Two-Dimensional Coincidence Techniques

For com plex m olecules, the ion-ion coincidence techniques used in the one-dim ensional experim ents do not unam biguously identify the ions responsible for the coincidence signals as there are a large num ber o f available fragm entation channels. H ow ever, the 2D coincidence technique involving the m easurem ent o f the actual flight tim es o f a pair o f ions, rather than tim e-of-flight difference, allows all the pairs of ions form ed upon dicationic dissociation to be identified.

2D coincidence spectra ( ‘p airs’ spectra) are generated by plotting the flight times o f a coincident pair of ions. F or the dissociation of a dication

r n ' ^ + rri2 (2.II)

the flight tim e of the lightest ion t\ is plotted on the y axis and that o f the second ion of the pair ti on the X axis. Thus a plot of t\ versus yields a pairs spectrum w here each peak in the pairs spectrum is a plot o f intensity as a function of the flight tim es o f the ion pair form ed by a dicationic dissociation

e v e n t , 1 5 - 2 2 illustrated in Fig. 2.15. N ote that under our experim ental geom etry, described in Section 2.3.1.2, the lightest ion will reach the detector first.

h = ti .. = 64 ns

+

Fig. 2.15 Schematic diagram of a pairs spectrum.

U nder the W iley-M cLaren focusing conditions,^ the flight tim e o f an ion t can be expressed in the follow ing way,

t = t o ~ k p COS0 Eq. 2.14

where to is the flight time for an ion with zero kinetic energy release (KER), /: is a constant inversely proportional to the source field, p is the m agnitude o f the initial m om entum on dissociation and 0 is the angle between the initial m om entum vector and the TO FM S axis (when 0 = 0 is parallel or anti­ parallel to the axis of the TOFM S). Therefore, the peaks observed in the pairs spectra will be lozenge-shaped (Fig. 2.15) because there will be a distribution o f ion flight tim es as a result of the K ER upon dissociation and the direction of the initial im pulse upon dissociation o f the dication. The axial length of the peaks is the result of the deviation ôr o f an ion flight tim e from as the ion flight tim e m ay be longer or shorter than to depending on the K ER and the direction o f the initial im pulse. The widths perpendicular to the axial lengths reflect both apparatus factors, such as tem poral resolution, the random therm al m otion of the parent dication before ionization and the dication dissociation m echanism followed. ^^-21 As will be discussed in the follow ing chapter, a pow erful m ethod for obtaining inform ation concerning the dissociation m echanism involved with a given fragm entation channel is to consider the slope of the relevant peak in the pairs spectrum. 1^-22

From Fig. 2.15 it can be seen that the peak corresponding to identical ion pairs (e.g.

from 02^"^) will be centred on the ti = diagonal of the pairs spectrum . H ow ever, any pairs of ions with identical or very sim ilar flight times will not be observed in the pairs spectrum as the dead tim e of the CFD prevents a second stop pulses being transm itted to the TD C within 64 ns of the first pulse received. Therefore, the first ion tim e will be counted by the TD C but that o f its correlated partner will not and so the single ion time will be plotted in the singles spectrum . As a result, there is a region in the pairs spectrum , corresponding to the dead tim e o f the CFD, w here no ion pair signals

are observed. For the majority of identical ion pairs, this dead time region does not obscure the entire peak since the ion pairs which have a large difference in flight times {i.e. greater than 64 ns) will still be detected and plotted in the pairs spectrum. As will be described in Chapter 3, it is possible to obtain an estimate of the area of the peak obscured by the dead time and hence the num ber of ion pairs which would be contained within the obscured area can be evaluated.

The use of the MCP detector, in place of the channeltron used in the one-dimensional experiments, should eliminate the angular discrimination introduced by the small aperture of the channeltron. Therefore, as indicated by the peaks in the pairs spectra, all the ions formed from dicationic dissociation reach the detector, regardless of the direction with respect to the TOFMS axis, of the initial dication dissociation impulse.^

2A .2.2 Calibration of the Pairs Spectra

Fig. 2.16 shows the pairs spectrum of CS]^"^ and, as can be seen in the figure, the following

— .W.V.:: .

,

- t t -

C S ^

pairs of ions are formed from the dissociation of CSz^^: CS^ + S^, C^ + and + S^.^2

Fig. 2.16 Pairs spectrum o f recorded at 250 eV.

As mentioned above, only a proportion of the peak corresponding to the identical ion pair + S"^ is observed in the spectrum as the dead time of the CFD obscures the region of the pairs spectrum where the flight times of the ions are similar.

Fig. 2.17 shows a 2D contour plot of the pairs spectrum in the region of the CS^ -f- ion pair to illustrate that each pairs peak is a plot of the intensity as a function of t\ and ti. As expected, the decreased angular discrimination induced by the M CP manifests itself in this 2D contour plot since the high intensity region is uniform along the axial length of the peak, indicating that there is no angular discrimination.

34c+

Fig. 2.17 2D contour plot o f the pairs peak corresponding to the CS^ + ion pair.