Science Poetry

Mass Spectrometry

“A method from lab, mass spectrometry,
Sends sample on fragmenting odyssey.
From mass-to-charge data,
A user can rate a 
First guess as to compound’s geometry.”   

The 12 July 2019 limerick addresses another common experimental method used to identify chemical compounds: mass spectrometry.  Unlike NMR or IR spectroscopy, mass spectrometry does not examine how a chemical sample interacts with light, but rather how a sample molecule breaks into component pieces.    

“A method from lab, mass spectrometry, / 
Sends sample on fragmenting odyssey.”
One type of mass spectrometry is electron impact ionization mass spectrometry.  In the experimental apparatus, a chemical species is first vaporized: converted to a gas.  Then, as the method’s name suggests, the species is bombarded (impacted) by a high-energy stream of electrons, resulting in the its ionization: the species becomes charged rather than neutral.  (In the common notation of this process, the molecule, represented as M, loses an electron through this process to form a molecular ion, represented as M+.  The molecular ion has a positive charge, because it lost a negatively charged electron.)  

As the molecular ion travels further through the apparatus, it fragments into common component pieces.  Through interaction with a magnet in the spectrometer, these pieces are deflected to various extents before they reach the detector of the instrument and data are collected.      

“From mass-to-charge data, / A user can rate a / 
First guess as to compound’s geometry.”  
The component pieces are also charged and thus also ions.  The mass spectrometer analyzes the mass-to-charge ratios of these smaller ions.  Generally, the ions formed in this type of instrument have a +1 charge, so the mass-to-charge ratios are equal to these ions’ masses. The resulting mass spectrum is a graph: showing the abundance (prevalence) of the component ions as a function of their masses.  

A mass spectrum provides a record of the ions generated by the fragmentation of a molecule. This helps a chemist to rate “a first guess as to [a] compound’s geometry,” providing evidence as to whether a target compound has been synthesized, by the presence or omission of expected fragments of that compound.