A mass spectrometer is an instrument which ionizes a sample, separates the ions depending on their mass-to-charge ratio, and measures the intensity of the ions present at different mass-to-charge ratios. This can be used to determine the elements or isotopes present in the sample.

The technique of mass spectrometry can be best understood by considering it’s application to determining the isotopes present in a pure elemental sample. Because the sample consists of a single element, all of the atoms will have the same number of protons and, if of neutral charge, an equal number of electrons. However, they may have different mass depending on the number of neutrons, resulting in different isotopes of the same element. If the molecules are ionized by removing an electron, they become positively charged and can then be accelerated using a magnetic accelerator. Because all the ions have the same charge, a magnetic field applied across the stream of ions exerts the same force on each ion. However, the differences in mass means this force causes more deflection for ions of some isotopes than others. A detector can then measure the intensity of ions striking different areas depending on this deflection. What is actually being measured is the intensity of ions with different mass-to-charge ratios.

The sample does not need to be composed of a single element, since the mass-to-charge ratio will indicate the elemental or isotopic signature for particles or molecules.

A typical mass spectrometer consists of an element to heat the sample, a radiation source which bombards the sample to ionize it, an electron trap to remove the free electrons, a charged accelerator to pull the ions from the ionized sample and accelerate them into a stream of high-velocity ions, magnets to deflect the stream of ions, and a detector to detect the intension of ions with different mass-to-charge ratios. More generically, all mass spectrometers must include an ion source, a mass analyzer and a detector.

The ion source ionizes the sample material. If the sample is a gas or has been vaporized by pre-heating, electron beam or chemical reaction may be used to ionize it. Liquid and solid samples can also be directly ionized using electrospray ionization or matrix-assisted laser desorption/ionization.

Ions are transported from the ion source using magnetic or electric fields. The combination of accelerator and magnets is a common implementation of a mass analyzer. There are many different arrangements of magnetic and electric fields which can be used for mass analyzers. This include the static magnetic fields described in the above example, but time-of-flight and other methods are also used. The ion detector may be an electron multiplier, a vacuum-tube which multiples incident charges.