The highest-mass ion in a spectrum is normally considered to be the molecular ion, and lower-mass ions are fragments from the molecular ion, assuming the sample is a single pure compound. Modern mass spectrometers easily distinguish (resolve) ions differing by only a single atomic mass unit (amu), and thus provide completely accurate values for the molecular mass of a compound. Most of the ions formed in a mass spectrometer have a single charge, so the m/z value is equivalent to mass itself. The most intense ion is assigned an abundance of 100, and it is referred to as the base peak. An animated display of this ionization process will appear if you click on the ion source of the mass spectrometer diagram.Ī mass spectrum will usually be presented as a vertical bar graph, in which each bar represents an ion having a specific mass-to-charge ratio (m/z) and the length of the bar indicates the relative abundance of the ion.

The molecular ion is a radical cation, but the fragment ions may either be radical cations (pink) or carbocations (orange), depending on the nature of the neutral fragment. Residual energy from the collision may cause the molecular ion to fragment into neutral pieces (colored green) and smaller fragment ions (colored pink and orange). This leaves behind a molecular ion (colored red in the following diagram). When a high energy electron collides with a molecule it often ionizes it by knocking away one of the molecular electrons (either bonding or non-bonding). By varying the strength of the magnetic field, ions of different mass can be focused progressively on a detector fixed at the end of a curved tube (also under a high vacuum). Lighter ions are deflected more than heavier ions. A perpendicular magnetic field deflects the ion beam in an arc whose radius is proportional to the mass of each ion. Some of these ions fragment into smaller cations and neutral fragments. Cations formed by the electron bombardment (red dots) are pushed away by a charged repeller plate (anions are attracted to it), and accelerated toward other electrodes, having slits through which the ions pass as a beam. Non-volatile solids and liquids may be introduced directly. Gases and volatile liquid samples are allowed to leak into the ion source from a reservoir (as shown). This is called an EI (electron-impact) source. Here molecules of the sample (black dots) are bombarded by electrons (light blue lines) issuing from a heated filament. The heart of the spectrometer is the ion source. A mass spectrometer operating in this fashion is outlined in the following diagram.

The ions are then detected electronically and the resulting information is stored and analyzed in a computer. In one common procedure, ionization is effected by a high energy beam of electrons, and ion separation is achieved by accelerating and focusing the ions in a beam, which is then bent by an external magnetic field. Each of the three tasks listed above may be accomplished in different ways. The pressure under which ions may be handled is roughly 10 -5 to 10 -8 torr (less than a billionth of an atmosphere). Atmospheric pressure is around 760 torr (mm of mercury). The Detectorīecause ions are very reactive and short-lived, their formation and manipulation must be conducted in a vacuum. The separated ions are then measured, and the results displayed on a chart. The ions are sorted and separated according to their mass and charge. A small sample is ionized, usually to cations by loss of an electron. The three essential functions of a mass spectrometer, and the associated components, are:ġ.

In order to measure the characteristics of individual molecules, a mass spectrometer converts them to ions so that they can be moved about and manipulated by external electric and magnetic fields.

Atomic spectra graph