Understanding Mass Spectrometry

May 24, 2016 0 Comments in lcms, mass spectrometry by
Understanding Mass Spectrometry

At National Laboratories, we use mass spectrometry to identify specific drugs or metabolites in a biological sample. Mass spectrometry allows us to measure the mass of molecules and atoms. Using the molecular ion peak and fragment ions, we can gather information on the mass of molecules in a biological sample, as well as their chemical structure.

Step One: Generating Ions

The first step in mass spectrometry is generating ions from the biological specimen. A small portion of the sample is injected into the mass spectrometer. An electron gun is fired, knocking electrons out of the sample’s molecules and creating positive ions.

Step Two: Acceleration

Once we’ve ionized the sample, we accelerate it through an electric field before passing it through a magnetic field. The positive ions are deflected by varying amounts as they pass through this field. Lighter ions will have greater deflection while heavier ions have less deflection. The ions deflect into the detector, allowing identification.

Step Three: Detection

The magnetic field strength is adjusted to make sure that all of the ion streams in the sample can be detected. Once the ions hit the detector, they begin accepting electrons, which causes a small electric current to flow. The size of that current is what allows us to determine how many ions are present. The rate of charge is printed on a graph that compares mass/charge ratio to current.

Step Four: Analysis and Identification

The heaviest peak in this chart allows us to identify molecular mass, which is important for identifying the specific molecule in the sample. Other, smaller peaks allow us to see the fragmentation of molecular ions in the sample. Each molecule has a range of different possible fragment ions. The varying fragmentation further helps us in identifying the molecule.

Every organic molecule has a different mass spectra. Some atoms have isotopes in specific natural abundance. This helps us identify an adulterated sample. For example, if a sample contains an abundance of chlorine from tap water or bleach, we will find peaks at two specific mass values in a 3:1 ratio. Similar effects are seen for bromine and other elements.

This is a brief and simplified introduction into mass spectrometry and how it can be used in detecting specific substances in a biological sample. For more information, we recommend Dr. Alison E. Ashcroft’s Introduction to Mass Spectrometry.

Image from http://www.compoundchem.com/2015/05/07/mass-spectrometry/ 

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