Mass spectrometry

Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample on the basis of the mass-to-charge ratio of charged particles.The method employs chemical fragmentation of a sample into charged particles (ions) and measurements of two properties, charge and mass, of the resulting particles, the ratio of which is deduced by passing the particles through electric and magnetic fields in a mass spectrometer. The design of a mass spectrometer has three essential modules: an ion source, which transforms the molecules in a sample into ionized fragments; a mass analyzer, which sorts the ions by their masses by applying electric and magnetic fields; and a detector, which measures the value of some indicator quantity and thus provides data for calculating the abundances each ion fragment present. The technique has both qualitative and quantitative uses, such as identifying unknown compounds, determining the isotopic composition of elements in a compound, determining the structure of a compound by observing its fragmentation, quantifying the amount of a compound in a sample using carefully designed methods (e.g., by comparison with known quantities of heavy isotopes), studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in vacuum), and determining other physical, chemical, or biological properties of compounds.

The word spectrograph has been used since 1884 as an "International Scientific Vocabulary". The linguistic roots, a combination and removal of bound morphemes and free morphemes, are closely related to the terms spectr-um and phot-ograph-ic plate. In fact, early spectrometry devices that measured the mass-to-charge ratio of ions were called mass spectrographs because they were instruments that recorded a spectrum of mass values on a photographic plate. A mass spectroscope is similar to a mass spectrograph except that the beam of ions is directed onto a phosphor screen.A mass spectroscope configuration was used in early instruments when it was desired that the effects of adjustments be quickly observed. Once the instrument was properly adjusted, a photographic plate was inserted and exposed. The term mass spectroscope continued to be used even though the direct illumination of a phosphor screen was replaced by indirect measurements with an oscilloscope.The use of the term mass spectroscopy is now discouraged due to the possibility of confusion with light spectroscopy. Mass spectrometry is often abbreviated as mass-spec or simply as MS. Thomson has also noted that a mass spectroscope is similar to a mass spectrograph except that the beam of ions is directed onto a phosphor screen. The suffix -scope here denotes the direct viewing of the spectra (range) of masses.

Simplified example

The following example describes the operation of a spectrometer mass analyzer, which is of the sector type. Consider a sample of sodium chloride (table salt). In the ion source, the sample is vaporized (turned into gas) and ionized (transformed into electrically charged particles) into sodium (Na+) and chloride (Cl-) ions. Sodium atoms and ions are monoisotopic, with a mass of about 23 amu. Chloride atoms and ions come in two isotopes with masses of approximately 35 amu (at a natural abundance of about 75 percent) and approximately 37 amu (at a natural abundance of about 25 percent). The analyzer part of the spectrometer contains electric and magnetic fields, which exert forces on ions traveling through these fields. The speed of a charged particle may be increased or decreased while passing through the electric field, and its direction may be altered by the magnetic field. The magnitude of the deflection of the moving ion's trajectory depends on its mass-to-charge ratio. By Newton's second law of motion, lighter ions get deflected by the magnetic force more than heavier ions. The streams of sorted ions pass from the analyzer to the detector, which records the relative abundance of each ion type. This information is used to determine the chemical element composition of the original sample (i.e. that both sodium and chlorine are present in the sample) and the isotopic composition of its constituents (the ratio of 35Cl to 37Cl).

Fourier transform ion cyclotron resonance

Fourier transform mass spectrometry, or more precisely Fourier transform ion cyclotron resonance MS, measures mass by detecting the image current produced by ions cyclotroning in the presence of a magnetic field. Instead of measuring the deflection of ions with a detector such as an electron multiplier, the ions are injected into a Penning trap (a static electric/magnetic ion trap) where they effectively form part of a circuit. Detectors at fixed positions in space measure the electrical signal of ions which pass near them over time, producing a periodic signal. Since the frequency of an ion's cycling is determined by its mass to charge ratio, this can be deconvoluted by performing a Fourier transform on the signal. FTMS has the advantage of high sensitivity (since each ion is "counted" more than once) and much higher resolution and thus precision.

Ion cyclotron resonance (ICR) is an older mass analysis technique similar to FTMS except that ions are detected with a traditional detector. Ions trapped in a Penning trap are excited by an RF electric field until they impact the wall of the trap, where the detector is located. Ions of different mass are resolved according to impact time.

Very similar nonmagnetic FTMS has been performed, where ions are electrostatically trapped in an orbit around a central, spindle shaped electrode. The electrode confines the ions so that they both orbit around the central electrode and oscillate back and forth along the central electrode's long axis. This oscillation generates an image current in the detector plates which is recorded by the instrument. The frequencies of these image currents depend on the mass to charge ratios of the ions. Mass spectra are obtained by Fourier transformation of the recorded image currents.