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The variation inaxial frequency with axial excursion, which is shown in322Intensity (ion signal − background)100%259334194241172160204183180200215220270231287250240306260280300348320340360m /zFigure 13 Mass spectrum of 2,3,7,8-T4 CDD. The [M C 2]Cž ion of the molecular cluster is shown at m/z 322, while that of a traceof 13 C12 -2,3,7,8-T4 CDD is shown at m/z 334; the primary fragmentation process is the loss of COClž giving rise to m/z 259 and 270,respectively.16Figure 14 Scan function for obtaining an EI (electron ionization) mass spectrum. The scan function shows the ionizationperiod, A, followed immediately by the analytical ramp withconcurrent axial modulation. Note that the prescan for theAGC algorithm is not shown.Figure 15 Variation of ion axial secular frequency as a functionof the axial excursion from the center of the ion trap. Iontrajectories were calculated for three m/z 134 ions, 1 (ž), 2()and 3(N), taken at random, with qz D 0.4 and subjected toresonant excitation.
While the three ions differed in their initialposition and velocity, all were close to the center of the iontrap and had been cooled collisionally. When specific axialexcursions had been reached, ion excitation was arrested andthe trajectory of each ion was subjected to frequency analysis.Figure 15 as obtained from analysis of simulated iontrajectories, arises from the superimposition of higherorder field components as a result of the axial ‘‘stretching’’MASS SPECTROMETRYof the ion trap..21,22/ The secular frequency variationnecessitates the development of strategies for optimizingCID, particularly when the duration of irradiation mustbe minimized as in GC/MS (gas chromatography/massspectrometry) analytical applications; these strategiesinclude secular frequency modulation, single frequencyirradiation with rf modulation, and multiple frequencyirradiation.Under the influence of the applied resonant excitationvoltage, ions are moved away from the center to aregion of higher potential whereupon they are accelerated(under the influence of the higher potential rather thanthe resonant excitation voltage alone) so that theirkinetic energies are increased.
Subsequent collisions withbuffer gas atoms lead to enhancement of ion internalenergy. Ions undergo rapid changes in kinetic energy onthe microsecond timescale whereas changes in internalenergy occur more slowly on the millisecond timescale.In analytical applications, the objective is usually todissociate all of the isolated ions and to maximize thetrapping of fragment ions produced; the achievement ofthis objective requires a balancing of ion kinetic energyuptake so that ion internal energy may be accumulatedincrementally and rapidly but ejection of both isolatedions and fragment ions is prevented. Incremental loss ofinternal energy in collisions can occur with both buffer gasatoms and sample molecules while total loss of internalenergy can occur upon charge exchange with samplemolecules; yet these loss mechanisms merely slow downthe overall accumulation of internal energy as there is nonet loss of charge involved.The role played by time is enormously important inCID.
The total time during which ions are subjected tocollisions and the total time during which ions are subjected to irradiation can be used as variable parametersto direct dissociation into a chosen fragmentation pathway. While the mode of irradiation is also influential indirecting fragmentation, alternating periods of irradiation (with collisions) and collisional cooling can affectsignificantly the fragment ion abundances. During anirradiation period, the time-averaged ion kinetic energiesincrease along with internal energy: during a subsequentcollisional cooling period in which the excitation voltage is removed, ion kinetic energies are quenched morereadily than are ion internal energies.
Thus ions becomekinetically relaxed during a cooling period and sampleonce more the lower reaches of the trapping potentialwell, yet the internal energies gained are virtually unimpaired. The internally excited ions, kinetically cool andfocused near the center of the ion trap, can be excitedfurther during the next irradiation period. In this manner,several electron volts of internal energy can be depositedin ions to allow access to fragmentation pathways of highactivation energy.QUADRUPOLE ION TRAP MASS SPECTROMETER1711 TANDEM MASS SPECTROMETRYsuch that the dissociation reaction channels of lowestenergy of activation are accessed almost exclusively; thisbehavior is highly advantageous in analytical chemistrybecause the total charge is conserved within a singlefragment ion species.
However, alternating periods ofexcitation and collisional cooling can be used to allowaccess to dissociation reaction channels of higher energyof activation. Furthermore, it is possible to dissociatecompletely the accumulated mass-selected ions and toconfine within the ion trap fragment ions arising fromsome 90% of the accumulated mass-selected ions infavorable cases.To achieve CID with such high efficiency (defined as100 times the ratio of the sum of product ion signalintensities to that of the parent ion) when the ion trapis interfaced with a gas chromatograph, it is imperativethat the resonant excitation conditions be optimized withrespect to the duration of the CID episode.
Here, we usemultifrequency irradiation (MFI) as it has been shownto be an effective method for CID because it exhibitshigh efficiency and requires a relatively short period ofirradiation, ca. 10 ms.Tandem (latin: at length) mass spectrometry, (MS/MS) or(MS)n , is the practice of carrying out one mass-selectiveoperation after another, much as one rider is seatedafter the other on a tandem bicycle. The objective of thefirst mass-selective operation is to isolate an ion speciesdesignated as the parent ion, while that of the secondoperation is to determine the mass/charge ratios of thefragment, or product, ions formed by CID of the parentions. MS/MS can be effected in space, by placing one massspectrometer after another, or by carrying out successivemass-selective operations in time in a quadrupole iontrap.
The ion trap offers two principal advantages whenused in the MS/MS mode. First, the ion trap operates ina pulsed mode, compared with sector and triple stagequadrupole instruments that operate in a continuousmode, so that it can accumulate ions mass-selectivelyover time. In this way, a target ion number can beselected so as to ensure constant signal/noise ratio overa wide range of eluent concentrations.
A disadvantageof ion accumulation is that the resultant product ionsignal intensity can correspond to integration of eluentconcentration over some 25 ms. Second, CID in the iontrap is wrought by some hundreds of collisions of massselected ions with helium buffer gas atoms. Under theseconditions, the energy transferred in a single collisionis seldom greater than that of a vibrational quantum11.1 Tandem Mass SpectrometryHere, MS/MS is illustrated using an extension of thedioxin example used in the above discussion. Thisapplication,.23/ while not illustrating the ultimate limitof sensitivity of the ion trap of some hundreds ofFigure 16 Scan function for MS/MS of dioxin T4 CDD; the prescan for AGC is not shown.18MASS SPECTROMETRYfemtograms of dioxin, does illustrate the high sensitivityof the ion trap for the determination of co-elutingdioxin congeners.
Polychlorodibenzo-p-dioxins (PCDDs)and polychlorodibenzofurans (PCDFs) are persistentorganochlorine compounds in the environment; most ofthese chlorocongeners are obtained from municipal andindustrial waste incinerators, automobile exhaust, and themanufacture of chlorophenol products. The compoundshaving the highest toxicity have been identified as thosecongeners having 2,3,7,8-tetrachloro-substitution. WhenGC is interfaced with MS, individual chlorocongenerscan be detected at the hundreds of femtograms level.The high specificity or informing power obtainablewith GC/MS/MS is achieved by observation of specificfragment ion signals, such as [M COClž ]C , from anisolated ion species MCž formed from M which eluteswithin a specified retention-time window.
The essentialstages of MS/MS are portrayed in the scan function fordioxin shown in Figure 16. It is instructive to examinethis scan function in advance of the following explanationof ionization, ion isolation, cooling period, CID, and theanalytical ramp over a selected mass range so as to detectproduct ions.We shall examine the MS/MS of [M]Cž and [M C 2]Cžfor T4 CDF (tetrachlorodibenzofurans), T4 CDD and theirinternal (labeled) standards following elution from a GCcolumn, where [M C 2]Cž is a molecular ion containing asingle 37 Clž atom.11.2 Scan FunctionsThe scan function employed for the MS/MS determinationof T4 CDDs is shown in Figure 16. The rf voltage is appliedto the ring electrode with a drive frequency f of 1.05 MHz.The LMCO value, as determined by the amplitude of therf potential, was set at m/z 160 during the ionizationperiod (A) for all the scan functions so that the qz valuefor MCž (m/z 320) was 0.45.
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