Mass spectrometry
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Mass spectrometry (MS) is an analyticaltechnique that measures the mass-to-chargeratio of charged particles.[1]It is used for determining masses of particles, for determining theelemental composition of a sample or molecule, and forelucidating the chemical structures of molecules, such as peptidesand other chemicalcompounds. The MS principle consists of ionizing chemicalcompounds to generate charged molecules or molecule fragments andmeasurement of their mass-to-chargeratios.[1]In a typical MS procedure:
MS instruments consist of three modules:
The technique has both qualitativeand quantitativeuses. These include identifying unknown compounds, determining theisotopic composition ofelements in a molecule, and determining the structure of acompound by observing its fragmentation. Other uses includequantifying the amount of a compound in a sample or studying thefundamentals of gas phaseion chemistry (the chemistry of ions and neutrals in a vacuum).MS is now in very common use in analytical laboratories that studyphysical, chemical, or biological properties of a great variety ofcompounds.
edit] Etymology
The word spectrograph had become part of theinternationalscientific vocabulary by 1884.[2][3]The linguisticroots are a combination and removal of bound morphemesand free morphemeswhich relate to the terms spectr-um andphot-ograph-ic plate.[4]Early spectrometry devices that measured the mass-to-chargeratio of ions were called massspectrographs which consisted of instruments that recordeda spectrum of mass valueson a photographicplate.[5][6]A mass spectroscope is similar to a mass spectrographexcept that the beam of ions is directed onto a phosphorscreen.[7]A mass spectroscope configuration was used in early instrumentswhen it was desired that the effects of adjustments be quicklyobserved. Once the instrument was properly adjusted, a photographicplate was inserted and exposed. The term mass spectroscopecontinued to be used even though the direct illumination of aphosphor screen was replaced by indirect measurements with anoscilloscope.[8]The use of the term mass spectroscopy is now discouraged dueto the possibility of confusion with light spectroscopy.[1][1][9]Mass spectrometry is often abbreviated as mass-spec orsimply as MS.[1]Thomson has also noted that a mass spectroscope is similarto a mass spectrograph except that the beam of ions isdirected onto a phosphorscreen.[10]The suffix -scope here denotes thedirect viewing of the spectra (range) ofmasses.
[edit]History
For more details on this topic, see Historyof mass spectrometry.
Replica of an early mass spectrometer
In 1886, Eugen Goldsteinobserved rays in gas dischargesunder low pressure that traveled away from the anode and throughchannels in a perforated cathode, opposite to thedirection of negatively charged cathode rays (whichtravel from cathode to anode). Goldstein called these positivelycharged anode rays"Kanalstrahlen"; the standard translation of this term into Englishis "canal rays".Wilhelm Wien foundthat strong electric or magnetic fields deflected the canal raysand, in 1899, constructed a device with parallel electric andmagnetic fields that separated the positive rays according to theircharge-to-mass ratio (Q/m). Wien found that thecharge-to-mass ratio depended on the nature of the gas in thedischarge tube. English scientist J.J. Thomson laterimproved on the work of Wien by reducing the pressure to create amass spectrograph.
The first application of mass spectrometry to theanalysis of amino acids and peptides was reported in1958.[11]Carl-Ove Andersson highlighted the main fragment ions observed inthe ionization of methyl esters.[12]
Some of the modern techniques of mass spectrometrywere devised by ArthurJeffrey Dempster and F.W. Aston in 1918and 1919 respectively. In 1989, half of the Nobel Prizein Physics was awarded to Hans Dehmelt andWolfgang Paul forthe development of the ion trap technique in the 1950s and 1960s.In 2002, the Nobel Prizein Chemistry was awarded to John BennettFenn for the development of electrosprayionization (ESI) and Koichi Tanaka forthe development of soft laserdesorption (SLD) and their application to the ionization ofbiological macromolecules, especially proteins.[13]
[edit]Simplified example
Schematics of a simple mass spectrometer withsector type mass analyzer. This one is for the measurement ofcarbon dioxide isotope ratios (IRMS) as in the carbon-13 urea breathtest
The following example describes the operation of aspectrometer mass analyzer, which is of the sector type.(Other analyzer types are treated below.) Consider a sample ofsodium chloride(table salt). In the ion source, the sample is vaporized (turnedinto gas) and ionized(transformed into electrically charged particles) into sodium(Na+) and chloride(Cl-) ions. Sodium atoms and ions are monoisotopic, witha mass of about 23 amu. Chloride atoms and ions come in twoisotopes with masses of approximately 35 amu (at a naturalabundance of about 75 percent) and approximately 37 amu (at anatural abundance of about 25 percent). The analyzer part of thespectrometer contains electric andmagnetic fields,which exert forces on ions traveling through these fields. Thespeed of a charged particle may be increased or decreased whilepassing through the electric field, and its direction may bealtered by the magnetic field. The magnitude of the deflection ofthe moving ion's trajectory depends on its mass-to-charge ratio.Lighter ions get deflected by the magnetic force more than heavierions (based on Newton'ssecond law of motion, F = ma). The streams of sorted ions passfrom the analyzer to the detector, which records the relativeabundance of each ion type. This information is used to determinethe chemical element composition of the original sample (i.e. thatboth sodium and chlorine are present in the sample) and theisotopic composition of its constituents (the ratio of35Cl to 37Cl).
[edit]Creating ions
Main article: Ion source
The ion source is the part of the mass spectrometerthat ionizes the material under analysis (the analyte). The ionsare then transported by magnetic orelectric fieldsto the mass analyzer.
Techniques for ionization have been key todetermining what types of samples can be analyzed by massspectrometry. Electronionization and chemicalionization are used for gases and vapors. Inchemicalionization sources, the analyte is ionized by chemicalion-molecule reactions during collisions in the source. Twotechniques often used with liquid and solidbiological samples include electrosprayionization (invented by JohnFenn[14])and matrix-assisted laser desorption/ionization (MALDI, developedby K. Tanaka[15]and separately by M. Karas and F. Hillenkamp[16]).
Inductively coupledplasma (ICP) sources are used primarily for cation analysis ofa wide array of sample types. In this type of Ion SourceTechnology, a 'flame' of plasma that is electrically neutraloverall, but that has had a substantial fraction of its atomsionized by high temperature, is used to atomize introduced samplemolecules and to further strip the outer electrons from thoseatoms. The plasma is usually generated from argon gas, since thefirst ionization energy of argon atoms is higher than the first ofany other elements except He, O, F and Ne, but lower than thesecond ionization energy of all except the most electropositivemetals. The heating is achieved by a radio-frequency current passedthrough a coil surrounding the plasma.
Others include glow discharge,fielddesorption (FD), fast atombombardment (FAB), thermospray,desorption/ionizationon silicon (DIOS), Direct Analysis inReal Time (DART), atmospheric pressure chemical ionization (APCI), secondaryion mass spectrometry (SIMS), sparkionization and thermalionization (TIMS).[17]
Ion attachmentionization is an ionization technique that allows forfragmentation free analysis.
[edit]Mass selection
Mass analyzers separate the ions according to theirmass-to-chargeratio. The following two laws govern the dynamics of chargedparticles in electric and magnetic fields in vacuum:
(Newton's secondlaw of motion in non-relativistic case, i.e. valid only at ionvelocity much lower than the speed of light).
Here 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, andv x B is the vector crossproduct of the ion velocity and the magnetic field
Equating the above expressions for the forceapplied to the ion yields:
This differentialequation is the classic equation of motion for chargedparticles. Together with the particle's initial conditions, itcompletely determines the particle's motion in space and time interms of m/Q. Thus mass spectrometers could be thought of as"mass-to-charge spectrometers". When presenting data, it is commonto use the (officially) dimensionlessm/z, where z is the number of elementarycharges (e) on the ion (z=Q/e). This quantity, althoughit is informally called the mass-to-charge ratio, more accuratelyspeaking represents the ratio of the mass number and the chargenumber, z.
There are many types of mass analyzers, usingeither static or dynamic fields, and magnetic or electric fields,but all operate according to the above differential equation. Eachanalyzer type has its strengths and weaknesses. Many massspectrometers use two or more mass analyzers for tandem massspectrometry (MS/MS). In addition to the more common mass analyzerslisted below, there are others designed for special situations.
There are several important analyserscharacteristics. The mass resolvingpower is the measure of the ability to distinguish two peaks ofslightly different m/z. The mass accuracy is the ratio ofthe m/z measurement error to the true m/z. Usually measuredin ppm ormilli massunits. The mass range is the range of m/z amenable toanalysis by a given analyzer. The linear dynamic range is the rangeover which ion signal is linear with analyte concentration. Speedrefers to the time frame of the experiment and ultimately is usedto determine the number of spectra per unit time that can begenerated.
[edit]Sector instruments
For more details on this topic, see sectorinstrument.
A sector field mass analyzer uses an electricand/or magnetic field to affect the path and/or velocity of thechargedparticles in some way. As shown above, sectorinstruments bend the trajectories of the ions as they passthrough the mass analyzer, according to their mass-to-chargeratios, deflecting the more charged and faster-moving, lighter ionsmore. The analyzer can be used to select a narrow range ofm/z or to scan through a range of m/z to catalog theions present.[18]
[edit]Time-of-flight
For more details on this topic, see time-of-flightmass spectrometry.
The time-of-flight(TOF) analyzer uses an electric field toaccelerate the ions through the same potential, and thenmeasures the time they take to reach the detector. If the particlesall have the same charge, thekinetic energieswill be identical, and their velocities will dependonly on their masses. Lighter ions willreach the detector first.[19]
[edit]Quadrupole mass filter
For more details on this topic, see Quadrupolemass analyzer.
Quadrupolemass analyzers use oscillating electrical fields to selectivelystabilize or destabilize the paths of ions passing through aradio frequency(RF) quadrupole fieldcreated between 4 parallel rods. Only the ions in a certain rangeof mass/charge ratio are passed through the system at any time, butchanges to the potentials on the rods allow a wide range of m/zvalues to be swept rapidly, either continuously or in a successionof discrete hops. A quadrupole mass analyzer acts as amass-selective filter and is closely related to the quadrupole iontrap, particularly the linear quadrupole ion trap except thatit is designed to pass the untrapped ions rather than collect thetrapped ones, and is for that reason referred to as a transmissionquadrupole. A common variation of the quadrupole is the triplequadrupole. Triple quadrupole mass spectrometers have threeconsecutive quadrupoles arranged in series to incoming ions. Thefirst quadrupole acts as a mass filter. The second quadrupole actsas a collision cell where selected ions are broken into fragments.The resulting fragments can once again be filtered by the thirdquadrupole or all be allowed to pass though to the detectoryielding an ms/ms fragmentation pattern.
[edit]Ion traps
[edit]Three-dimensional quadrupole ion trap
For more details on this topic, see quadrupole iontrap.
The quadrupole iontrap works on the same physical principles as the quadrupolemass analyzer, but the ions are trapped and sequentially ejected.Ions are trapped in a mainly quadrupole RF field, in a spacedefined by a ring electrode (usually connected to the main RFpotential) between two endcap electrodes (typically connected to DCor auxiliary AC potentials). The sample is ionized eitherinternally (e.g. with an electron or laser beam), or externally, inwhich case the ions are often introduced through an aperture in anendcap electrode.
There are many mass/charge separation and isolationmethods but the most commonly used is the mass instability mode inwhich the RF potential is ramped so that the orbit of ions with amass a > b are stable while ions with massb become unstable and are ejected on the z-axis ontoa detector. There are also non-destructive analysis methods.
Ions may also be ejected by the resonanceexcitation method, whereby a supplemental oscillatory excitationvoltage is applied to the endcap electrodes, and the trappingvoltage amplitude and/or excitation voltage frequency is varied tobring ions into a resonance condition in order of their mass/chargeratio.[20][21]
The cylindrical ion trap mass spectrometer is a derivative of thequadrupole ion trap mass spectrometer.
[edit]Linear quadrupole ion trap
A linearquadrupole ion trap is similar to a quadrupole ion trap, but ittraps ions in a two dimensional quadrupole field, instead of athree-dimensional quadrupole field as in a 3D quadrupole ion trap.Thermo Fisher's LTQ ("linear trap quadrupole") is an example of thelinear ion trap.[22]
A toroidal ion trap can be visualized as a linearquadrupole curved around and connected at the ends or as a crosssection of a 3D ion trap rotated on edge to form the toroid, donutshaped trap. The trap can store large volumes of ions bydistributing them throughout the ring-like trap structure. Thistoroidal shaped trap is a configuration that allows the increasedminiaturization of an ion trap mass analyzer. Additionally all ionsare stored in the same trapping field and ejected togethersimplifying detection that can be complicated with arrayconfigurations due to variations in detector alignment andmachining of the arrays.[23]
[edit]Orbitrap
For more details on this topic, see Orbitrap.
Very similar nonmagnetic FTMS has been performed,where ions are electrostaticallytrapped in an orbit around a central, spindle shaped electrode. Theelectrode confines the ions so that they both orbit around thecentral electrode and oscillate back and forth along the centralelectrode's long axis. This oscillation generates an image current in the detector plates which is recorded by theinstrument. The frequencies of these image currents depend on themass to charge ratios of the ions. Mass spectra are obtained byFouriertransformation of the recorded image currents.
Similar to Fourier transform ion cyclotron resonance mass spectrometers,Orbitraps have a high mass accuracy, high sensitivity and a gooddynamic range.[24]
[edit]Fourier transform ion cyclotron resonance
An FT-ICR mass spectrometer
For more details on this topic, see Fouriertransform mass spectrometry.
Fourier transform mass spectrometry (FTMS), or more preciselyFourier transform ion cyclotron resonance MS, measures mass bydetecting the image current produced by ions cyclotroning in thepresence of a magnetic field. Instead of measuring the deflectionof ions with a detector such as an electronmultiplier, the ions are injected into a Penning trap (astatic electric/magnetic ion trap) where theyeffectively form part of a circuit. Detectors at fixed positions inspace measure the electrical signal of ions which pass near themover time, producing a periodic signal. Since the frequency of anion's cycling is determined by its mass to charge ratio, this canbe deconvoluted byperforming a Fouriertransform on the signal. FTMS has the advantage ofhigh sensitivity (since each ion is "counted" more than once) andmuch higher resolutionand thus precision.[25][26]
Ioncyclotron resonance (ICR) is an older mass analysis techniquesimilar to FTMS except that ions are detected with a traditionaldetector. Ions trapped in a Penning trap areexcited by an RF electric field until they impact the wall of thetrap, where the detector is located. Ions of different mass areresolved according to impact time.
[edit]Detectors
A continuous dynode particle multiplierdetector.
The final element of the mass spectrometer is thedetector. The detector records either the charge induced or thecurrent produced when an ion passes by or hits a surface. In ascanning instrument, the signal produced in the detector during thecourse of the scan versus where the instrument is in the scan (atwhat m/Q) will produce a mass spectrum, arecord of ions as a function of m/Q.
Typically, some type of electronmultiplier is used, though other detectors including Faraday cups andion-to-photon detectors are also used. Because the number ofions leaving the mass analyzer at a particular instant is typicallyquite small, considerable amplification is often necessary to get asignal. Microchannelplate detectors are commonly used in modern commercialinstruments.[27]In FTMS and Orbitraps, the detectorconsists of a pair of metal surfaces within the mass analyzer/iontrap region which the ions only pass near as they oscillate. No DCcurrent is produced, only a weak AC image current is produced in acircuit between the electrodes. Other inductive detectors have alsobeen used.[28]
[edit]Tandem mass spectrometry
Main article: Tandemmass spectrometry
A tandem mass spectrometer is one capable ofmultiple rounds of mass spectrometry, usually separated by someform of molecule fragmentation. For example, one mass analyzer canisolate one peptide from manyentering a mass spectrometer. A second mass analyzer thenstabilizes the peptide ions while they collide with a gas, causingthem to fragment by collision-induceddissociation (CID). A third mass analyzer then sorts thefragments produced from the peptides. Tandem MS can also be done ina single mass analyzer over time, as in a quadrupole iontrap. There are various methods for fragmentingmolecules for tandem MS, including collision-induceddissociation (CID), electroncapture dissociation (ECD), electrontransfer dissociation (ETD), infraredmultiphoton dissociation (IRMPD), blackbody infrared radiative dissociation (BIRD), electron-detachmentdissociation (EDD) and surface-induceddissociation (SID). An important application using tandem massspectrometry is in protein identification.[29]
Tandem mass spectrometry enables a variety ofexperimental sequences. Many commercial mass spectrometers aredesigned to expedite the execution of such routine sequences asselectedreaction monitoring (SRM), multiplereaction monitoring (MRM), and precursor ion scan. In SRM, thefirst analyzer allows only a single mass through and the secondanalyzer monitors for a single user defined fragment ion. MRMallows for multiple user defined fragment ions. SRM and MRM aremost often used with scanning instruments where the second massanalysis event is duty cycle limited.These experiments are used to increase specificity of detection ofknown molecules, notably in pharmacokinetic studies. Precursor ionscan refers to monitoring for a specific loss from the precursorion. The first and second mass analyzers scan across the spectrumas partitioned by a user defined m/z value. This experimentis used to detect specific motifs within unknownmolecules.
Another type of tandem mass spectrometry used forradiocarbondating is AcceleratorMass Spectrometry (AMS), which uses very high voltages, usuallyin the mega-volt range, to accelerate negative ions into a type oftandem mass spectrometer.
[edit]Common mass spectrometer configurations and techniques
When a specific configuration of source, analyzer,and detector becomes conventional in practice, often a compoundacronymarises to designate it, and the compound acronym may be betterknown among nonspectrometrists than the component acronyms. Theepitome of this is MALDI-TOF, whichsimply refers to combining a matrix-assisted laser desorption/ionization source with atime-of-flightmass analyzer. The MALDI-TOF moniker is more widely recognized bythe non-mass spectrometrists than MALDI or TOF individually. Otherexamples include inductively coupledplasma-mass spectrometry (ICP-MS), acceleratormass spectrometry (AMS), Thermalionization-mass spectrometry (TIMS) and spark source massspectrometry (SSMS). Sometimes the use of the generic "MS"actually connotes a very specific mass analyzer and detectionsystem, as is the case with AMS, which is always sector based.
Certain applications of mass spectrometry havedeveloped monikers that although strictly speaking would seem torefer to a broad application, in practice have come instead toconnote a specific or a limited number of instrumentconfigurations. An example of this is isotoperatio mass spectrometry (IRMS), which refers in practice to theuse of a limited number of sector based mass analyzers; this nameis used to refer to both the application and the instrument usedfor the application.
[edit]Chromatographic techniques combined with mass spectrometry
An important enhancement to the mass resolving andmass determining capabilities of mass spectrometry is using it intandem with chromatographicseparation techniques.
[edit]Gas chromatography
A gas chromatograph (right) directly coupled to amass spectrometer (left)
See also: Gas chromatography-mass spectrometry
A common combination is gas chromatography-massspectrometry (GC/MS or GC-MS). In this technique, a gaschromatograph is used to separate different compounds. Thisstream of separated compounds is fed online into the ion source, a metallicfilament towhich voltage is applied. Thisfilament emits electrons which ionize the compounds. The ions canthen further fragment, yielding predictable patterns. Intact ionsand fragments pass into the mass spectrometer's analyzer and areeventually detected.[30]
[edit]Liquid chromatography
See also: Liquid chromatography-mass spectrometry
Similar to gas chromatography MS (GC/MS), liquidchromatography mass spectrometry (LC/MS or LC-MS) separatescompounds chromatographically before they are introduced to the ionsource and mass spectrometer. It differs from GC/MS in that themobile phase is liquid, usually a mixture of water and organicsolvents, instead of gasand the ions fragments cannot yield predictable patterns. Mostcommonly, an electrosprayionization source is used in LC/MS. There are also some newlydeveloped ionization techniques like laser spray.
[edit]Ion mobility
See also: Ion mobility spectrometry-mass spectrometry
Ionmobility spectrometry/mass spectrometry (IMS/MS or IMMS) is atechnique where ions are first separated by drift time through someneutral gas under an applied electrical potential gradient beforebeing introduced into a mass spectrometer.[31]Drift time is a measure of the radius relative to the charge of theion. The duty cycle of IMS(the time over which the experiment takes place) is longer thanmost mass spectrometric techniques, such that the mass spectrometercan sample along the course of the IMS separation. This producesdata about the IMS separation and the mass-to-charge ratio of theions in a manner similar to LC/MS.[32]
The duty cycle of IMS is short relative to liquidchromatography or gas chromatography separations and can thus becoupled to such techniques, producing triple modalities such asLC/IMS/MS.[33]
[edit]Data and analysis
Mass spectrum of a peptide showing the isotopicdistribution
[edit]Data representations
See also: Massspectrometry data format
Mass spectrometry produces various types of data.The most common data representation is the massspectrum.
Certain types of mass spectrometry data are bestrepresented as a masschromatogram. Types of chromatograms include selected ionmonitoring (SIM), total ion current (TIC), and selected reactionmonitoring chromatogram (SRM), among many others.
Other types of mass spectrometry data are wellrepresented as a three-dimensional contour map. In thisform, the mass-to-charge, m/z is on the x-axis,intensity the y-axis, and an additional experimentalparameter, such as time, is recorded on the z-axis.
[edit]Data analysis
Basics
Mass spectrometry data analysis is a complicatedsubject that is very specific to the type of experiment producingthe data. There are general subdivisions of data that arefundamental to understanding any data.
Many mass spectrometers work in either negativeion mode or positive ion mode. It is very important toknow whether the observed ions are negatively or positivelycharged. This is often important in determining the neutral massbut it also indicates something about the nature of themolecules.
Different types of ion source result in differentarrays of fragments produced from the original molecules. Anelectron ionization source produces many fragments and mostlysingle-charged (1-) radicals (odd number of electrons), whereas anelectrospray source usually produces non-radical quasimolecularions that are frequently multiply charged. Tandem mass spectrometrypurposely produces fragment ions post-source and can drasticallychange the sort of data achieved by an experiment.
By understanding the origin of a sample, certainexpectations can be assumed as to the component molecules of thesample and their fragmentations. A sample from asynthesis/manufacturing process will probably contain impuritieschemically related to the target component. A relatively crudelyprepared biological sample will probably contain a certain amountof salt, which may form adducts with the analytemolecules in certain analyses.
Results can also depend heavily on how the samplewas prepared and how it was run/introduced. An important example isthe issue of which matrix is used for MALDI spotting, since much ofthe energetics of the desorption/ionization event is controlled bythe matrix rather than the laser power. Sometimes samples arespiked with sodium or another ion-carrying species to produceadducts rather than a protonated species.
The greatest source of trouble when non-massspectrometrists try to conduct mass spectrometry on their own orcollaborate with a mass spectrometrist is inadequate definition ofthe research goal of the experiment. Adequate definition of theexperimental goal is a prerequisite for collecting the proper dataand successfully interpreting it. Among the determinations that canbe achieved with mass spectrometry are molecular mass, molecularstructure, and sample purity. Each of these questions requires adifferent experimental procedure. Simply asking for a "mass spec"will most likely not answer the real question at hand.
Interpretation of mass spectra
Main article: Massspectrum analysis
Since the precise structure orpeptidesequence of a molecule is deciphered through the set offragment masses, the interpretation of mass spectrarequires combined use of various techniques. Usually the firststrategy for identifying an unknown compound is to compare itsexperimental mass spectrum against a library of mass spectra. Ifthe search comes up empty, then manual interpretation[34]or softwareassisted interpretation of mass spectra are performed. Computersimulation of ionization andfragmentation processes occurring in mass spectrometer is theprimary tool for assigning structure or peptide sequence to amolecule. An apriori structural information is fragmented insilico and the resulting pattern is compared with observedspectrum. Such simulation is often supported by a fragmentationlibrary[35]that contains published patterns of known decomposition reactions.Softwaretaking advantage of this idea has been developed for both smallmolecules and proteins.
Another way of interpreting mass spectra involvesspectra with accurate mass. A mass-to-charge ratio value (m/z) withonly integer precision can represent an immense number oftheoretically possible ion structures. More precise mass figuressignificantly reduce the number of candidate molecularformulas, albeit each can still represent large number ofstructurally diverse compounds. A computer algorithm called formulagenerator calculates all molecular formulas that theoretically fita given mass with specified tolerance.
A recent technique for structure elucidation inmass spectrometry, called precursor ion fingerprinting identifies individual pieces ofstructural information by conducting a search of the tandemspectra of the molecule under investigation against a libraryof the product-ion spectra of structurally characterized precursorions.
[edit]Applications
[edit]Isotope ratio MS: isotope dating and tracking
Mass spectrometer to determine the16O/18O and 12C/13Cisotope ratio on biogenous carbonate
Main article: Isotoperatio mass spectrometry
Mass spectrometry is also used to determine theisotopic composition ofelements within a sample. Differences in mass among isotopes of anelement are very small, and the less abundant isotopes of anelement are typically very rare, so a very sensitive instrument isrequired. These instruments, sometimes referred to as isotope ratiomass spectrometers (IR-MS), usually use a single magnet to bend abeam of ionized particles towards a series of Faraday cups whichconvert particle impacts to electriccurrent. A fast on-line analysis of deuterium content ofwater can be done using Flowingafterglow mass spectrometry, FA-MS. Probably the most sensitiveand accurate mass spectrometer for this purpose is the acceleratormass spectrometer (AMS). Isotope ratios are important markersof a variety of processes. Some isotope ratios are used todetermine the age of materials for example as in carbon dating.Labeling with stable isotopes is also used for proteinquantification. (see protein characterization below)
[edit]Trace gas analysis
Several techniques use ions created in a dedicatedion 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 chemicalionization dedicated for trace gas analysis of air, breath orliquid headspace using well defined reaction time allowingcalculations of analyte concentrations from the known reactionkinetics without the need for internal standard or calibration.
[edit]Atom probe
Main article: Atom probe
An atom probe is aninstrument that combines time-of-flightmass spectrometry and field ionmicroscopy (FIM) to map the location of individual atoms.
[edit]Pharmacokinetics
Main article: Pharmacokinetics
Pharmacokinetics is often studied using massspectrometry because of the complex nature of the matrix (oftenblood or urine) and the need for high sensitivity to observe lowdose and long time point data. The most common instrumentation usedin this application is LC-MS with a triplequadrupole mass spectrometer. Tandem mass spectrometry isusually employed for added specificity. Standard curves andinternal standards are used for quantitation of usually a singlepharmaceutical in the samples. The samples represent different timepoints as a pharmaceutical is administered and then metabolized orcleared from the body. Blank or t=0 samples taken beforeadministration are important in determining background and ensuringdata integrity with such complex sample matrices. Much attention ispaid to the linearity of the standard curve; however it is notuncommon to use curve fitting withmore complex functions such as quadratics since the response ofmost mass spectrometers is less than linear across largeconcentration ranges.[36][37][38]
There is currently considerable interest in the useof very high sensitivity mass spectrometry for microdosing studies,which are seen as a promising alternative to animalexperimentation.
[edit]Protein characterization
Main article: Proteinmass spectrometry
Mass spectrometry is an important emerging methodfor the characterization and sequencing ofproteins. The two primary methods for ionization of whole proteinsare electrosprayionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). In keepingwith the performance and mass range of available massspectrometers, two approaches are used for characterizing proteins.In the first, intact proteins are ionized by either of the twotechniques described above, and then introduced to a mass analyzer.This approach is referred to as "top-down"strategy of protein analysis. In the second, proteins areenzymatically digested into smaller peptides usingproteases such astrypsin or pepsin,either in solution or in gel afterelectrophoreticseparation. Other proteolytic agents are also used. The collectionof peptide products are then introduced to the mass analyzer. Whenthe characteristic pattern of peptides is used for theidentification of the protein the method is called peptidemass fingerprinting (PMF), if the identification is performedusing the sequence data determined in tandemMS analysis it is called de novosequencing. These procedures of protein analysis are alsoreferred to as the "bottom-up"approach.
[edit]Glycan Analysis
Mass spectrometry (MS), with its low samplerequirement and high sensitivity, has been the predominantly usedin glycobiology forcharacterization and elucidation of glycanstructures.[39]Mass spectrometry provides a complementary method to HPLC for the analysis of glycans. Intact glycans may bedetected directly as singly charged ions by matrix-assisted laser desorption/ionization mass spectrometry(MALDI-MS) or, following permethylation or peracetylation, byfast atombombardment mass spectrometry (FAB-MS).[40]Electrospray ionization mass spectrometry (ESI-MS) also givesgood signals for the smaller glycans.[41]Various free and commercial software are now available which interpretMS data and aid in Glycan structure characterization.
[edit]Space exploration
As a standard method for analysis, massspectrometers have reached other planets and moons. Two were takento Mars by the Viking program.In early 2005 the Cassini-Huygensmission delivered a specialized GC-MS instrument aboardthe Huygens probethrough the atmosphere of Titan, the largestmoon of the planet Saturn. This instrumentanalyzed atmospheric samples along its descent trajectory and wasable to vaporize and analyze samples of Titan's frozen, hydrocarboncovered surface once the probe had landed. These measurementscompare the abundance of isotope(s) of each particle comparativelyto earth's natural abundance.[42]Also onboard the Cassini-Huygensspacecraft is an ion and neutral mass spectrometer which has beentaking measurements of Titan's atmospheric composition as well asthe composition of Enceladus'plumes. A Thermaland Evolved Gas Analyzer mass spectrometer was carried by theMars PhoenixLander launched in 2007.[43]
Mass spectrometers are also widely used in spacemissions to measure the composition of plasmas. For example, theCassini spacecraft carries the Cassini Plasma Spectrometer(CAPS),[44]which measures the mass of ions in Saturn's magnetosphere.
[edit]Respired gas monitor
Mass spectrometers were used in hospitals forrespiratory gas analysis beginning around 1975 through the end ofthe century. Some are probably still in use but none are currentlybeing manufactured.[45]
Found mostly in the operating room,they were a part of a complex system, in which respired gas samplesfrom patients undergoing anesthesia were drawninto the instrument through a valve mechanism designed tosequentially connect up to 32 rooms to the mass spectrometer. Acomputer directed all operations of the system. The data collectedfrom the mass spectrometer was delivered to the individual roomsfor the anesthesiologist to use.
The uniqueness of this magnetic sector massspectrometer may have been the fact that a plane of detectors, eachpurposely positioned to collect all of the ion species expected tobe in the samples, allowed the instrument to simultaneously reportall of the gases respired by the patient. Although the mass rangewas limited to slightly over 120 u,fragmentation of some of the heavier molecules negated the need fora higher detection limit.[46]
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