quadr-iontrap1 (1248334), страница 2
Текст из файла (страница 2)
Because of the cylindricalsymmetry of the ion trap, the x- and y-components ofthe field are often combined to yield a single radialr-component using x2 C y2 D r2 . As the rf potential oscillates sinusoidally with a frequency of the order of 1 MHz,the field is periodically reversed; this situation can beenvisaged as Figure 3 inverted with, say, 1000 V0 papplied to the ring electrode and the end-cap electrodesgrounded. The ion in Figure 3 rolls down the slope andapproaches the ion trap center radially, that is, the ionis being focused towards the center; the ion may continue to roll downhill and moves axially away from the3Figure 2 Quadrupole ion trap; (a) photograph of an ion trapcut in half along the axis of cylindrical symmetry; (b) schematicdiagram of the three-dimensional ideal ion trap showing theasymptotes and the dimensions r0 and z0 .center, that is, the ion is being defocused.
Upon reversal of the field in Figure 3, the ion is defocused radiallyand focused axially. When the focusing and defocusingforces are balanced by the imposition of appropriate conditions, the ion can remain confined on the surface as4MASS SPECTROMETRYFigure 5 Apparatus for producing Lissajous figures.Figure 3 Instantaneous pure quadrupole field, or potentialsurface, for a quadrupole ion trap. Note the four poles ofthe surface and the similarity of the field shape to the trajectoryin Figure 6.
The open circle represents an ion on the potentialsurface.though it were trapped in a pseudo-potential well. Thus, atrapping pseudo-potential well, parabolic in cross-section,can be created when an rf potential is applied to the ringelectrode and the two end-cap electrodes are grounded.The trapping pseudo-potential well is shown inFigure 4; the potential well in the axial direction is ofdepth Dz while in the radial direction the depth is Dr . Thevalue of Dz is about twice that of Dr so that the potentialwell resembles more a flower vase than it does a bowl. Thedepth of the trapping pseudo-potential well represents ameasure of the kinetic energy that is required in orderfor an ion to escape the well; for an ion of m/z 200, underthe conditions of Figure 3, the kinetic energy required isca. 58 eV.In the ion trap, the trajectory or path of an ion ischaracterized by two frequencies, or secular frequencies,Figure 4 Representation of the parabolic trapping potentialwells of depths Dz and Dr .that arise from the axial and radial motions of the ioninduced by the trapping field.
At a given instant of time,an ion is focused axially towards the center of the iontrap while it is simultaneously defocused radially so thatit moves away from the ion trap center; some half amicrosecond later the situation is reversed, and so on. Theresultant trajectory from these axial and radial oscillationsresembles a figure-of-eight; such a figure is known as aLissajous figure. A Lissajous figure can be demonstratedreadily with a light beam reflected in turn from each oftwo mirrors, M1 and M2 attached respectively to twotuning forks, F1 and F2 , arranged orthogonally as shownin Figure 5.
In the case of the ion trajectory, there isa high-frequency ripple superimposed on the Lissajousfigure; the ripple is due to the rf potential that oscillatesat 1 MHz.In Figure 6(a) is shown the trajectory of a single ionin an ion trap; an enlargement of this trajectory is shownin Figure 6(b) where the Lissajous-type motion and thehigh-frequency ripple are seen clearly. The projection ofthe trajectory onto the x – y plane forms a straight line asshown in Figure 6(b); this line shows that the ion motionis restricted to a plane.Mass-selective ejection of ions from the potential wellwithin the ion trap is accomplished by ramping in alinear fashion the amplitude of a rf potential appliedto one of the ion trap electrodes; each ion species isejected from the potential well at a specific rf amplitudeand, because the initial amplitude and ramping rate areknown, the mass/charge ratio can be determined for eachion species upon ejection.
This relatively simple methodfor measuring the mass/charge ratios of confined ionsupon ejection was developed by Stafford et al..2/ andis known as the ‘‘mass-selective axial instability mode’’;this method made possible the commercialization of thequadrupole ion trap around 1984. A prerequisite of thismethod of mass-selective ion ejection is that ions beherded initially to the center of the ion trap under theaction of momentum-dissipating collisions; helium atomsare used for this purpose.5QUADRUPOLE ION TRAP MASS SPECTROMETER300−2202y-axis (mm)33)(mm−2axis−5−3(b)−3−3x-z -axis (mm)5Figure 6 (a) Trajectory of a trapped ion of m/z 105 in aquadrupole ion trap.
(b) Expanded view of the ion trajectoryshowing the magnitude of the trajectory. The projection of thetrajectory onto the x – y plane forms a straight line and illustratesplanar motion in three-dimensional space. The trajectorydevelops a shape that resembles a flattened boomerang.
Takenfrom Nappi et al..1/published by Dawson and Whetten.5/ and by Dawson;.6/the latter publication has now been reprinted as part ofthe series of ‘‘American Vacuum Society Classics’’ by theAmerican Institute of Physics under ISBN 1563964554. Afull treatment of ion trap theory can be found in the nowstandard but somewhat dated text by March, Hughes andTodd;.7/ the historical account in this text by Todd hasbeen expanded into a full-scale review..8/ Other reviewson specific topics have been contributed by Cooks,et al..9,10/ and a special collection of papers reportingupon recent developments has appeared also..11/The field of quadrupole ion trap MS was reviewedextensively for the 12th International Mass Spectrometry Conference,.12/ held in Amsterdam in 1991.
Sincethat time, three volumes have been published in theCRC Series on Modern Mass Spectrometry..13/ Volume 1of this series covers the history of the quadrupole iontrap, nonlinear ion traps, ion activation, ion/moleculereactions and ion trajectory simulations; the reader isreferred to Chapter 2 of Volume 1 for a detailed exposition of the mathematical basis of the operation of the iontrap. Volume 2 deals with enhancement of ion trap performance, ion trap confinement of externally generatedions, ion structure differentiation, ion photodissociation,lasers and the ion trap, and ion traps in the study ofphysics. Volume 3 includes a review of fundamentals inaddition to extensive expositions on gas chromatography/ion trap tandem mass spectrometry (GC/MS/MS)and liquid chromatography/ion trap tandem mass spectrometry (LC/MS/MS); examples of applications in eachof the three areas above are given.
In all, the above threevolumes contain 30 chapters that originated from 18 of theleading ion trap research laboratories. An introductionto the quadrupole ion trap written by this author in atutorial form has appeared..14/4 TRAPPING PSEUDO-POTENTIAL WELL3 LITERATUREThe quadrupole ion trap and the related quadrupole massfilter were invented by Paul and Steinwedel.3/ in 1960 andtheir pioneering work was recognized by the award of a1989 Nobel Prize in Physics to Wolfgang Paul..4/ Yet thebasis of the theory of operation of quadrupole deviceswas laid down about 140 years ago by Mathieu to whosework we shall return.
In this presentation, references aregiven to those works that are regarded as landmarks inthe field of ion trap MS and to those publications thathave been used to illustrate specific modes of operationof the ion trap.Detailed accounts of the early development of thequadrupole-type devices as mass spectrometers wereIn a quadrupole ion trap, ions are focused by collisionswith helium buffer gas towards the center of the iontrap.
This focusing process imposes a degree of orderupon the ions much like that of a flock of sheep that hasbeen shepherded together by sheepdogs. Ions of lowestmass/charge ratio have the smallest excursions from thecenter of the ion trap so that they reside closest to the iontrap center; since these ions have the least kinetic energy,they populate the lowest region in the potential well. Theions of next higher mass/charge ratio are arranged in theion trap about the ions of lowest mass/charge ratio andpopulate the next higher region of the potential well, andso on. Under these conditions, trapped ion species arearranged about the ion trap center rather like the layers6MASS SPECTROMETRYof an onion with the ions of lowest mass/charge ratioresiding in the center of the onion. Mass-selective ionejection in order of increasing mass/charge ratio requiresthat ions of lowest mass/charge ratio be ejected first sothat the mass-selective ion ejection process is akin toremoving the layers of the onion, one by one, startingwith the layer nearest the onion center.When ions of lowest mass/charge ratio are extractedfrom the center of the onion, they experience space chargeperturbation while passing through the layers of ionsof higher mass/charge ratio, become spatially dispersedand, upon ejection, produce ion signals with poor massresolution.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
