Ion Trap Mass Spectrometry - Current Separations

1 mass Spectrometry , the science andtechnology of gaseous ions (1), hasas its basis the measurement ofmass-to-charge ratios (m/z) of atomic and molecular ions are,in principle, accessible by massspectrometry, making it a universalmethod for chemical analysis. Itsimplementation requires suitablemethods of ion generation, ionanalysis, and ion detection. We treateach of these processes in turn andshow below that there are multiplemethods of accomplishing first step in recording amass spectrum is to convert analytemolecules (or atoms) into gas phaseions. In biological applications, themost common ionization tech-niques are electrospray ionization(ES) (2), atmospheric pressurechemical ionization (APCI) (3,4),and matrix-assisted laser desorptionionization (MALDI) (5). These aresoft ionization methods in the sensethat at least some analyte moleculesare converted, intact, into corre-sponding ions.

2 Solution phase sam-ples are examined with ES andAPCI while MALDI is particularlyappropriate for solid phase successfully generated gasphase ions, they must then be massanalyzed. There are several differ-ent types of mass analyzers, allbased on the interactions of chargedparticles with electric and/ or mag-netic themost common mass analyzers andlists some analytical performancecharacteristics by which they canbe compared (6). As with the vari-ous ionization methods, there is nosingle right choice the nature ofthe problem and the resources ofthe laboratory will dictate whichmass analyzer is most uses of mass spectrome-try are made in combination withchromatographic separation, princi-pally in the form of the GC/MS orLC/MS technique. These combina-tions have been used, for example,in organic analysis in the environ-mental sciences and in charac-terization of biological compounds,including molecular weight (MW)determinations, and sequenceanalyses of biopolymers (7).

3 In-creasingly important applicationshave been found in drug metabo-lism and protein sequencing due tothe high sensitivity and chemicalspecificity of mass advantages apply even whenthe samples are presented to themass spectrometer as mixturessince the two-stage tandem massspectrometry (MS/MS) experimentserves as a method of separation aswell as characterization of the sepa-rated and Ion TrapsWith the above background onionization and mass analysis, wecan now introduce a family of massanalyzers whose operation is basedon ion motion in rf electric quadrupole mass filter (8), orlinear quadrupole, consists of a lin-ear array of four symmetrically ar-ranged rods (F1) to which rf and dcvoltages are supplied. Forces areexerted in a plane normal to the di-rection (z-direction) in which theions drift through the array in theirjourney from the ion source to thedetector.

4 The rf potential gives riseto a field which alternatively rein-forces and then dominates the dcfield, also applied by coupling op-posite sets of rods. Ions oscillate inthe x,y-plane with frequencieswhich depend on their m/z valuesand with excursions which dependon the amplitudes of the appliedpotentials and their initial the oscillations of an ion in thisplane are stable, the ion will con-tinue to drift down the rod assem-bly and reach the detector. Stableoscillations are only achieved byions of given m/z values for a givenrod assembly, oscillation frequency,rf voltages, and dc voltage. Therange of values of m/z which corre-spond to stable motion can be madeIon Trap mass SpectrometryPhilip WongBioanalytical SystemsWest Lafayette, IN47906-1382R. Graham Cooks*Department of ChemistryPurdue UniversityWest Lafayette, IN47907*Corresponding operating principles of linear quadrupoles and quadrupole ion trapsare described, and the performance characteristics of triple quadrupolesand ion trap instruments are compared.

5 The theoretical basis for massanalysis using quadrupole fields is also described. The highperformance of quadrupole ion traps is illustrated by introducing somenew developments, including mass range extension, high resolutionexperiments, MSnexperiments, selective ion manipulation techniques,and non-destructive ion large (wide band pass) or itcan be a single m/z value (narrowband pass). In practice, ions of aparticular m/z value are often se-lected, and mass scanning is usuallyachieved by sweeping the dc and rfvoltages, keeping their ratio and theoscillator frequency quadrupole ion trap ,(9,10), the subject of this article, isthe three dimensional analogue ofthe linear quadrupole mass filter. Inthis device too, ions are subjected toforces applied by an rf field but theforces occur in all three, instead ofjust two, dimensions. Stable motionof ions in the linear quadrupole al-lowed ions freedom of motion inone dimension (z-direction); in theion trap, stable motion allows nodegrees of freedom.

6 Hence, ions aretrapped within the system of threeelectrodes-a ring electrode and twoend-cap electrodes of hyperboliccross-section (F2). The principaladvantages of the quadrupole iontrap in chemical analysis can besummarized as follows:(i) high sensitivity,(ii) compactness and mechanicalsimplicity in a device which isnevertheless capable of highperformance,(iii) tandem mass Spectrometry ex-periments are available by per-forming sequential mass analy-sis measurements,(iv) ion/molecule reactions can bestudied for mass -selected ions,(v) high resolution (>106at m/z>1000) is accessible throughslow scans, but mass measure-ment accuracy is relatively poor,MethodQuantityMeasuredMass/charge(m /z) rangeResolution atm/z = 1,000 DynamicRangeSector Magnetmomentum/charge104105107 Time of Flightflight time106103-104104 Ion CyclotronResonancecyclotronfrequency1051 06104 Ion Trapfrequency104104104 Quadrupolemass filterfiltersfor m/z103-104103-104105T1 Characteristics of Differ-ent mass Analyzers.

7 (Adapted fromreference 6.)Ion SourceDetectorZYXQ uadrupole mass FilterF1 Schematic diagramshowing the operationof the quadrupole massfilter. Note that as ionsdrift through the arrayof rods they are sub-jected to forces whichcause oscillation in thex,y-plane (shaded).Ion SourceIons inEndcapEndcapRingIons outDetectorEndcapEndcapCeramic postand spacerRingZoroF2 The ion trap consists ofthree electrodes with hy-perbolic surfaces, thecentral ring electrode,and two adjacent end-cap electrodes. Theschematic of the assem-bly shows how the elec-trodes are aligned andisolated using ceramicspacers and posts. Thedevice is radially sym-metrical, and roand zorepresent its size.(vi) ions of high mass /charge areaccessible using resonance ex-periments, and(vii) non-destructive detection isavailable using Fourier trans-form of the Ion Trapwith the Triple QuadrupoleThe differences in operatingprinciples of the linear quadrupoleand the ion trap have just been de-scribed.

8 In comparing their per-formance characteristics, one im-mediately notes that a unique fea-ture of an ion trap is that MS/MSexperiments are possible. Evenwhen compared with a triple quad-rupole MS/MS instrument, the iontrap can perform multiple stagemass Spectrometry (MSn) simplyby the use of additional operationswhich are performed sequentiallyin time. The triple quadrupole hasthe advantage of access to parention and neutral loss scans, and ana-lytically useful versions of theMS/MS experiment; however, MSnexperiments can only be performedin multi-quadrupole ion/molecule reactionscan be studied in both instruments,the reaction time can only be variedin the ion trap . This allows the ki-netics and equilibrium of ion-mole-cule reactions to be studied. On theother hand, the triple-quadrupoleinstrument provides good controlover the kinetic energies of the ionsF=f(x)E=f(x)Field = E = -2 d dxXF3 Potential and fieldstrength in a hypotheticalone-dimensional quadru-pole field.

9 The slope ofthe plot of potential ( )against position (x) yieldsthe field strength E(x).VV(A)(B)roro-Zo-ZoZoZo-ro-roAxial DimensionAxial DimensionRadial DimensionRadial DimensionF4 Potential used for trap-ping ions (A) in the radialdirection and (B) in the ax-ial direction. An ion in theposition shown is acceler-ated away from the trapcenter in the axial direc-tion at the rf phaseshown in (A) and towardsit in (B) (Adapted from ref-erence 13).which are important for thermo-chemical recent, instructive compari-son of the Finnigan LCQ ion trapwith the Finnigan TSQ 700 triplequadrupole mass spectrometer wasmade using an LC/APCI/MS assayfor several spinosyns (11). Theoverall sensitivity of the LCQ in thefull-scan mode was found to be 5-10 times greater than the TSQ. Incontrast, in the selected ion moni-toring mode, in which a single ionis monitored, the TSQ was found tobe 3-5 times more sensitive than theLCQ.

10 Similar results were obtainedin a comparative study of the LCQion trap and the PE/Sciex API 300triple quadrupole instrument usingLC/MS/MS quantitation of orlistatin human plasma (12). Clearly, bothinstruments have unique the small size, relatively lowcost, modest pressure requirements,and experimental flexibility of thequadrupole ion trap , an increasingnumber of analyses will be per-formed with ion traps coupled withion sources (ES or APCI) which al-low solution Principle ofQuadrupole Ion TrapsQuadrupole FieldsA quadrupole field is one inwhich the field strength E varieslinearly with displacement x,The applied potential which es-tablishes the electric field must varyquadratically in order that the fieldstrength vary linearly with x. HenceandIf it were possible to employ asystem of electrodes and construct aone-dimensional quadrupole field,then the potential distribution andthe field strength would be asshown inF3.