What’s in an Oil Drop?

Horseshoes and Hand Grenades

We’ve talked about the high resolution powers of FT-ICR. Let’s take a minute to admire its high mass accuracy.

Mass spectrometers, including FT-ICR, often don’t hit a bull’s eye when they’re measuring mass. Sometimes, pretty close is close enough: given the sample, you can be reasonably sure what the substance is. Sometimes, pretty close doesn’t cut it.

Let’s take another look at the crude oil sample from the previous section.

Enlarged region of the mass spectrum of a crude oil sample,
identifying individual hydrocarbons.
Click on image to enlarge.

Now let's look at the weights of the hydrocarbons identified above, as determined by FT-ICR.

Crude Oil Sample, Mass Spectrum Results Chemical Formula Measured Mass Calculated Mass C32H28N1 426.22177 426.22163 C29H32N1S1 426.22511 426.22500 C30H36N1O1 426.27925 426.27914 C29H37N213C1 426.29854 426.29848 C31H40N1 426.31562 426.31553 C28H44N1S1 426.31895 426.31890 C30H52N1 426.40949 426.40943

Look at C₃₁H₄₀N₁ (fifth from the top). The actual mass of this hydrocarbon (at least as best as scientists can determine) is 426.31562 Dalton. A less accurate type of MS would give you a reading of 426, or maybe 426.3. Clearly, that’s not accurate enough to distinguish this molecule from others in the sample, such as nearby C₂₈H₄₄N₁S₁, with a mass of 426.31895 Dalton. A reading to the next decimal spot would also be iffy in some instances.

FT-ICR can determine masses five figures past the decimal point. With this kind of accuracy, it is possible to determine beyond the shadow of a doubt the identity of unknowns, based simply on mass. This accuracy is all the more impressive when you consider that the machine is not just weighing one element at a time, but may well be measuring upwards of 10,000 elements at once (as in this example), and can determine the composition of each of them with extremely high confidence.

Also of great importance is that FT-ICR MS can simultaneously separate and identify great numbers of substances – more than 10,000 separate chemical constituents within a single sample. Other mass analyzers are unable to cover such a vast spectrum.

Finally, unlike other MS methods, FT-ICR doesn’t destroy molecules in the measuring process (we’ll see this shortly, when we discuss how the machine works). That means you can use them again. For example, after measuring the molecules once, you can break them down into smaller pieces, and remeasure those pieces for a new level of information on the sample.

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