Mass Spectrometry is one of the simplest physical methods that Chemists use to identify substances. Mass Spectrometers vary in how they work, but I will describe one of the most common, as well as the machine that we have at Augustana College.
Molecules are held together by bonding orbitals of electrons. If a high energy electron beam or photon beam bombards a molecule, it will knock an electron off the molecule and cause the molecule to break apart. Molecules break apart in characteristic ways depending on how the molecule is bonded together and what atoms and groups of atoms make up the molecule. If we can measure the masses of fragments that we get, we can make a good guess as to what substance we are dealing with.
The basic principles of Mass Spectrometery can be illustrated in the following way. A sample is placed a small resevoir and inserted into an inlet port. The sample must be volatile enough to go into the gas phase once inside the machine. Substances are thus prepared in the liquid or gas phase, and injected automatically or manually. Manual injection is a good choice for less volatile substances, because it is pure sample rather than a mixture of solvent and sample. Once injected, the sample passes through an electron beam typically of 70eV, which is strong enough to knock off an electron and initiate fragmentation. The fragments are directed by a magnetic field towards a detector, which records the masses of the various fragments. Most mass spec machines use a semi-circular path for the fragments to travel as they move towards the detector. Since charge is typically +1, m/e almost always is the mass of the fragment.
The Mass Spectrometer in our department is a Quadrupole system. It uses electron impact (EI) and a rapid scan technique. In EI spectrometry, a sample is bombarded with high energy electrons resulting in the removal of a valence electron and the production of a cation radical (M+), also called a molecular ion.
A volatile substance in the liquid or gaseous phase is injected into the GCD's inlet system where it passes through an electron beam of 70 eV. Electrons from the electron beam collide with the compound, knocking off an electron and initiating compound fragmentation. Next, the volatile substance passes through a series of accelerating plates in an ionization chamber (due to a potential difference between the plates). The compound then travels in a spiraling manner down the center of four alternately charged rods which are continually changing in voltage magnitude. These changes allow charged fragments of differing masses to pass through the center of the rods successfully at different intervals. Finally, the charged fragments are recorded after passing through the detector located at the end of the four Quadrupole rods. Charged fragments of incorrect masses will collide with the rods or escape the rods before reaching the detector.
Here are some GC/Mass Spectra of my unkowns...
Cool places to find out more....
Murray's Links to Mass Spec pages
North Carolina State University Mass Spec Page
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