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The magnitude of the potential well inthe z-direction may be estimated from the approximationthat Dz ³ qz V/8 thus, for our example of m/z 134 atqz D 0.450, Dz ³ 0.45 ð 757 V/8 D 43 V.10 MASS SPECTROMETRIC OPERATION OFTHE QUADRUPOLE ION TRAPA schematic diagram of the QITMS is shown in Figure 11.This figure does not show the end of the gas chromatograph capillary column from which elutes the sampleof interest in the helium carrier gas.
The gate controlin Figure 11 permits the passage of a 200-µs burst ofelectrons from the filament for automatic gain control(AGC). The fundamental rf voltage is ramped rapidlyto eject ions formed during the brief burst of electrons.Under AGC, the instrument calculates the duration ofionization appropriate to the stipulated ion count and theprevailing partial pressure of sample in the ion trap.
Allforms of resonant excitation are effected with the supplementary AC voltage applied to the end-cap electrodes asshown in Figure 11. When this supplementary AC voltageis used for axial modulation, ions are ejected in order ofmass/charge ratio, impinge upon the electron multiplierdetector, and produce a signal after preamplification. All9.6 Mass ResolutionThe normal mass scan rate of Finnigan and Varian iontraps prior to 1995 was 5555 Da s 1 and over the normalmass range of 10 – 650 Da, peak width was maintainedat ca. 0.5 Da.
Mass resolution is defined as the ratio ofthe mass of an ion to the peak width at half height;hence mass resolution increases with mass. However thisstatement is somewhat misleading as can be seen from thefollowing example; mass resolution for m/z 65 is ca. 130while that for m/z 650 is 1300. The real performance ofthe instrument is gauged from the width of the ion signalsproduced upon ion ejection. Upon reducing the scan rate,it was observed.20/ that the peak width was reduced so thatFigure 11 Schematic diagram of the QITMS.14of the voltages applied to the ion trap are controlled bythe scan acquisition processor.Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin T4 CDD)is used for illustrating the use of the ion trap as aMS; dioxin is an excellent example of the applicationof ion trap MS to analytical chemistry.
Compoundssuch as dioxin are analyzed from complex mixtureswhere they are present in trace amounts and, undersuch circumstances, gas chromatography (GC) is acommon method for sample introduction; in this example,ca. 400 pg of dioxin dissolved in 2 µL of n-nonane is used.The pressure of helium in the ion trap is set at about10 3 Torr to provide adequate cooling of ions formed;while helium can be used directly from a cylinder, thehelium carrier gas flowing through a chromatographprovides an adequate background pressure in the ion trap.Dioxin molecules elute directly into the ion trap wherethey are bombarded with electrons of 50 – 80 eV emittedfrom a heated filament and gated into the ion trap,as shown in Figure 11. The duration of ionization isdetermined by AGC; the ion number formed duringan ionization burst is used to scale the ionization time inorder to produce the required number of ions.
The ionsthus formed come immediately under the influence of thetrapping potential within the ion trap. During ionization,the ring electrode is driven at an initial rf voltage V0 anda fixed frequency (f ³ 1 MHz in many instruments) sothat all ions in a given range of mass/charge ratio aretrapped within the imposed quadrupole field. The valueof the initial rf voltage V0 imposes a LMCO, usually in therange m/z 20 – 50, so that ions of lower mass/charge ratioare not stored.
No DC potential is applied between thering and end-cap electrodes (U D 0) so that the confiningfield is purely oscillatory. During and after ionization,ions follow Lissajous-type trajectories and are subjectedsimultaneously to about 20 000 collisions per second withhelium; those ions that are not lost from the ion trapbecome focused near the trap center. The rf amplitude isramped during the analytical scan and as the axial secularfrequency of ion motion comes into resonance with theaxial modulation frequency, mass-selective ion ejectionand mass analysis occur.Each ion species confined within the ion trap isassociated with a qz value which is calculated accordingto Equation (26) and which lies on the qz -axis on thestability diagram; ions of relatively high mass/chargeratio have qz values near the origin while ions of lowermass/charge ratio have qz values which extend towardsthe bz D 1 stability boundary, as shown diagrammaticallyusing stick-people of various sizes in Figure 12(a).
Atthe intersection of the bz D 1 stability boundary and theqz -axis, where qz D 0.908 (see Figure 10), the trajectoriesof trapped ions become unstable axially such that ions ofmass/charge ratio less than the LMCO are not stored.MASS SPECTROMETRYFigure 12 (a) Schematic representation of working points (thatis, coordinates in az , qz space) in the stability diagram forseveral species of ions stored concurrently. The arrangement ofworking points with respect to mass/charge ratio is depicted byfigures which differ in size. (b) Ions are shown residing near thebottom of their respective axial potential wells of depth Dz ; theladder represents the opportunity for resonant ejection of anion species.Once the ion cloud within the ion trap has been focusedcollisionally to the center of the ion trap over a period ofsome 1 – 30 ms, the amplitude of the rf potential is ramped.The ramping of the rf potential amplitude, which isdescribed as an analytical ramp or analytical scan, causesthe qz values of all ion species to increase throughoutthe ramp.
As the qz value for each ion species reachesa value of 0.908, the ions are ejected axially through theend-cap electrodes. This method of ion ejection, whichcan occur only at a boundary of the stability diagram andis referred to as mass-selective axial instability, has beensupplanted by the use of axial modulation. Originally,axial modulation was the name given to resonant ejectionof ions at a frequency of 485 kHz and at a qz value onlyslightly less than 0.908.When the rf amplitude is ramped, ions come intoresonance at 485 kHz as their qz values approach 0.908; inthis manner, ions are ejected axially in order of increasingmass/charge ratio. As ions are focused into a region at thecenter of the ion trap, they are arranged in concentriclayers rather in the fashion of an onion. Resonanceejection affects first the ions of low mass/charge ratioresiding in the innermost layer of the onion by removingthem axially from ions of higher mass/charge ratio. Inthis manner, the influence of space charge perturbationsinduced by the ion of higher mass/charge ratio is sharply15QUADRUPOLE ION TRAP MASS SPECTROMETER13C-labeled 13 C12 -2,3,7,8-T4 CDD, MCž of m/z 332, addedto the sample injected.reduced so that the ions of low mass/charge ratio areejected free of space charge and with enhanced massresolution.
An additional advantage of resonant ejectionis that it can be carried out at any frequency. Resonantejection is depicted pictorially in Figure 12(b) where ionsare shown residing near the bottom of their respectiveaxial potential wells of depth Dz ; the ladder representsthe opportunity for resonant ejection of an ion speciesat any frequency. For axial modulation, the ladder ispositioned at a qz value just a little less than 0.908.Resonantly ejected ions pass through holes in the endcap electrodes so that only half of them impinge uponan electron multiplier located behind one of the end-capelectrodes; ion signals are created which produce a massspectrum in order of increasing mass/charge ratio. Thequadrupole ion trap functions as a mass spectrometerwhen operated in this manner.A mass spectrum is obtained by running the scanfunction (see below) a number of times specified bythe number of microscans; the signals which composethe mass spectrum are the result of averaging thoseobtained from each microscan.
In the following exampleof obtaining a mass spectrum, one mass spectral filewas generated each second. A mass spectrum of 2,3,7,8T4 CDD obtained in this manner is shown in Figure 13.MCž is of m/z 320 and the [M C 2]Cž ion of the molecularcluster which is shown at m/z 322 corresponds to theinclusion of one 37 Cl atom; the ion of m/z 259 is aprimary fragment ion and is due to the loss of COClžfrom [M C 2]Cž and of CO37 Clž from [M C 4]Cž . The peakat m/z 334 is [M C 2]Cž from a relatively small amount of10.1 Scan FunctionThe sequence of events described above can be expressedsuccinctly in a scan function which shows the temporalvariation of all of the potentials applied to the ion trapelectrodes. A scan function is a visual representation ofthe sequence of program segments in the software thatcontrols ion trap operation.
The scan function for themass spectrometric operation of the ion trap describedabove is shown in Figure 14.10.2 Collision-induced DissociationCID of an isolated ion species in a quadrupole ion trap hasbecome a powerful technique for both the determinationof ion structures and the analytical determination of,for example, compounds of environmental interest suchas dioxin. Before we explore each of these uses, let usconsider the CID process, how ion dissociation is effected,the efficiency of the process, and the duration.While CID is effected by resonant excitation of aselected ion species in the ion trap, the process is mademore complex since the frequencies of ion motion changeslightly as an ion moves away from the ion trap center;the values of the frequencies of ion motion are dependentalso on space charge, that is, on the number and natureof other ions present in the ion trap.
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