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What is mass spectrometry (MS)?

1 Answer
Jul 24, 2015

http://www.mhhe.com/

Mass Spectroscopy (MS), in the most basic sense, is for tracing the fragmentation patterns of molecular ions in order to identify them. This tends to be more useful when coupled with other processes, such as Gas Chromatography and Liquid Chromatography.

MS has an interesting process by which we do the following (in a vacuum):

  • Inject liquid sample (might be a few #muL# if you are injecting into a GC-MS setup; depends on the injection method)
  • Vaporize sample (must be a gas to minimize undesirable fragmentation)
  • Ionize sample (must be an ion to interact with electric/magnetic fields) to facilitate fragmentation
  • Accelerate fragments into field (electric and/or magnetic) to separate ions by #m/z# ratio
  • Detect the ions to get a count for the abundance of each ion
  • Acquire mass spectrum

This is the essential process of Mass Spectroscopy.

In further depth:

Inject the sample via some method, such as the 30-ft long tube you use in Gas Chromatography (i.e. the GC is interfaced with MS and you have a GC-MS setup), for example.

Some sort of sample vaporization occurs so that you have a gaseous sample. This may be done with, perhaps, a coil with electric current flowing through it, or maybe a hot flame (#> 3000 K# or so), for example.

Some sort of ionization occurs (which facilitates fragmentation), such as:

  • Chemical Ionization (soft/indirect ionization via the presence of ions in the system)
  • Matrix-Assisted Laser-Desorption Ionization (a sample matrix is hit with a laser, ionized, and the matrix itself soft-ionizes the sample embedded within and shielded by the matrix)
  • Electron "Impact", where an electron beam allows electrons to interact with the sample to knock off an electron and thus ionize it (hard ionization).

The point of having both soft and hard ionization is that soft ionization better-retains the parent peak during fragmentation, so that you can find the peak that corresponds to the molecular mass of the original ion.

The fragments are then accelerated into an electric and/or magnetic field for the purpose of separating it by a mass-to-charge ratio, #m"/"z#. The ions then separate and then spiral towards some sort of collection surface that counts ions. This separation may be done with, for example, a quadrupole filter ("quadrupole" literally means "four [magnetic] poles").

The ions must reach a detector, such as a Faraday Plate or Faraday Cup. Behind it would be some sort of transducer to convert/encode the number of ions that are counted into a current that acts as a signal for a computer to read, so that it can generate some sort of display to give you your mass spectrum.