Mass spectrometry

Lab Note #19
Apr 18, 2014
The image below shows the mass spectrometer (mass spec) that we are using for this project.  This mass spec is named "Irene", after a character in the children's book Brave Irene by William Steig.

It's hard to tell how it works because all of the important parts are inside the box.   To the right of Irene is another mass spec, "The Lorax" (Dr. Seuss of course), which is pictured below.  Even though we are not using The Lorax for this project, its picture is useful here because all of the important parts are clearly visible on top of the box.

All mass specs of this type have three important parts:
1) An ionizer
2) A mass selector
3) An ion detector

The ionizer is on the left size of the Lorax.  It ionizes (imparts a positive charge on to) the N2 and O2 gases as they enter the mass spec.  This is done by bombarding the gases with electrons using a heated filament.  The ions are accelerated in an electric field, after which they pass through a magnetic field.  The big, black thing in the middle of the picture is a large electro-magnet.  As the ions pass through the magnetic field, they are deflected into circular paths.  The radius of curvature is determined by their mass, so the different masses (or isotopes) are separated.  The detector is the silver thing on the right.  You can find a conceptual diagram here:

We can "see" our sample as it is detected by the mass spec on the computer.  A picture of one sample's "trace" is shown below.

Focus on the bottom half of the trace (labeled Intensity).  The first rectangular looking peak is our standard O2 gas, the second smaller peak is the O2 from our sample, followed by our sample N2, followed by another rectangular looking peak of standard N2 gas.  Here you can see that the O2 and N2 gases are separated, as they enter the detector over 100 seconds apart.  The O2 peaks (standard and sample) have three different lines (blue, red and green) because O2 has three isotopes (resulting in O2 masses of 32, 33, and 34).  The N2 peaks have two lines (red and green) because N2 has two isotopes (N2 masses of 28 and 29).  Note that all of the masses of a given gas are detected simultaneously, which is key for precise determinations of the mass ratios.  From this we calculate the isotope ratios, which is what we will eventually use to calculate the flux of reactive nitrogen out of the Utah snow.

You may be wondering why we name our mass specs after characters in children's books.  My brilliant colleague and collaborator's uncle is William Steig (most famous for Shrek), and my husband's last name is Suess (no relation in this case, note slightly different spelling).  That is the origin of our children's book theme.  You can tell who "owns" what mass spec by whether the character is from a William Steig or Dr. Seuss book. (Really, we do not "own" the mass specs, they are owned by the UW of course!  But we do get to name them.)

There are not just two but six different research groups in the IsoLab, where we are performing the measurements for this project.    There is a lot of exciting work going on in this lab, as you can see for yourself here:  It is a fun place to work.

Soon we will make plots of our preliminary isotope data, and I will give you our initial thoughts on them.  Thanks again for your support!

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