What's in a mass?

We are analyzing the extracted dissolved organic matter (DOM) from our water samples using mass spectrometry. The goal of this is to figure out what molecules are in the pool of organic nutrients on the coral reef and how the components of this pool change when the water is filtered through a sponge.

The use of mass spectrometry allows us to separate different compounds based on their mass. There are many ways to do this and many different types of mass spectrometers. The important thing for our goal is that we need to detect as many different compounds as possible in our sample. The ultra high-resolution mass spectrometer that we use in the "untargeted" analysis allows us to do just that.

The resolution of this mass spectrometer is so high that we can obtain the 'exact mass' for each of thousands of different compounds in a sample. The exact mass differs from the 'nominal mass' of a compound because there is higher precision in the mass value. For example, a water molecule consists of two hydrogen (H

 

But if you calculate the exact mass of water, it is 18.063390.

HUGE difference right?! Well, it may not seem like a big difference at first but here is why the exact mass is important.

Let's use the molecule glucose, a sugar you may be familiar with if you have ever had your blood sugar level tested. Glucose consists of six carbon atoms (shown in black in the image below), 12 hydrogen atoms (white), and six oxygen atoms (red) (C

 

If we search a database of metabolites (compounds that our bodies use for energy and growth) using a mass of 180 (180.0000 -180.9999) we get 121 possible compounds (I did this using a database called METLIN).

If we use the exact mass we find only 5 molecules with this mass and these all have the same chemical formula as glucose.

We have now narrowed down our search from 121 possibilities to 5. That is a big deal!

There are a few methods we can use to narrow down those 5 compounds to identify our "unknown" compound with a mass of 180.063390. We'll save that for a later lab note, for now at least, you can see why having an exact mass will help us identify different compounds in our samples. This is the first step to see how sponges are changing the water chemistry on coral reefs.

 

Want to know a little more?

 

While the above image is true there is more to the story. Each of these elements has different forms called isotopes. An isotope of an element has the same number of protons but a different number of neutrons. Usually there is one isotope that is highly abundant in the environment. Carbon for example usually has 6 neutrons – represented as

The (monoisotopic) exact mass is calculated for a single isotope for all atoms in the compound. For water this means:

1.0078 + 1.0078 + 15.9949 = 18.0105