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Mass spectrometry is an analytical technique that identifies the chemical composition of a sample on the basis of the mass-to-charge ratio of charged particles. The mass-to-charge ratio, is a Physical quantity that is widely used in the Electrodynamics of charged particles e [1] The technique has both qualitative and quantitative uses, including:

Main steps of measuring with a mass spectrometer
Main steps of measuring with a mass spectrometer

The underlying principle of mass spectrometry is that as charged particles (ions) pass through electric and magnetic fields, their paths vary according to their mass-to-charge ratios. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides Structure is a fundamental and sometimes Intangible notion covering the Recognition, Observation, nature, and Stability of Gas phase ion chemistry is a field of science encompassed dually within both Chemistry and Physics. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Devices which operate according to this principle are called mass spectrometers. Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample based on the Mass-to-charge ratio of charged particles The design of a mass spectrometer has three essential modules: an ion source, which transforms the molecules of a sample into a variety of ionized fragments; an analyzer, which sorts the ions by their masses, using internally generated electric and magnetic fields; and a detector, which measures the value of some indicator quantity and thus provides data for calculating the abundances of respective ion fragments.

Contents

Etymology

Early 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 spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. Photographic plates preceded Photographic film as a mean of photography [2][3] A mass spectroscope is similar to a mass spectrograph except that the beam of ions is directed onto a phosphor screen. A phosphor is a substance that exhibits the phenomenon of Phosphorescence (sustained glowing after exposure to energized particles such as Electrons [4] 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. An oscilloscope (commonly abbreviated to scope or O-scope) is a type of Electronic test equipment that allows signal Voltages to be viewed [5] The use of the term mass spectroscopy is now discouraged due to the possibility of confusion with light spectroscopy. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ [1] [6][1] Mass spectrometry is often abbreviated as mass-spec or simply as MS. [1]

History

For more details on this topic, see History of mass spectrometry. The history of mass spectrometry dates back more than one hundred years and has its roots in physical and chemical studies regarding the nature of matter
Francis William Aston won the 1922 Nobel Prize in Chemistry for his work in mass spectrometry
Francis William Aston won the 1922 Nobel Prize in Chemistry for his work in mass spectrometry

In 1886, Eugen Goldstein observed rays in gas discharges under low pressure that travelled through the channels in a perforated cathode toward the anode, in the opposite direction to the negatively charged cathode rays. Year 1886 ( MDCCCLXXXVI) was a Common year starting on Friday (link will display the full calendar of the Gregorian calendar (or a Common Eugen Goldstein ( September 5, 1850 &ndash December 25, 1930) was a German Physicist. Gas discharge lamps are a family of artificial light sources that generate light by sending an Electrical discharge through an ionized gas i A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device Cathode rays (also called an electron beam or e-beam) are streams of Electrons observed in Vacuum tubes i Goldstein called these positively charged anode rays "Kanalstrahlen"; the standard translation of this term into English is "canal rays". Anode rays (or Canal rays) were observed in experiments by a German Scientist, Eugen Goldstein, in 1886 Anode rays (or Canal rays) were observed in experiments by a German Scientist, Eugen Goldstein, in 1886 Wilhelm Wien found that strong electric or magnetic fields deflected the canal rays and, in 1899, constructed a device with parallel electric and magnetic fields that separated the positive rays according to their charge-to-mass ratio (Q/m). Wilhelm Carl Werner Otto Fritz Franz Wien ( German:) ( 13 January 1864 &ndash 30 August 1928) was a German physicist Wien found that the charge-to-mass ratio depended on the nature of the gas in the discharge tube. English scientist J.J. Thomson later improved on the work of Wien by reducing the pressure to create a mass spectrograph. Sir Joseph John “JJ” Thomson, OM, FRS (18 December 1856 &ndash 30 August 1940 was a British Physicist and Nobel laureate

Some of the modern techniques of mass spectrometry were devised by Arthur Jeffrey Dempster and F.W. Aston in 1918 and 1919 respectively. Arthur Jeffrey Dempster ( August 14 1886 - March 11 1950) was a Canadian - American Physicist best known for Francis William Aston ( September 1 1877 &ndash November 20 1945) was a British Chemist and Physicist who won the Year 1918 ( MCMXVIII) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian calendar (or a Common Year 1919 ( MCMXIX) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian calendar (or a Common In 1989, half of the Nobel Prize in Physics was awarded to Hans Dehmelt and Wolfgang Paul for the development of the ion trap technique in the 1950s and 1960s. Year 1989 ( MCMLXXXIX) was a Common year starting on Sunday (link displays 1989 Gregorian calendar) The Nobel Prize in Physics (Nobelpriset i fysik is awarded once a year by the Royal Swedish Academy of Sciences. In 2002, the Nobel Prize in Chemistry was awarded to John Bennett Fenn for the development of electrospray ionization (ESI) and Koichi Tanaka for the development of soft laser desorption (SLD) in 1987. See also 2002 (disambiguation Year 2002 ( MMII) was a Common year starting on Tuesday of the Gregorian calendar. The Nobel Prize in Chemistry (Nobelpriset i kemi is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of Chemistry. Electrospray ionization (ESI is a technique used in Mass spectrometry to produce ions Koichi Tanaka (田中 耕一 Tanaka Kōichi, born August 3, 1959) is a Japanese scientist who won the Nobel Prize in Chemistry in 2002 for developing Soft laser desorption is Laser desorption of large molecules that results in Ionization without fragmentation Year 1987 ( MCMLXXXVII) was a Common year starting on Thursday (link displays 1987 Gregorian calendar) An improved SLD method, matrix-assisted laser desorption/ionization (MALDI), was developed in 1987 by Franz Hillenkamp and Michael Karas. Matrix-assisted laser desorption/ionization (MALDI is a soft Ionization technique used in Mass spectrometry, allowing the analysis of Biomolecules ( [7]

Simplified example

Schematics of a simple mass spectrometer with sector type mass analyzer
Schematics of a simple mass spectrometer with sector type mass analyzer

The following example will describe the operation of a spectrometer mass analyzer which is of the sector type. A sector instrument is a general term for a class of Mass spectrometer that utilizes a static electric or magnetic sector or some combination of the two (separately in space (Other analyzer types are treated below. ) Consider a sample of sodium chloride, (NaCl) (table salt). Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and In the ion source, the sample is vaporized (turned into gas) and ionized (transformed into electrically charged particles) into (Na+ and Cl-). A vapor or vapour (see Spelling differences) is a substance in the Gas phase at a Temperature lower than its Critical temperature This page is about the physical properties of gas as a state of matter Sodium atoms and ions are monoisotopic, with a mass of about 23 amu. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides Chloride atoms and ions come in two isotopes with masses of approximately 35 amu (at a natural prevalence of about 75 percent) and approximately 37 amu (at a natural prevalence of about 25 percent). The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus The analyzer of the spectrometer contains electric and magnetic fields, which exert forces on ions traveling through the fields. The speed of a charged particle may be increased or reduced by passing it through an electric field and its direction may be altered by passing it through a magnetic field. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges 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. Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the The streams of sorted ions pass from the analyzer to the detector, which determines the relative abundance of each ion type. By interpreting this information, the chemical element composition of the original sample (i. e. that both sodium and chlorine are present in the sample) and the isotopic compositions of its constituents (i. e. whether the ratio of 35Cl to 37Cl has been changed by some process) can be determined.

Instrumentation

Ion source technologies

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 electric fields to the mass analyzer. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can

Techniques for ionization have been key to determining what types of samples can be analyzed by mass spectrometry. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Electron ionization and chemical ionization are used for gases and vapors. Electron ionization ( EI, formerly known as electron impact) is an Ionization technique widely used in Mass spectrometry, particularly for Chemical ionization ( CI) is an Ionization technique used in Mass spectrometry. This page is about the physical properties of gas as a state of matter A vapor or vapour (see Spelling differences) is a substance in the Gas phase at a Temperature lower than its Critical temperature In chemical ionization sources, the analyte is ionized by chemical ion-molecule reactions during collisions in the source. Chemical ionization ( CI) is an Ionization technique used in Mass spectrometry. Two techniques often used with liquid and solid biological samples include electrospray ionization (innovated by John Fenn) and matrix-assisted laser desorption/ionization (MALDI, due to K. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Electrospray ionization (ESI is a technique used in Mass spectrometry to produce ions John Bennett Fenn (born June 15, 1917, New York City New York) is a research professor of analytical chemistry who was awarded the Nobel Prize Matrix-assisted laser desorption/ionization (MALDI is a soft Ionization technique used in Mass spectrometry, allowing the analysis of Biomolecules ( Tanaka and separately, M. Karas and F. Hillenkamp). Inductively coupled plasma sources are used primarily for metal analysis on a wide array of sample types. ICP-MS ( Inductively coupled plasma mass spectrometry) is a type of Mass spectrometry that is highly sensitive and capable of the determination of a range of Metals Others include glow discharge, field desorption (FD), fast atom bombardment (FAB), thermospray, desorption/ionization on silicon (DIOS), Direct Analysis in Real Time (DART), atmospheric pressure chemical ionization (APCI), secondary ion mass spectrometry (SIMS), spark ionization and thermal ionisation. Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas usually Argon or another Noble Field desorption (FD/field ionization (FI refers to an Ion source for Mass spectrometry first reported by Beckey in 1969 Fast atom bombardment (FAB is an Ionization technique used in Mass spectrometry. Thermospray is a form of atmospheric pressure ionization in Mass spectrometry. Soft laser desorption is Laser desorption of large molecules that results in Ionization without fragmentation A DART ion source ( Direct analysis in real time) refers to an atmospheric-pressure Ion source for Mass spectrometry or Ion mobility spectrometry Atmospheric pressure chemical ionization (APCI is an ionization method used in Mass spectrometry. Secondary ion mass spectrometry (SIMS is a technique used to analyze the composition of solid surfaces and Thin films by Sputtering the surface of the specimen with Spark ionization (also known as spark source ionization ( deprecated) is a method used to produce gas phase Ions from a solid sample In thermal ionization, also referred to as surface ionization, chemically-purified material loaded onto a filament which is then heated to cause some of the material [8] Ion Attachment Ionization is a newer soft ionization technique that allows for fragmentation free analysis. Ion-attachment mass spectrometry ( IAMS) is a form of Mass spectrometry that uses a "soft" form of ionization similar to Chemical ionization in

Mass analyzer technologies

Mass analyzers separate the ions according to their mass-to-charge ratio. The mass-to-charge ratio, is a Physical quantity that is widely used in the Electrodynamics of charged particles e The following two laws govern the dynamics of charged particles in electric and magnetic fields in vacuum:

\mathbf{F} = Q (\mathbf{E} + \mathbf{v} \times \mathbf{B}) (Lorentz force law)
\mathbf{F}=m\mathbf{a} (Newton's second law of motion)

where F is the force applied to the ion, m is the mass of the ion, a is the acceleration, Q is the ion charge, E is the electric field, and v x B is the vector cross product of the ion velocity and the magnetic field

Equating the above expressions for the force applied to the ion yields:

(m/Q)\mathbf{a} = \mathbf{E}+ \mathbf{v} \times \mathbf{B}

This differential equation is the classic equation of motion for charged particles. In Physics, the Lorentz force is the Force on a Point charge due to Electromagnetic fields It is given by the following equation Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the In Mathematics, the cross product is a Binary operation on two vectors in a three-dimensional Euclidean space that results in another vector which Together with the particle's initial conditions, it completely determines the particle's motion in space and time in terms of m/Q. Thus mass spectrometers could be thought of as "mass-to-charge spectrometers". When presenting data, it is common to use the (officially) dimensionless m/z, where z is the number of elementary charges (e) on the ion (z=Q/e). In Dimensional analysis, a dimensionless quantity (or more precisely a quantity with the dimensions of 1) is a Quantity without any Physical units The elementary charge, usually denoted e, is the Electric charge carried by a single Proton, or equivalently the negative of the electric charge carried This quantity, although it is informally called the mass-to-charge ratio, more accurately speaking represents the ratio of the mass number and the charge number, z.

There are many types of mass analyzers, using either static or dynamic fields, and magnetic or electric fields, but all operate according to the above differential equaltion. Each analyzer type has its strengths and weaknesses. Many mass spectrometers use two or more mass analyzers for tandem mass spectrometry (MS/MS). In addition to the more common mass analyzers listed below, there are others designed for special situations.

Sector

For more details on this topic, see sector instrument. A sector instrument is a general term for a class of Mass spectrometer that utilizes a static electric or magnetic sector or some combination of the two (separately in space

A sector field mass analyzer uses an electric and/or magnetic field to affect the path and/or velocity of the charged particles in some way. In Physics, velocity is defined as the rate of change of Position. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. As shown above, sector instruments bend the trajectories of the ions as they pass through the mass analyzer, according to their mass-to-charge ratios, deflecting the more charged and faster-moving, lighter ions more. A sector instrument is a general term for a class of Mass spectrometer that utilizes a static electric or magnetic sector or some combination of the two (separately in space The analyzer can be used to select a narrow range of m/z or to scan through a range of m/z to catalog the ions present. [9]

Time-of-flight

For more details on this topic, see time-of-flight mass spectrometry. Time-of-flight mass spectrometry (TOFMS is a method of Mass spectrometry in which Ions are accelerated by an Electric field of known strength

The time-of-flight (TOF) analyzer uses an electric field to accelerate the ions through the same potential, and then measures the time they take to reach the detector. The time of flight (TOF describes the method used to measure the time that it takes for a particle object or stream to reach a detector while traveling over a known distance In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical If the particles all have the same charge, the kinetic energies will be identical, and their velocities will depend only on their masses. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. The kinetic energy of an object is the extra Energy which it possesses due to its motion In Physics, velocity is defined as the rate of change of Position. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object Lighter ions will reach the detector first. [10]

Quadrupole

For more details on this topic, see Quadrupole mass analyzer. The quadrupole mass analyzer is one type of mass analyzer used in Mass spectrometry.

Quadrupole mass analyzers use oscillating electrical fields to selectively stabilize or destabilize ions passing through a radio frequency (RF) quadrupole field. The quadrupole mass analyzer is one type of mass analyzer used in Mass spectrometry. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz A quadrupole or quadrapole is one of a sequence of configurations of — for example — electric charge or current or gravitational mass that can exist in ideal form but it A quadrupole mass analyzer acts as a mass selective filter and is closely related to the Quadrupole ion trap, particularly the linear quadrupole ion trap except that it operates without trapping the ions and is for that reason referred to as a transmission quadrupole. A quadrupole ion trap exists in both linear and 3D ( Paul Trap QIT) varieties and refers to an Ion trap that uses constant DC and Radio A common variation of the quadrupole is the triple quadrupole.

Quadrupole ion trap

For more details on this topic, see quadrupole ion trap. A quadrupole ion trap exists in both linear and 3D ( Paul Trap QIT) varieties and refers to an Ion trap that uses constant DC and Radio

The quadrupole ion trap works on the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected. A quadrupole ion trap exists in both linear and 3D ( Paul Trap QIT) varieties and refers to an Ion trap that uses constant DC and Radio Ions are created and trapped in a mainly quadrupole RF potential and separated by m/Q, non-destructively or destructively.

There are many mass/charge separation and isolation methods but most commonly used is the mass instability mode in which the RF potential is ramped so that the orbit of ions with a mass a > b are stable while ions with mass b become unstable and are ejected on the z-axis onto a detector.

Ions may also be ejected by the resonance excitation method, whereby a supplemental oscillatory excitation voltage is applied to the endcap electrodes, and the trapping voltage amplitude and/or excitation voltage frequency is varied to bring ions into a resonance condition in order of their mass/charge ratio. [11][12]

The cylindrical ion trap mass spectrometer is a derivative of the quadrupole ion trap mass spectrometer. A quadrupole ion trap exists in both linear and 3D ( Paul Trap QIT) varieties and refers to an Ion trap that uses constant DC and Radio

Linear quadrupole ion trap

A linear quadrupole ion trap is similar to a quadrupole ion trap, but it traps ions in a two dimensional quadrupole field, instead of a three dimensional quadrupole field as in a quadrupole ion trap. A quadrupole ion trap exists in both linear and 3D ( Paul Trap QIT) varieties and refers to an Ion trap that uses constant DC and Radio Thermo Fisher's LTQ ("linear trap quadrupole") is an example of the linear ion trap. [13]

Fourier transform ion cyclotron resonance

A FT-ICR mass spectrometer
A FT-ICR mass spectrometer
For more details on this topic, see Fourier transform mass spectrometry. Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer (or Mass spectrometer) for

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. Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer (or Mass spectrometer) for Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer (or Mass spectrometer) for A cyclotron is a type of Particle accelerator. Cyclotrons accelerate Charged particles using a high- Frequency, alternating Voltage (potential 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. An electron multiplier (continuous dynode electron multiplier is a vacuum-tube structure that multiplies incident charges Penning traps are devices for the storage of charged particles using a constant static Magnetic field and a spatially inhomogeneous static Electric field. An ion trap is a combination of electric or magnetic fields that captures Ions in a region of a vacuum system or tube 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. In Mathematics, deconvolution is an algorithm-based process used to reverse the effects of Convolution on recorded data This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and FTMS has the advantage of high sensitivity (since each ion is "counted" more than once) and much higher resolution and thus precision. Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer (or Mass spectrometer) for [14][15]

Ion cyclotron resonance (ICR) is an older mass analysis technique similar to FTMS except that ions are detected with a traditional detector. Ion cyclotron resonance is a phenomenon related to the movement of Ions in a Magnetic field. 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. Penning traps are devices for the storage of charged particles using a constant static Magnetic field and a spatially inhomogeneous static Electric field. Ions of different mass are resolved according to impact time.

Similarly, nonmagnetic FTMS has been performed, where ions are electrostatically trapped in an orbit around a central, spindle shaped electrode. Electrostatics is the branch of Science that deals with the Phenomena arising from what seems to be stationary Electric charges Since Classical 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. This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and

Similar to Fourier transform ion cyclotron resonance mass spectrometers, Orbitraps have a high mass accuracy, high sensitivity and a good dynamic range. Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer (or Mass spectrometer) for [16]

Detector

A continuous dynode particle multiplier detector.
A continuous dynode particle multiplier detector.

The final element of the mass spectrometer is the detector. The detector records either the charge induced or the current produced when an ion passes by or hits a surface. In a scanning instrument, the signal produced in the detector during the course of the scan versus where the instrument is in the scan (at what m/Q) will produce a mass spectrum, a record of ions as a function of m/Q. A mass spectrum is an intensity vs m/z ( Mass-to-charge ratio) plot representing a chemical analysis

Typically, some type of electron multiplier is used, though other detectors including Faraday cups and ion-to-photon detectors are also used. An electron multiplier (continuous dynode electron multiplier is a vacuum-tube structure that multiplies incident charges A Faraday cup is a Metal (conductive cup designed to catch charged particles in vacuum Because the number of ions leaving the mass analyzer at a particular instant is typically quite small, considerable amplification is often necessary to get a signal. Microchannel Plate Detectors are commonly used in modern commercial instruments. A micro-channel plate (MCP is a planar component used for detection of particles ( Electrons or Ions and impinging Radiation ( Ultraviolet [17] In FTMS and Orbitraps, the detector consists of a pair of metal surfaces within the mass analyzer/ion trap region which the ions only pass near as they oscillate. Fourier transform ion cyclotron resonance mass spectrometry, also known as Fourier transform mass spectrometry, is a type of mass analyzer (or Mass spectrometer) for An orbitrap is a type of Mass spectrometer invented by Alexander Makarov. No DC current is produced, only a weak AC image current is produced in a circuit between the electrodes. Other inductive detectors have also been used. [18]

Tandem mass spectrometry

Main article: Tandem mass spectrometry

A tandem mass spectrometer is one capable of multiple rounds of mass spectrometry, usually separated by some form of molecule fragmentation. Tandem mass spectrometry, also known as MS/MS, involves multiple steps of Mass spectrometry selection with some form of fragmentation occurring in between the stages For example, one mass analyzer can isolate one peptide from many entering a mass spectrometer. Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino A second mass analyzer then stabilizes the peptide ions while they collide with a gas, causing them to fragment by collision-induced dissociation (CID). In Mass spectrometry, collision-induced dissociation (CID referred to by some as collisionally activated dissociation (CAD is a mechanism by which to fragment A third mass analyzer then sorts the fragments produced from the peptides. Tandem MS can also be done in a single mass analyzer over time, as in a quadrupole ion trap. A quadrupole ion trap exists in both linear and 3D ( Paul Trap QIT) varieties and refers to an Ion trap that uses constant DC and Radio There are various methods for fragmenting molecules for tandem MS, including collision-induced dissociation (CID), electron capture dissociation (ECD), electron transfer dissociation (ETD), infrared multiphoton dissociation (IRMPD) and blackbody infrared radiative dissociation (BIRD). In Mass spectrometry, collision-induced dissociation (CID referred to by some as collisionally activated dissociation (CAD is a mechanism by which to fragment In Mass spectrometry, electron capture dissociation (ECD is a method of fragmenting gas phase ions for tandem mass spectrometric analysis (structural elucidation Electron transfer dissociation (ETD is a method to fragment ions in a Mass spectrometer. Infrared multiphoton dissociation (IRMPD is a technique used in Mass spectrometry to fragment molecules in the gas phase usually for structural analysis of the original (parent In Mass spectrometry blackbody infrared radiative dissociation (BIRD is a method for fragmenting gas phase ions for tandem mass spectrometric analysis (structural elucidation An important application using tandem mass spectrometry is in protein identification. Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample based on the Mass-to-charge ratio of charged particles [19]

Tandem mass spectrometry enables a variety of experimental sequences. Many commercial mass spectrometers are designed to expedite the execution of such routine sequences as single reaction monitoring (SRM), multiple reaction monitoring (MRM), and precursor ion scan. In SRM, the first analyzer allows only a single mass through and the second analyzer monitors for a single user defined fragment ion. MRM allows for multiple user defined fragment ions. SRM and MRM are most often used with scanning instruments where the second mass analysis event is duty cycle limited. In Telecommunications and Electronics, the term duty cycle is used to describe the fraction of time that a system is in an "active" state These experiments are used to increase specificity of detection of known molecules, notably in pharmacokinetic studies. Precursor ion scan refers to monitoring for a specific loss from the precursor ion. The first and second mass analyzers scan across the spectrum as partitioned by a user defined m/z value. This experiment is used to detect specific motifs within unknown molecules.

Common mass spectrometer configurations and techniques

When a specific configuration of source, analyzer, and detector becomes conventional in practice, often a compound acronym arises to designate it, and the compound acronym may be more well known among nonspectrometrists than the component acronyms. The epitome of this is MALDI-TOF, which simply refers to combining a Matrix-assisted laser desorption/ionization source with a Time-of-flight mass analyzer. Matrix-assisted laser desorption/ionization (MALDI is a soft Ionization technique used in Mass spectrometry, allowing the analysis of Biomolecules ( Matrix-assisted laser desorption/ionization (MALDI is a soft Ionization technique used in Mass spectrometry, allowing the analysis of Biomolecules ( The time of flight (TOF describes the method used to measure the time that it takes for a particle object or stream to reach a detector while traveling over a known distance The MALDI-TOF moniker is more widely recognized by the non-mass spectrometrist scientist than MALDI or TOF individually. Other examples include inductively coupled plasma-mass spectrometry (ICP-MS), accelerator mass spectrometry (AMS), Thermal ionization-mass spectrometry (TIMS) and spark source mass spectrometry (SSMS). ICP-MS ( Inductively coupled plasma mass spectrometry) is a type of Mass spectrometry that is highly sensitive and capable of the determination of a range of Metals Accelerator mass spectrometry (AMS differs from other forms of Mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass In thermal ionization, also referred to as surface ionization, chemically-purified material loaded onto a filament which is then heated to cause some of the material Spark ionization (also known as spark source ionization ( deprecated) is a method used to produce gas phase Ions from a solid sample Sometimes the use of the generic "MS" actually connotes a very specific mass analyzer and detection system, as is the case with AMS, which is always sector based.

Certain applications of mass spectrometry have developed monikers that although strictly speaking they would seem to refer to a broad application, in practice have come instead to connote a specific or a limited number of instrument configurations. An example of this is isotope ratio mass spectrometry (IRMS), which refers in practice to the use of a limited number of sector based mass analyzers; this name is used to refer to both the application and the instrument used for the application. Isotope ratio mass spectrometry (IRMS is a specialization of Mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of

Chromatographic techniques combined with mass spectrometry

An important enhancement to the mass resolving and mass determining capabilities of mass spectrometry is using it in tandem with chromatographic separation techniques. Chromatography (from Greek χρώμα chroma, color and γραφειν"graphein" to write is the collective term for a family of Laboratory

Gas chromatography

A gas chromatograph (right) directly coupled to a mass spectrometer (left)
A gas chromatograph (right) directly coupled to a mass spectrometer (left)
See also: Gas chromatography-mass spectrometry

A common combination is gas chromatography-mass spectrometry (GC/MS or GC-MS). Gas chromatography-mass spectrometry ( GC-MS) is a method that combines the features of Gas-liquid chromatography and Mass spectrometry to identify different This page is about the physical properties of gas as a state of matter In this technique, a gas chromatograph is used to separate different compounds. Gas-liquid chromatography (GLC, or simply gas chromatography (GC, is a type of Chromatography in which the mobile phase is a carrier gas usually an Inert This stream of separated compounds is fed online into the ion source, a metallic filament to which voltage is applied. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across An electrical filament is a thread of Metal, usually Tungsten, which is used to convert Electricity into light in Incandescent light bulbs (as developed Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical This filament emits electrons which ionize the compounds. The ions can then further fragment, yielding predictable patterns. Intact ions and fragments pass into the mass spectrometer's analyzer and are eventually detected. [20]

Liquid chromatography

See also: Liquid chromatography-mass spectrometry

Similar to gas chromatography MS (GC/MS), liquid chromatography mass spectrometry (LC/MS or LC-MS) separates compounds chromatographically before they are introduced to the ion source and mass spectrometer. Liquid chromatography-mass spectrometry ( LC-MS) is an Analytical chemistry technique that combines the physical separation capabilities of liquid chromatography It differs from GC/MS in that the mobile phase is liquid, usually a mixture of water and organic solvents, instead of gas. Water is a common Chemical substance that is essential for the survival of all known forms of Life. A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Most commonly, an electrospray ionization source is used in LC/MS. Electrospray ionization (ESI is a technique used in Mass spectrometry to produce ions

Ion mobility

Ion mobility spectrometry/mass spectrometry (IMS/MS or IMMS) is a technique where ions are first separated by drift time through some neutral gas under an applied electrical potential gradient before being introduced into a mass spectrometer. An ion mobility spectrometer ( IMS) is a Spectrometer capable of detecting and identifying very low concentrations of chemicals based upon the differential migration [21] Drift time is a measure of the radius relative to the charge of the ion. The duty cycle of IMS (the time over which the experiment takes place) is longer than most mass spectrometric techniques, such that the mass spectrometer can sample along the course of the IMS separation. In Telecommunications and Electronics, the term duty cycle is used to describe the fraction of time that a system is in an "active" state This produces data about the IMS separation and the mass-to-charge ratio of the ions in a manner similar to LC/MS. Liquid chromatography-mass spectrometry ( LC-MS) is an Analytical chemistry technique that combines the physical separation capabilities of liquid chromatography [22]

The duty cycle of IMS is short relative to liquid chromatography or gas chromatography separations and can thus be coupled to such techniques, producing triple modalities such as LC/IMS/MS. [23]

Data and analysis

Mass spectrum of a peptide showing the isotopic distribution
Mass spectrum of a peptide showing the isotopic distribution

Data representations

See also: Mass spectrometry data formats

Mass spectrometry produces various types of data. Here the main Mass spectrometry data formats are listed mzXML mzXML is a XML (eXtensible Markup Language based common file format for Proteomics The most common data representation is the mass spectrum. A mass spectrum is an intensity vs m/z ( Mass-to-charge ratio) plot representing a chemical analysis

Certain types of mass spectrometry data are best represented as a mass chromatogram. A Mass chromatogram is a representation of Mass spectrometry data as a Chromatogram, where the X-axis represents time and the y-axis represents signal intensity Types of chromatograms include selected ion monitoring (SIM), total ion current (TIC), and selected reaction monitoring chromatogram (SRM), among many others.

Other types of mass spectrometry data are well represented as a three dimensional contour map. A contour line (also Level set, isopleth, isoline, isogram or isarithm) of a function of two In this form, the mass-to-charge, m/z is on the x-axis, intensity the y-axis, and an additional experimental parameter, such as time, is recorded on the z-axis.

Data analysis

Basics

Mass spectrometry data analysis is a complicated subject matter that is very specific to the type of experiment producing the data. There are general subdivisions of data that are fundamental to understand any data.

Many mass spectrometers work in either negative ion mode or positive ion mode. It is very important to know whether the observed ions are negatively or positively charged. This is often important in determining the neutral mass but it also indicates something about the nature of the molecules.

Different types of ion source result in different arrays of fragments produced from the original molecules. An electron ionization source produces many fragments and mostly odd electron species with one charge, whereas an electrospray source usually produces quasimolecular even electron species that may be multiply charged. Tandem mass spectrometry purposely produces fragment ions post-source and can drastically change the sort of data achieved by an experiment.

By understanding the origin of a sample, certain expectations can be assumed as to the component molecules of the sample and their fragmentations. A sample from a synthesis/manufacturing process will likely contain impurities chemically related to the target component. A relatively crudely prepared biological sample will likely contain a certain amount of salt, which may form adducts with the analyte molecules in certain analyses. See also Adduction, one of the Anatomical terms of motion. An adduct (from the Latin adductus, "drawn toward" is

Results can also depend heavily on how the sample was prepared and how it was run/introduced. An important example is the issue of which matrix is used for MALDI spotting, since much of the energetics of the desorption/ionization event is controlled by the matrix rather than the laser power. Sometimes samples are spiked with sodium or another ion-carrying species to produce adducts rather than a protonated species.

The greatest source of trouble when non-mass spectrometrists try to conduct mass spectrometry on their own or collaborate with a mass spectrometrist is inadequate definition of the research goal of the experiment. Adequate definition of the experimental goal is a prerequisite for collecting the proper data and successfully interpreting it. Among the determinations that can be achieved with mass spectrometry are molecular mass, molecular structure, and sample purity. Each of these questions requires a different experimental procedure. Simply asking for a "mass spec" will most likely not answer the real question at hand.

Interpretation of mass spectra

Main article: Mass spectrum analysis

Since the precise structure or peptide sequence of a molecule is deciphered through the set of fragment masses, the interpretation of mass spectra requires combined use of various techniques. Mass spectrum analysis is an integral part of Spectroscopy and Mass spectroscopy dealing with the interpretation of mass spectra. Chemical structure refers to Molecular geometry, Electronic structure and Crystal structure. Peptide sequence or amino acid sequence is the order in which Amino acid residues connected by Peptide bonds lie in the chain in Peptides A mass spectrum is an intensity vs m/z ( Mass-to-charge ratio) plot representing a chemical analysis Usually the first strategy for identifying an unknown compound is to compare its experimental mass spectrum against a library of mass spectra. If the search comes up empty, then manual interpretation[24] or software assisted interpretation of mass spectra are performed. Mass spectrometry software is any Software for data acquisition analysis or data representation in Mass spectrometry. Computer simulation of ionization and fragmentation processes occurring in mass spectrometer is the primary tool for assigning structure or peptide sequence to a molecule. Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons An a priori structural information is fragmented in silico and the resulting pattern is compared with observed spectrum. In Silico is the second full-length studio album by Australian Drum and bass band Pendulum, released in Australia and Such simulation is often supported by a fragmentation library[25] that contains published patterns of known decomposition reactions. Software taking advantage of this idea have been developed for both small molecules and proteins. Mass spectrometry software is any Software for data acquisition analysis or data representation in Mass spectrometry. Peptide mass fingerprinting (PMF (also known as protein fingerprinting) is an analytical technique for Protein identification that was developed in 1993 by several

Another way of interpreting mass spectra involves spectra with accurate mass. The mass recorded by a Mass spectrometer can refer to different physical quantities depending on the characteristics of the instrument and the manner in which the Mass An mass-to-charge ratio value (m/z) with only integer precision can represent an immense number of theoretically possible ion structures. More "accurate" (actually, "precise") mass figures significantly reduce the number of candidate molecular formulas, albeit each can still represent large number of structurally diverse compounds. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by A computer algorithm called formula generator calculates all molecular formulas that theoretically fit a given mass with specified tolerance. The mass recorded by a Mass spectrometer can refer to different physical quantities depending on the characteristics of the instrument and the manner in which the Mass

A recent technique for structure elucidation in mass spectrometry, called precursor ion fingerprinting identifies individual pieces of structural information by conducting a search of the tandem spectra of the molecule under investigation against a library of the product-ion spectra of structurally characterized precursor ions. Tandem mass spectrometry, also known as MS/MS, involves multiple steps of Mass spectrometry selection with some form of fragmentation occurring in between the stages Tandem mass spectrometry, also known as MS/MS, involves multiple steps of Mass spectrometry selection with some form of fragmentation occurring in between the stages

Applications

Isotope ratio MS: isotope dating and tracking

Mass spectrometer to determine the 16O/18O and 12C/13C isotope ratio on biogenous carbonate
Mass spectrometer to determine the 16O/18O and 12C/13C isotope ratio on biogenous carbonate
Main article: Isotope ratio mass spectrometry

Mass spectrometry is also used to determine the isotopic composition of elements within a sample. Isotope ratio mass spectrometry (IRMS is a specialization of Mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides 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, sometimes referred to as isotope ratio mass spectrometers (IR-MS), usually use a single magnet to bend a beam of ionized particles towards a series of Faraday cups which convert particle impacts to electric current. A Faraday cup is a Metal (conductive cup designed to catch charged particles in vacuum Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. A fast on-line analysis of deuterium content of water can be done using Flowing afterglow mass spectrometry, FA-MS. Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Flowing afterglow mass spectrometry, FA-MS is a sensitive and quantitative Mass spectrometry analytical approach that offers a route to on-line real-time Deuterium Probably the most sensitive and accurate mass spectrometer for this purpose is the accelerator mass spectrometer (AMS). Accelerator mass spectrometry (AMS differs from other forms of Mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass Isotope ratios are important markers of a variety of processes. Some isotope ratios are used to determine the age of materials for example as in carbon dating. Radiocarbon dating is a Radiometric dating method that uses the naturally occurring Radioisotope Carbon-14 (14C to determine the age of Labelling with stable isotopes is also used for protein quantification. (see Protein quantitation below)

Trace gas analysis

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 chemical ionization 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 calibration. Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample based on the Mass-to-charge ratio of charged particles Selected ion flow tube mass spectrometry is a sensitive and quantitative Mass spectrometry technique for trace gas analyses using chemical ionisation of sample trace gases by Chemical ionization ( CI) is an Ionization technique used in Mass spectrometry.

Atom probe

Main article: Atom probe

An atom probe is an instrument that combines time-of-flight mass spectrometry and field ion microscopy (FIM) to map the location of individual atoms. The atom probe is an atomic-resolution Microscope used in Materials science that was invented in 1967 by Erwin Müller, J The atom probe is an atomic-resolution Microscope used in Materials science that was invented in 1967 by Erwin Müller, J The time of flight (TOF describes the method used to measure the time that it takes for a particle object or stream to reach a detector while traveling over a known distance Field ion microscopy (FIM is an analytical technique used in Materials science.

Pharmacokinetics

Main article: Pharmacokinetics

Pharmacokinetics is often studied using mass spectrometry because of the complex nature of the matrix (often blood or urine) and the need for high sensitivity to observe low dose and long time point data. Pharmacokinetics (in Greek: “pharmacon” meaning drug and “kinetikos” meaning putting in motion the study of time dependency sometimes abbreviated as “PK” is a The most common instrumentation used in this application is LC-MS with a triple quadrupole mass spectrometer. Liquid chromatography-mass spectrometry ( LC-MS) is an Analytical chemistry technique that combines the physical separation capabilities of liquid chromatography The quadrupole mass analyzer is one type of mass analyzer used in Mass spectrometry. Tandem mass spectrometry is usually employed for added specificity. Standard curves and internal standards are used for quantitation of usually a single pharmaceutical in the samples. The samples represent different time points as a pharmaceutical is administered and then metabolized or cleared from the body. Blank or t=0 samples taken before administration are important in determining background and insuring data integrity with such complex sample matrices. Much attention is paid to the linearity of the standard curve; however it is not uncommon to use curve fitting with more complex functions such as quadratics since the response of most mass spectrometers is less than linear across large concentration ranges. [26][27][28]

There is currently considerable interest in the use of very high sensitivity mass spectrometry for microdosing studies, which are seen as a promising alternative to animal experimentation. Microdosing is a technique for studying the behaviour of compounds In vivo through the administration of doses so low they are unlikely to produce whole-body effects but high Animal testing or animal research is the use of non-human Animals in scientific experimentation.

Protein characterization

Main article: Protein mass spectrometry

Mass spectrometry is an important emerging method for the characterization of proteins. Protein mass spectrometry refers to the application of Mass spectrometry to the study of Proteins. The two primary methods for ionization of whole proteins are electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). Electrospray ionization (ESI is a technique used in Mass spectrometry to produce ions Matrix-assisted laser desorption/ionization (MALDI is a soft Ionization technique used in Mass spectrometry, allowing the analysis of Biomolecules ( In keeping with the performance and mass range of available mass spectrometers, two approaches are used for characterizing proteins. In the first, intact proteins are ionized by either of the two techniques described above, and then introduced to a mass analyser. This approach is referred to as "top-down" strategy of protein analysis. Top-down proteomics is a method of Protein identification that uses an Ion trapping Mass spectrometer to store an isolated protein ion for mass measurement In the second, proteins are enzymatically digested into smaller peptides using proteases such as trypsin or pepsin, either in solution or in gel after electrophoretic separation. Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino A protease is any Enzyme that conducts Proteolysis, that is begins protein Catabolism by Hydrolysis of the Peptide bonds that link Trypsin ( is a Serine protease found in the Digestive system, where it breaks down Proteins Trypsin predominantly cleaves peptide chains at the carboxyl Pepsin is a digestive Protease ( released by the chief cells in the Stomach that functions to degrade food Proteins into In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances The in-gel digestion is part of the sample preparation for the mass spectrometric identification of Proteins in course of proteomic analysis. Electrophoresis is the most well-known electrokinetic phenomenon. Other proteolytic agents are also used. The collection of peptide products are then introduced to the mass analyser. When the characteristic pattern of peptides is used for the identification of the protein the method is called peptide mass fingerprinting (PMF), if the identification is performed using the sequence data determined in tandem MS analysis it is called de novo sequencing. Peptide mass fingerprinting (PMF (also known as protein fingerprinting) is an analytical technique for Protein identification that was developed in 1993 by several Tandem mass spectrometry, also known as MS/MS, involves multiple steps of Mass spectrometry selection with some form of fragmentation occurring in between the stages These procedures of protein analysis are also referred to as the "bottom-up" approach.

Space exploration

As a standard method for analysis, mass spectrometers have reached other planets and moons. Two were taken to Mars by the Viking program. NASA 's Viking program consisted of a pair of space probes sent to Mars, Viking 1 and Viking 2. In early 2005 the Cassini-Huygens mission delivered a specialized GC-MS instrument aboard the Huygens probe through the atmosphere of Titan, the largest moon of the planet Saturn. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its Gas chromatography-mass spectrometry ( GC-MS) is a method that combines the features of Gas-liquid chromatography and Mass spectrometry to identify different The Huygens probe, supplied by the European Space Agency (ESA and named after the Dutch 17th century astronomer Christiaan Huygens, is an atmospheric entry TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as This instrument analyzed atmospheric samples along its descent trajectory and was able to vaporize and analyze samples of Titan's frozen, hydrocarbon covered surface once the probe had landed. These measurements compare the abundance of isotope(s) of each particle comparatively to earth's natural abundance. [29]

Mass spectrometers are also widely used in space missions to measure the composition of plasmas. For example, the Cassini spacecraft carries the Cassini Plasma Spectrometer (CAPS),[30] which measures the mass of ions in Saturn's magnetosphere. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object.

Respired gas monitor

Mass spectrometers were used in hospitals for respiratory gas analysis beginning around 1975 through the end of the century. Some are likely still in use but none are currently being manufactured. [31]

Found mostly in the operating room, they were a part of a complex system in which respired gas samples from patients undergoing anesthesia were drawn into the instrument through a valve mechanism designed to sequentially connect up to 32 rooms to the mass spectrometer. An operating theater (or theatre) was a tiered Theater or Amphitheater in which students and other spectators could watch surgeons perform surgery Anesthesia, or anaesthesia (see spelling differences; from Greek grc αν- an-, "without" and grc αἲσθησις A computer directed all operations of the system. The data collected from the mass spectrometer was delivered to the individual rooms for the anesthesiologist to use.

This magnetic sector mass spectrometer's uniqueness may have been the fact that a plane of detectors, each purposely positioned to collect all of the ion species expected to be in the samples, allowed the instrument to simultaneously report all of the patient respired gases. Although the mass range was limited to slightly over 120 u, fragmentation of some of the heavier molecules negated the need for a higher detection limit. The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express [32]

See also

Manufacturers

References

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Bibliography

External links

Professional organizations

Dictionary

mass spectrometry

-noun

  1. (physics, analytical chemistry) An analytical technique that measures the mass / charge ratio of the ions formed when a molecule or atom is ionized, vaporized and introduced into a vacuum. Mass spectrometry may also involve breaking molecules into fragments - thus enabling its structure to be determined.
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