In physics, a spectrometer is an apparatus to measure a spectrum. Generally, a spectrum is a graph that shows intensity as a function of wavelength, of frequency, of energy, of momentum, or of mass.

Optical spectrometer

Optical spectrometers (often simply called "spectrometers"), in particular, show the intensity of light as a function of wavelength or of frequency. The deflection is produced either by refraction in a prism or by diffraction in a diffraction grating.

Mass spectrometer

A mass spectrometer is an analytical instrument that is used to identify the amount and type of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of gas-phase ions.[1]

Time-of-flight spectrometer

The energy spectrum of particles of known mass can also be measured by determining the time of flight between two detectors (and hence, the velocity) in a time-of-flight spectrometer. Alternatively, if the velocity is known, masses can be determined in a time-of-flight mass spectrometer.

Magnetic spectrometer

A positive charged particle moving in a circle under the influence of the Lorentz force F

When a fast charged particle (charge q, mass m) enters a constant magnetic field B at right angles, it is deflected into a circular path of radius r, due to the Lorentz force. The momentum p of the particle is then given by

Focus of a magnetic semicircular spectrometer

where m and v are mass and velocity of the particle. The focussing principle of the oldest and simplest magnetic spectrometer, the semicircular spectrometer,[2] invented by J. K. Danisz, is shown on the left. A constant magnetic field is perpendicular to the page. Charged particles of momentum p that pass the slit are deflected into circular paths of radius r = p/qB. It turns out that they all hit the horizontal line at nearly the same place, the focus; here a particle counter should be placed. Varying B, this makes possible to measure the energy spectrum of alpha particles in an alpha particle spectrometer, of beta particles in a beta particle spectrometer,[3] of particles (e.g., fast ions) in a particle spectrometer, or to measure the relative content of the various masses in a mass spectrometer.

Since Danysz' time, many types of magnetic spectrometers more complicated than the semicircular type have been devised.[3]


Generally, the resolution of an instrument tells us how well two close-lying energies (or wavelengths, or frequencies, or masses) can be resolved. Generally, for an instrument with mechanical slits, higher resolution will mean lower intensity.


  1. "mass spectrometer" (PDF). 2009. doi:10.1351/goldbook.M03732.
  2. Jan Kazimierz Danysz, Le Radium 9, 1 (1912); 10, 4 (1913)
  3. 1 2 K. Siegbahn, Alpha-, Beta- and Gamma-Ray Spectroscopy, North-Holland Publishing Co. Amsterdam (1966)
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