quadr-iontrap1 (1248334)
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Quadrupole Ion Trap Mass SpectrometerRaymond E. MarchinEncyclopedia of Analytical ChemistryR.A. Meyers (Ed.)pp. 11848–11872 John Wiley & Sons Ltd, Chichester, 20001QUADRUPOLE ION TRAP MASS SPECTROMETERQuadrupole Ion Trap MassSpectrometerRelated Articles23References24Raymond E. MarchTrent University, Peterborough, CanadaThe quadrupole ion trap is constructed of three electrodesthat, when held at appropriate potentials, cause theformation of a trapping pseudo-potential well so thatcharged particles, or gaseous ions, may be confinedor stored for long periods of time. The two end-capelectrodes resemble saucers while the ring electroderesembles a napkin ring; all of the electrodes are ofhyperbolic geometry. The ion trap itself functions asa mass spectrometer when the ion-confining conditionsare modified such that ions are ejected mass-selectivelyfrom the trapping potential well.
As ions of successivemass/charge ratios are ejected in turn from the ion trap,they impinge upon an external detector whereby ion signalsare created in proportion to the ion number of eachspecies; in this manner, a mass spectrum is generated.The QITMS (quadrupole ion trap mass spectrometer) isan extraordinary instrument in that it is physically small(the entire electrode assembly can be held in the palm ofone’s hand) compared with magnetic and electric sectorinstruments, it is relatively inexpensive, it is one of themost, if not the most, sensitive mass spectrometers and,since several mass-selective operations can be carried insuccession, the ion trap can function as a tandem massspectrometer.
Tandem mass spectrometric operation isdescribed as (MS)n . With the QITMS, (MS)n is carriedout in time in the same volume of space whereas (MS)nin sector instruments is carried out in space. With sectorinstruments, the maximum value of n is n D 4 yet withthe ion trap, (MS)n where n D 4 – 6 can be carried outroutinely and n D 13 has been achieved. The QITMSshares several similarities with the ion cyclotron resonancemass spectrometer yet the cost of the former is aboutone-tenth that of the latter.
One striking difference betweenthe QITMS and all other mass spectrometers is that theQITMS operates at a pressure of 10 3 Torr compared with10 6 – 10 9 Torr for other mass spectrometers.The theory of ion confinement and ion trajectorymanipulation in the QITMS has been explained relativelysimply so far. Since the theory differs widely from thoseof sector instruments and ion cyclotron resonance massspectrometry (ICR/MS), it will not be familiar to thosewho have not had the opportunity to examine ion motionin quadrupole fields. Optimum operation of the QITMSis effected by collisional focusing of the ion cloud tothe center of the ion trap under the influence of heliumbuffer gas. Since the movement of ions confined in theion trap is periodic, the trajectories of collisionally focusedions can be expanded by resonance excitation effectedby the imposition of supplementary radio frequency (rf)1 Introduction22 Structure of the Quadrupole Ion TrapMass Spectrometer235Literature4 Trapping Pseudo-potential Well55 Structure of the Quadrupole Ion Trap66 Theory of Quadrupole Ion TrapOperation6.1 An Ion in a Quadrupole Field6.2 The Mathieu Equation6.3 Potentials on the Electrodes6.4 Stretched Ion Trap677897 Regions of Ion Trajectory Stability7.1 Stability Region7.2 Secular Frequencies8 Resonant Excitation91010119 Calculations9.1 qz and Low-mass Cut-off9.2 bz9.3 wz9.4 Mass Range9.5 Mass Range Extension9.6 Mass Resolution9.7 Dz121212131313131310 Mass Spectrometric Operation of theQuadrupole Ion Trap10.1 Scan Function10.2 Collision-induced Dissociation13151511 Tandem Mass Spectrometry11.1 Tandem Mass Spectrometry11.2 Scan Functions11.3 Tandem Mass Spectrometry Determination of Eluting and Co-elutingCompounds11.4 Tandem Mass SpectrometricOperation1717182012 Chemical Ionization and Ion/MoleculeReactions2113 Conclusions23Acknowledgments23Abbreviations and Acronyms2319Encyclopedia of Analytical ChemistryR.A.
Meyers (Ed.) Copyright John Wiley & Sons Ltd2potentials of low amplitude to the end-cap electrodes ofthe ion trap. This excitation operation permits isolationof selected ion species, by ejection of unwanted ionspecies, followed by selective ion/molecule reaction orby collision-induced dissociation (CID) with subsequentmass analysis of the product/fragment ions formed. Samplecalculations are given of all of the relevant trappingparameters. Applications of the QITMS as a single stagemass spectrometer and as a tandem mass spectrometerare discussed.
The operation of the QITMS for theidentication of dioxins and furans co-eluting from a gaschromatograph is described. In addition, the applicationof chemical ionization (CI) for the identification ofco-eluting polychlorinated biphenyl (PCB) congeners isdiscussed. The QITMS is an extraordinary instrumentthat is capable of great sensitivity, high mass range andhigh mass resolution.
Since the QITMS is compatible withmethods for generating ions externally, such as electrosprayionization (ESI), its continued growth in many areas ofmass spectrometry (MS) is assured.1 INTRODUCTIONThe QITMS functions not only as a mass spectrometerof high sensitivity and high specificity but also as an ionstore in which gaseous ions can be confined for periodsof some hundreds of milliseconds; both functions areemployed in normal ion trap operation. While othermass spectrometers operate at pressures <10 6 Torr, theion trap operates at a pressure of 1 mTorr of heliumbuffer gas. As a storage device, the ion trap acts as an‘‘electric field test tube’’ for the confinement of gaseousions, either positively charged or negatively charged, inthe absence of solvent molecules.
The confining capacityof the ion trap arises from the formation of a trappingpotential well when appropriate potentials are applied tothe electrodes of the ion trap. As an ion storage device, theion trap permits the study of gas-phase ion chemistry andthe dissociation of ions for elucidation of ion structures.When stages of mass selectivity are employed repeatedly,the ion trap functions as a tandem mass spectrometer.The ion trap functions as a mass spectrometer when thefield within the device is changed, so that the trajectoriesof simultaneously trapped ions of consecutive specificmass/charge ratio (m/z) become sequentially unstable,and ions leave the trapping field in order of mass/chargeratio.
Upon ejection from the ion trap, ions strike adetector and provide an output signal. This relativelysimple method of mass-selective operation of the ion traphas led to a revolution in MS. It is estimated that some4500 ion trap instruments have been sold thus far at atotal cost of about a quarter of a billion US dollars; thevolume of these sales has led to a reorganization of theMASS SPECTROMETRYMS industry.
The combination of a quadrupole ion trapinterfaced with a gas chromatograph is now availablecommercially at a price which permits the acquisitionof these instruments by most academic departments ofchemistry; thus these instruments are now becomingaccessible to relatively large numbers of students, bothgraduate and undergraduate.With the advent of new methods by which ions canbe formed in the gas phase from polar as well ascovalent molecules and introduced subsequently into anion trap, the range of applications of the quadrupoleion trap is now considerable. The coupling of liquidchromatography (LC) with ESI and with MS in theearly 1980s, together with the rapid advancement inion trap technology, have led to the development ofnew ion trap instruments for the analysis of nonvolatile,polar, and thermally labile compounds.
In 1995, newion trap instruments (Finnigan’s LCQ and GCQ ,and Bruker-Franzen’s ESQUIRE ) were introducedwhich employ external ion sources with injection ofexternally generated ions into the ion trap. The majorfocus for the application of these new instruments,using LC/ESI/MS (liquid chromatography/electrosprayionization mass spectrometry), has been the analysisof high-molecular-weight biopolymers such as proteins,peptides, and oligodeoxyribonucleotides.2 STRUCTURE OF THE QUADRUPOLEION TRAP MASS SPECTROMETERThe quadrupole ion trap consists essentially of threeelectrodes which are shown in open array in Figure 1.
TwoFigure 1 Three electrodes of the quadrupole ion trap shown inopen array.QUADRUPOLE ION TRAP MASS SPECTROMETERof the three electrodes are virtually identical and, whilehaving hyperboloidal geometry, resemble small invertedsaucers; these saucers are called end-cap electrodes andin Figure 1 are distinguishable by the number of holes inthe center of each electrode. One end-cap electrode hasa single small central aperture through which electronsand/or ions can be gated periodically while the otherhas several small apertures arranged centrally also andthrough which ions pass to a detector; in ion trapinstruments with external ion sources, each end-capelectrode has a single perforation.
The third electrodeis also of hyperboloidal geometry (of two sheets, in thiscase, rather than one) and is called the ring electrode;it not only resembles a serviette or napkin holder but isof roughly the same size in that the radius in the centralplane, r0 , of the ring electrode is approximately 1 cm. Thering electrode is positioned symmetrically between thetwo end-cap electrodes as shown in Figure 2; Figure 2(a)shows a photograph of an ion trap cut in half along theaxis of cylindrical symmetry, while Figure 2(b) is a crosssection of an ideal ion trap showing the asymptotes andthe dimensions r0 and z0 , where 2z0 is the separation ofthe two end-cap electrodes measured along the axis ofthe ion trap.The electrodes in Figure 2 are necessarily truncatedfor practical purposes but, in theory, they extend toinfinity and meet the asymptotes shown in Figure 2.
Theasymptotes arise from the hyperboloidal geometries ofthe three electrodes and, in the ideal case, are inclinedat an angle of 53° 340 to the cylindrical axis (the z-axis)of the electrode arrangement. The geometries of theelectrodes have been chosen so as to produce a near idealquadrupole field when an rf potential is applied to the ringelectrode and the two end-cap electrodes are grounded.In a quadrupole field, the forces acting on an ion dependlinearly upon the displacement, x, y, or z from the iontrap center.In Figure 3 is shown a representation of the instantaneous saddle-shaped potential surface generated bythe application of an rf potential of, say, 1000 V0 p(zero-to-peak) to the ring electrode; the open circle represents an ion on this surface.
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