Mass spectrometric time scale

Fig. 2.9. The mass spectrometric time scale. It is important to note the logarithmic time scale for the ion source spanning over nine orders of magnitude. Reproduced from Ref. [9] with permission. John Wiley Sons Ltd., 1985.

The terminology for ions has been coined as a direct consequence of the classical mass spectrometric time scale. Nondecomposing molecular ions and molecular ions decomposing at rates below about 10 s will reach the detector without fragmentation and are therefore termed stable ions. Consequently, ions dissocial-... 

The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2) shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contributions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation of adsorbed Hupd (dotted lines in Fig. 13.2a), but no measurable increase in the CO2 partial pressure (m/z = 44 current) above the background level (dotted lines in Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base electrolyte can be ruled out on the time scale of our experiments. Moreover, this experiment also demonstrates the advantage of mass spectrometric transient measurements compared with faradaic current measurements, since the initial reaction signal is not obscured by pseudocapacitive effects and the related faradaic current spike. 

The mass spectrometric currents follow largely, but not completely the faradaic current signals. The contributions to the respective faradaic currents resulting from complete oxidation to CO2, which are calculated using the calibration constant K (see Section 13.2), are plotted as dashed lines in the top panels in Fig. 13.3. For the calculations of the partial reaction currents, we assumed six electrons per CO2 molecule formation and considered the shift in the potential scale caused by the time... 

Mass spectrometric studies are not limited to the investigation of stable intermediates they have also been carried out on reaction transition states. The ultrafast studies by Zewail, for example, are nominally mass spectrometric based, where photoionization is used to detect reactive species on exceedingly short (femtosecond) time scales.Time resolved studies provide insight into the rates of unimo-lecular reactions, but do not provide direct thermochemical insight. 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

LC-MS/MS has dramatically changed the way bionalysis is conducted. Accurate and precise quantitation in the pg ml scale is nowadays possible however one has to be aware of certain issues which are specific to mass spectrometric detection such as matrix effects and metabolite crosstalk. With the current growing interest in the analysis of endogenous biomarkers in biological matrices, quantitative bioanalysis with MS has certainly the potential to contribute further in this field with the development of multicomponent assays. Modern triple quadrupole instruments have the feature to use very short dwell times (5-10 ms), allowing the simultaneous determination of more than 100 analytes within the timescale of an HPLC peak. Due to the selectivity of the MS detection the various analytes... 

Due to the time-resolution limitation of the method, FPTRMS can be used to determine the kinetics of only those unimolecular reactions that occur on millisecond time scales or longer. However, even if a unimolecular reaction occurs too rapidly for time resolution of the kinetics, the occurrence of a reaction can be shown by mass spectrometric detection of the products. If the unimolecular reaction is rate limited by a preceding slow step so that the product formation rates are time resolved, then a lower limit to the unimolecular rate coefficient can be estimated. In the case of atmospheric reactions this will frequently be enough information to permit reaction mechanisms to be sorted out. 

Mass spectrometry involves the detection of charged particles, and, in the present case, a portion of the neutral cluster beam is ionized. Ionization essentially involves electronic excitation and occurs on the time scale of the order of 10 16 s (Haberland 1985 Mark 1987). The mass spectrometric detection of the ions is usually achieved on a microsecond time scale after the ionization event. As a result, the ionization process is taken to be time zero in the discussion of the processes which occur following the actual ionization of the neutral clusters, yet before the mass selection of the cluster ions. That is, the resulting cluster ion will incubate in the ionizer for microseconds before being accelerated into the mass filter. On that time scale, the cluster ion may lose monomer units, and the cation within the cluster may fragment or react chemically with the adjacent molecules. 

We have gained an appreciation for the ionization bias observed between MALDI and LSIMS. The utilization of PSD to identify known peptides and provide sequence information has been investigated for conotoxins. This approach to obtaining sequence information on novel peptides is attractive because of the low amount of material required. A number of mass spectrometric based derivatizations have been used to scan fractions of venoms in order to characterize peptides of interest. For closely related components (based on HPLC retention time and mass), the small scale derivatization schemes can be used to test hypotheses about peptides with otherwise novel masses (i.e. which may be homologs). The mass accuracy of the TOF technique, with a gridless reflector, was important for identifying and calling these substitutions. 

Elucidation of glycan composition is a combinatorial problem where the number of compositions that must be tested scales exponentially with the number of different monomers that can form the solution. Therefore, the composition for molecules of high mass can take a very long time to be derived. The number of solutions will also grow at a similar ratio, thus offering an enormously large amount of alternatives to the user. Furthermore, when adducts and other mass spectrometric losses are taken into account, a number of compositions may be indistinguishable within a relatively low mass delta threshold (less than 0.05 Da). Therefore, it is often... 

Fig. 6 Mass spectrometric intensity vs time for etch products and reflected Xcl . with a square wave modulated ion beam. Purely physically sputtered products would have the same time dependence as the Si+ SIMS signal. The longer time scales evident in the plots for the etch products (SiF+ and SiF+) suggest chemical sputtering. (From Winters and Coburn, 1992.)...

Figure 19. (a) P E ) for process 24. (b) P E ) for process 25. (c) Solid curve P E ) derived from the TOF spectrum for SSCHj dashed curve P(E derived from TOF spectrum for CHj for process 26 the shaded area indicates the experimental uncertainty. The difference of the two P E ) curves represents the portion of excited SSCHj, which has undergone unimolecular decay (to form Sj + CHj) within the time scale of the TOF mass spectrometric experiment, (d) P(E ) for the unimolecular dissociation of excited SSCHj formed by process 26. Taken from ref. 53. 

Several techniques have been used to follow explosive reactions on a shorter time scale, such as mass spectrometry [6,7,31,46] and emission spectroscopy [11-14]. Mass spectrometry seems more universal as it can identify many species relatively easily. But due to the inherent instability of especially explosive molecules and some of the decomposition intermediates, the ionisation process can generate secondary species. This influence of the ionisation process is difficult to avoid. Also, the time resolution achieved in mass spectrometric techniques is not high enough, unless sophisticated and expensive techniques are used. On the other hand, emission spectroscopy is a fast, non-intrusive, sensitive detection technique. Furthermore, there is little spectroscopic information about the decomposition reactions of explosives. It is this technique (emission spectroscopy) that has been used in the experiments described below to investigate fast (explosive) decomposition reactions induced by a nanosecond laser light pulse. 

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