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Proton transfer CI, involves the transferto a neutral species of a proton from an ion which hasbeen formed in an ion/molecule reaction. Parenthetically,CI can be effected by the transfer of other even-electroncharged particles.Let us consider the CI of the co-eluting PCB congeners 77 and 110 shown in Scheme 1..24/ Congener 77,which is highly toxic, is a nonortho compound because themolecule is not chlorine-substituted in the ortho positionsvicinal to the phenyl – phenyl bond. On the other hand,congener 110 is a diortho compound and is much lesstoxic.
Since the concentration of toxic congener 77 in environmental samples is often <1% of that of congener 110,quantitative analysis of congener 77 is a challenge.Intensity (ion signal − background)100%The EI mass spectrum, averaged over the GC composite peak of the co-eluting congeners 77 and 110, is shownin Figure 20.
The base peak group in the mass spectrum isthe molecular ion cluster of the tetrachloro-congener 77,where m/z 292 is [M77 C 2]Cž ; this peak is superimposedon the fragment ion cluster owing to the loss of one chlorine atom from pentachloro congener 110 [M110 C 2]Cž ,m/z 326, where m/z 291 is [M110 C 2 Cl]Cž . The peakat m/z 220 is due to loss of 2Clž from [M77 C 2]Cž and3Clž from [M110 C 2]Cž , while the peak at m/z 256 is dueprincipally to the loss of 2Clž from [M110 C 2]Cž . It is notpossible to determine quantitatively the relative contributions of the two congeners to this mass spectrum.
Nowlet us examine the CI of these congeners using methaneas the CI reagent.The ionization of methane yields the molecular ion thatreacts with methane to form CH5 C , Equation (31)CH4 Cž C CH4! CH5 C C CHž3292220256326200220240260280300320340360380400m/zFigure 20 EI mass spectrum averaged over the GC composite peak of the co-eluting polychlorinated congeners 77 and 110..31/22MASS SPECTROMETRYIntensity (ion signal − background)100%293327207200220220235240256260270 281280343 355300320340360380400380400m/z(a)100%293Intensity (ion signal − background)327355321200220240260280300320340360m/z(b)Figure 21 (a) CI mass spectrum averaged over the GC composite peak of the co-eluting PCB congeners 77 and 110; methane CIgas at a pressure and reaction time such that the ratio of signal intensities of m/z 29 and m/z 17 was 1 : 1.
(b) Selected reagent ion CImass spectrum averaged over the GC composite peak of the co-eluting PCB congeners 77 and 110 using m/z 29 from methane.and the CH3 C ion which reacts with methane to formC2 H5 C , Equation (32)CH3 C C CH4! C2 H5 C C H2.32/In practice, an LMCO of ca. m/z 12 is established by therf amplitude then methane is introduced briefly into theion trap and ionized simultaneously with the congeners.Owing to the preponderance of methane, CH5 C andC2 H5 C are formed rapidly during a short interval insertedinto the scan function for ion/molecule reactions andcan be isolated; during the isolation process, primaryions of methane and the congeners together with othersecondary ions from methane are ejected and surplusmethane is pumped away.
Proton transfer then occursduring a reaction period of ca. 10 ms.When methane is used as a CI reagent and the reactiontime is varied such that the signal intensities of CH5 Cand C2 H5 C are similar, CI of a mixture of congeners 77and 110 yields a mixture of proton transfer CI (to form[M C H]C ) and charge transfer (to form MCž ), as shownin Figure 21(a). However, when a single reagent ion23QUADRUPOLE ION TRAP MASS SPECTROMETERspecies, in this case C2 H5 C of m/z 29, is isolated forCI, proton transfer only is observed for both congeners,([M77 C 2 C H]C , m/z 293 and [M110 C 2 C H]C , m/z 327),as shown in Figure 21(b). The isotopic ratios for eachcluster of protonated congeners are in good agreementwith those expected for pure CI: the contribution ofcharge exchange to the congener 77 molecular cluster is3%. The mass spectrum is entirely free of fragment ionsbelow [M77 C H]C .
The adduct formation channel can besuppressed by reducing reagent gas pressure.Now that external ion sources can be used with anion trap, CI reagent ions can be created externally andinjected subsequently into the ion trap, isolated massselectively and allowed to react therein with samplemolecules.ICR/MSITSIMLCLC/ESI/MSLC/MS/MSLMCOMFIMSMS/MS(MS)nPCBPCDDPCDFQITMS13 CONCLUSIONSThe QITMS is a versatile instrument of high sensitivityand high specificity. The relatively low cost of commercialinstrumentation has permitted a substantial growth in thepractice of MS and a pronounced diminution of theaverage cost per mass spectrum.
The theory of ion trapoperation differs from those of other mass spectrometersand presents a challenge to the MS community.ACKNOWLEDGMENTSThe author acknowledges gratefully the support of TrentUniversity and the Natural Sciences and EngineeringResearch Council of Canada. The author is indebtedto Denise McMaster, Amy Trang, Dr Mehran Sharifi,Glenn Legault, and Douglas Simmons who have assistedby reading and criticizing the text and by the preparationof figures.ABBREVIATIONS AND ACRONYMSACAGCCICIDDCEIESIGCGC/MSGC/MS/MSAlternating CurrentAutomatic Gain ControlChemical IonizationCollision-induced DissociationDirect CurrentElectron IonizationElectrospray IonizationGas ChromatographyGas Chromatography/MassSpectrometryGas Chromatography/Ion TrapTandem Mass SpectrometryrfT4 CDDT4 CDFIon Cyclotron ResonanceMass SpectrometryIon Trajectory Simulation ProgramLiquid ChromatographyLiquid Chromatography/ElectrosprayIonization Mass SpectrometryLiquid Chromatography/IonTrap Tandem Mass SpectrometryLow-mass Cut-offMultifrequency IrradiationMass SpectrometryTandem Mass SpectrometryTandem Mass Spectrometric OperationPolychlorinated BiphenylPolychlorodibenzo-p-dioxinPolychlorodibenzofuranQuadrupole Ion TrapMass SpectrometerRadio FrequencyTetrachlorodibenzo-p-dioxinTetrachlorodibenzofuranRELATED ARTICLESBiomolecules Analysis (Volume 1)Mass Spectrometry in Structural BiologyEnvironment: Trace Gas Monitoring (Volume 3)Environmental Trace Species Monitoring: Introduction žLaser Mass Spectrometry in Trace AnalysisEnvironment: Water and Waste (Volume 3)Dioxin-like Compounds, Screening Assays ž ExplosivesAnalysis in the EnvironmentEnvironment: Water and Waste cont’d (Volume 4)Polychlorinated Biphenyls Analysis in EnvironmentalSamples ž Trace Organic Analysis by Gas Chromatography with Quadrupole Mass SpectrometryField-portable Instrumentation (Volume 5)Mobile Mass Spectrometry used for On-site/In situEnvironmental MeasurementsForensic Science (Volume 5)Mass Spectrometry for Forensic ApplicationsNucleic Acids Structure and Mapping (Volume 6)Mass Spectrometry of Nucleic AcidsPesticides (Volume 7)Carbamate and Carbamoyloxime Insecticides: Singleclass, Multiresidue Analysis of ž High-performance24MASS SPECTROMETRYLiquid Chromatography/Mass Spectrometry Methods inPesticide Analysis6.7.Petroleum and Liquid Fossil Fuels Analysis (Volume 8)Mass Spectrometry, High-resolution, (Homolog)-typeAnalysis of Petroleum and Synfuel Distillates andRefinery Streams ž Mass Spectrometry, Low-resolutionElectron Impact, for the Rapid Analysis of PetroleumMatricesPharmaceuticals and Drugs (Volume 8)Mass Spectrometry in Pharmaceutical Analysis8.9.10.11.Process Instrumental Methods (Volume 9)Mass Spectrometry in Process AnalysisGas Chromatography (Volume 12)Hyphenated Gas Chromatography ž Instrumentation ofGas ChromatographyMass Spectrometry (Volume 13)Mass Spectrometry: Overview and History ž ArtificialIntelligence and Expert Systems in Mass Spectrometryž Atmospheric Pressure Ionization Mass Spectrometryž Chemical Ionization Mass Spectrometry: Theory andApplications ž Discrete Energy Electron Capture Negative Ion Mass Spectrometry ž Electron Ionization MassSpectrometry ž Gas Chromatography/Mass Spectrometryž High-resolution Mass Spectrometry and its Applicationsž Liquid Chromatography/Mass Spectrometry ž Literature of Mass Spectrometry ž Tandem Mass Spectrometry:Fundamentals and InstrumentationREFERENCES1.2.3.4.5.M.
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