that weighs 118.69. Some weigh 116, some 118, some 120, to name a few. These are all different isotopes – atoms of the same element that vary in the number of neutrons in their nuclei, and hence in their mass (most elements don’t have as many isotopes as tin does). The atomic mass listed on the periodic table is a weighted average of the masses of all the naturally occurring isotopes of a chemical element.

One challenge of MS is to tell such isotopes apart, starting with the lightest – hydrogen isotopes weighing in the neighborhood of 1 Dalton. But big, cumbersome molecules also pose a substantial challenge. They may contain upwards of 1,000 atoms, which translates into atomic masses that exceed 100,000 Dalton! That’s a phenomenal range, and measuring accurately from one extreme to the other is a very tall order.

FT-ICR is up to the task.

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