What is the mass spectrum?

Jan 24, 2016

A mass "spectrum" that you get from a Mass Spectrometer is basically a graph of abundance vs. molecular mass.

First, if you aren't comfortable with it yet, it would help you to read at least a little bit of this background on Mass Spectroscopy.

In Mass Spectroscopy, the aim is to identify a molecule by its fragmentation pattern.

When analyzing the sample, it is vaporized so that it can be ionized (otherwise, it won't be affected by electromagnetic fields), and then it is able to be accelerated through a magnetic and/or electric field (sequentially) to be separated according to the mass-to-charge ($m \text{/} z$) ratios.

Fragmentation allows you to determine the structure of the molecule because it breaks up into smaller structures in a patterned way such that you can piece it back together based on the molecular masses of specific groups.

You can see examples of fragmentation here, of 2-pentanone:

Once the ions are separated, the molecules are sent into a detector that basically counts the ions that hit it, converting the received signal into a form that a computer can use to make the mass spectrum.

Shown below is the mass spectrum for 2-pentanone, the compound depicted above. I've overlayed the structures to correspond to their proper peaks.

This signal sent to the computer is literally the abundance of each ion that came from the solution. It just says how much of each ion is there relative to the others. That is the y-axis.

This is plotted against the $m \text{/} z$ ratio on the x-axis. Rarely are the produced ions anything more than a $+ 1$ charge, so the $m \text{/} z$ ratio can often be approximated as the actual molecular mass.