What’s in an Oil Drop?
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Ready for our Close-Up
Sometimes the difference between the masses of two isotopes, or two molecules, is quite tiny. This is where resolution power makes all the difference in mass spectrometry. It’s like having a camera with a series of super duper zoom lenses. The more you zoom, the more you see. If all you’ve got to work with, though, is a cheap disposable, you’re not going to capture much detail.
Take the series of images below, for instance. The first is an oak leaf. Examine it more closely, though, and you'll see it's made up of cells. Look even closer, and you'll see inside each cell a nucleus. (These images, by the way, come from an outstanding tutorial well worth checking out called Secret Worlds: The Universe Within, which explains the powers of 10. It's found on the excellent microscopy Web site, Molecular Expressions).
The closer you look, the more you see.
Get the picture? Now, let's transfer this concept to mass spectrometry, viewing the next three images in the same way as you've just examined the above three images.
Take a look at the graph below, which depicts the MS results of a sample of South American crude oil.
Broadband mass spectrum of a crude oil sample.
Click on image to enlarge.
This is a mass spectrum, the type of reading you’ll get from any mass spectrometer. Each peak represents a type of atom or, in this case, molecule. The numbers running across the bottom of the graph – what’s called the mass to charge ratio, or m/z – refer to the molecule’s atomic mass. The height of the peaks tells us how many there are.
As you can see, these peaks are packed in pretty densely. In the example, the range of molecular masses covered starts at about 225 Dalton and goes up to 1,000 Dalton – and there’s an awful lot happening in between. The crude oil is complex; it’s hard to differentiate the molecules.
Zoom in a bit closer, though, and you can see a lot more.
Enlarged region of the mass spectrum of a crude oil sample.
Click on image to enlarge.
We’ve honed in significantly on our sample here, covering not a range of 775 Dalton but of 50. It becomes clear that the blur we saw in the broadband spectrum actually can be broken down into individual, discernible peaks – just as the leaf is broken down into discrete cells.
However, the lines are still pretty fuzzy. To be able to tell those peaks apart – or even to know that there is more than one peak there – we need to zoom in even closer. Let's pull out a magnifying glass and examine the area around the 426 Dalton mark.
Enlarged region of the mass spectrum of a crude oil sample,
identifying individual hydrocarbons.
Click on image to enlarge.
Aha! What before looked to be a single line is actually several, depicting several separate hydrocarbons. C31N40N1 is the most numerous of the bunch, but the sample also contains (in order of abundance), C30H52N1, C28H44N1S1 and C29H32N1S1, among others. Like the leaf cell nuclei in the example above, these peaks emerge under the FT-ICR microscope to reveal what's really going on.
In fact, when you look at this crude oil sample with the help of FT-ICR, you will find upwards of 11,000 different hydrocarbons.
Other machines cannot produce this level of detail. Instead, they essentially lump together several different molecules in a single peak, which represents their weighted average – something quite different than what is truly there. This capacity is of extreme interest to scientists, and something the MagLab does exceptionally well. In fact, the instruments here hold the world’s record in mass resolution! You may be surprised to hear that such a thing exists, but MagLab scientists were able to differentiate between two distinct molecules that were only about .0005 Dalton apart in atomic mass. That’s about as much as an electron weighs, which is so close to nothing as to boggle the mind.
World record for mass resolution.
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