6.4:
Mass Spectrometers
The mass spectrometer ionizes molecules and produces a mass spectrum from the fragment distribution.
It performs three functions: ionization of molecules or atoms, separation of the ion fragments, and detection of ions.
In electron ionization, a common ionization method, a vaporized sample in the ionization chamber bombarded with a high-energy electron beam produces radical cations. The radical cation quickly fragments into neutral molecules, radicals, and cations.
The cations are then accelerated into an analyzing chamber by a series of negatively charged accelerator plates.
Many common analyzers apply a magnetic or electrical field to the analyzing chamber, which bends the trajectories of ions based on their mass and charge.
Only ions of particular m/z values have the exact trajectory to pass through the narrow slit placed in front of the ion detector for a given magnetic or electrical field.
By altering the field, the mass analyzer scans through a range of ion masses, with their abundances recorded in the mass spectrum.
6.4:
Mass Spectrometers
This lesson details the instrumentation of a mass spectrometer—a physical instrument to perform mass spectrometry on analyte molecules and record the characteristic mass spectra. This is achieved via three chief functions:
- Conversion of the gas-phase analyte atoms/molecules into a beam of positive or negative charged ions by ionization.
- Separation of the charged species based on their mass-to-charge ratio.
- Recording the relative abundance of each type of ion.
In the ionization chamber of the mass spectrometer, the vaporized analyte in a vacuum is struck with high-energy electrons. The electron's energy is around 70 eV, sufficient to strip an electron from the analyte. The resultant molecular ion further fragments into charged species and neutral molecules. The mass spectrometer only records the mass of charged species, as the charge enables the control of molecules by an electric or magnetic field.
The molecular ion and its charged fragments are accelerated by a series of negatively charged accelerator plates positioned appropriately into the detector in the analyzing chamber. A magnetic field is applied on the path between the accelerator plate and detector, which causes a curve in the path of charged species. At a constant magnetic field, the radius of curvature depends on the molecular mass of the charged species. Placing a slit in front of the detector ensures that only charged species of a particular molecular weight reach the detector.
By varying the magnetic field, the charged species of all molecular weights can be recorded at the detector, each unique molecular weight species at a time. By scanning through all the magnetic fields, the mass spectrometer provides information on the relative abundance of all charged species as the mass spectrum.