Mass spectrometry is a technique for separating ions by their mass-to-charge (m/z) ratios. It allows the detection of compounds by separating ions by their unique mass. It can be divided into two broad applications:
- identification of compounds by the mass of one or more elements in a compound
- determination of the isotopic composition of one or more elements in a compound
A mass spectrometer is a device used for mass spectrometry, and produces a mass spectrum of a sample to find its composition. This is normally achieved by ionizing the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. A typical mass spectrometer comprises three parts: an ion source, a mass analyzer, and a detector.
The ion source is the part of the mass spectrometer that ionizes the material under analysis (the analyte). The ions are then transported by magnetic or electrical fields to the mass analyzer.
Techniques for ionization have been key to determining what types of samples can be analyzed by mass spectrometry. Electron ionization and chemical ionization are used for gases and vapors. Two techniques often used with liquid and solid biological samples include electrospray ionization (due to John Fenn) and Matrix-assisted Laser Desorption Ionization (MALDI, due to M. Karas and F. Hillenkamp]).
The mass analyzer is the most flexible part of the mass spectrometer. It uses an electric or magnetic field to deflect the charged particles, while the kinetic energy imparted by motion through an electric field gives the particles inertia dependent on their mass. Thus, the analyzer steers the particles to the detector, based on their mass-to-charge ratios (m/z) by varying an electric or magnetic field. The smallest, most highly charged ions move most rapidly. It can be used to select a narrow range of m/z's or to scan through a range of m/z's to catalog the ions present. Besides the original magnetic-sector types, several types are currently in more common use, including time-of-flight, ion trap , and quadrupole mass analyzers.
Various kinds of mass analyzers have been produced. Perhaps the easiest to understand is the Time-of-flight (TOF) analyzer, typically integrated with MALDI ion sources. It boosts ions to the same kinetic energy by passage through an electric field, and the times they take to reach the detector are measured. Quadrupoles and quadrupole ion traps use electrical fields to selectively stabilize or destabilize ions falling within a narrow window of m/z values. Sector instruments change the direction ions are flying through the mass analyzer. Fourier Transform Mass Spectrometry measures mass by detecting the image current produced by ions spinning (cyclotron) in the presence of a magnetic field. This technique provides extremely high resolution and mass measurement accuracy. The best mass analyzer for an experiment depends upon the type of information to be gleaned from the experiment.
The final element of the mass spectrometer is the detector. The detector records the charge induced when an ion passes by or hits a surface. If a scan is conducted in the mass analyzer, the charge induced in the detector during the course of the scan will produce a mass spectrum, a record of the m/z's at which ions are present.
Typically, some type of electron multiplier is used, though others (such as Faraday cups ) have been employed. Because the number of ions leaving the mass analyzer at a particular instant is typically quite small, significant amplification is necessary to get a signal.
Kinds of MS
The most common form of mass spectrometry is gas chromatography-mass spectroscopy (GC-MS). In this technique a gas chromatograph is used to separate compounds. This stream is fed into the ion source, a metallic filament to which voltage is applied. This filament emits electrons which ionize the compounds. The ions can then further fragment, yielding predictable patterns. The stream then passes into the detector.
MS for large molecules
For large molecules typical of biological applications, special techniques are used. The ion source subjects a sample of material to an Electric charge that causes the material to be ionized. Types of ion sources include electrospray ionization(ESI), chemical ionization (CI), fast atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI), Thermal ionisation (TI), Secondary ionisation (SI), and inductively coupled Plasma source (ICP).
Chemical Ionization MS
The term chemical ionization comes from the fact that the analyte is ionized by chemical ion-molecule reactions during collisions in the source.
One form of chemical ionization is Atmospheric Pressure chemical Ionization (APCI) allowing for high flow rates typical of HPLC to be used directly, often without diverting the larger fraction of volume to waste. Typically the mobile phase containing eluting analyte is heated above 400 degrees Celsius, sprayed with high flow rates of nitrogen and the entire aerosol cloud is subjected to a corona discharge that creates ions. Often APCI can be performed in a modified ESI source.
Several techniques use ions created in a dedicated ion source injected into a flow tube or a drift tube: Selected Ion Flow Tube (SIFT-MS), and Proton Transfer Reaction (PTR-MS), are variants of CI dedicated for trace gas analysis of air, breath or liquid headspace using well defined reaction time allowing calculations of analyte concentrations from the known reaction kinetics without the need for internal standard or callibration.
A tandem mass spectrometer is one capable of multiple rounds of mass spectrometry. For example, one mass analyzer can isolate one peptide from many entering a mass spectrometer. A second mass analyzer then stabilizes the peptide ions while gas collides with them, causing them to fragment. A third mass analyzer then catalogs the fragments produced from the peptides. This process is called collision-induced dissociation and is used for many experiments in proteomics.
Isotope ratio MS
Mass spectrometry is also used to determine the isotopic composition of elements within a sample. Differences in mass among isotopes of an element are very small, and the less abundant isotopes of an element are typically very rare, so a very sensitive instrument is required. These instruments are called isotope ratio mass spectrometers (IR-MS) and use a single magnet to bend a beam of ionized particles towards a series of cups which convert particle impacts to electric current.
The first mass spectography technique was described in an 1899 article by English scientist J.J. Thomson. The processes that more directly gave rise to the modern version were devised by A.J. Dempster and F.W. Aston in 1918 and 1919 respectively.
In 2002 John Fenn received the Nobel Prize in Chemistry for electrospray ionization. The same year Koichi Tanaka received the Nobel Prize for Macromolecule Ionization by Laser Irradiation, however, Matrix-assisted Laser Desorption Ionization (MALDI) was created by M. Karas and F. Hillenkamp.
Last updated: 05-13-2005 07:56:04