Fast transient signals detection

Of all atomic spectroscopic methods, ICP-MS is unrivalled concerning its detection power (Bencs et al. 2003). Its capability to process fast transient signals is crucial for the combination with sample preparation methods that generate impulse signals, like e.g. ablation techniques or most on-line preconcentration systems. 

As interesting as these few experiments conducted for chromatography with MC-ICP-MS detection are, the results are not conclusive. Most experiments reveal a drift of the analyte isotope ratio during peak elution, and no common rule can be applied to explain this effect. So far, it has not been determined if this drift is of instrumental origin, as a result of the fact that multicollector detection systems are by default not conceived for fast transient signal acquisition. Also, it is not yet clear what effect this drift may have on the accuracy of isotope ratios obtained on transient signals. 

Quadrupole and magnetic-sector instruments are the conunon type of mass analyzers for the detection of the GD-formed ions. The use of quadrupole ion traps, FT-ICR, and TOF instruments has also been explored [11-13], Specifically, quadrupole ion traps and FT-ICR mass spectrometers are of particular interest because they can accumulate and store ions for a desired length of time for subsequent CID or ion-molecule reactions that will eliminate isobaric interferences. The fast-scan facility of a TOF mass analyzer has the advantage that it can be used to monitor even short-lived transient signals. 

Instead of direct observation or measurement of the amplitude, for detection of very fast transients it is more convenient to observe the time integrated signal as a function of the delay-time. At the output of a bandwidth limited detector we then expect a signal of the form... 

Recently, the femtosecond time-resolved spectroscopy has been developed and many interesting publications can now be found in the literature. On the other hand, reports on time-resolved vibrational spectroscopy on semiconductor nanostructures, especially on quantum wires and quantum dots, are rather rare until now. This is mainly caused by the poor signal-to-noise ratio in these systems as well as by the fast decay rates of the optical phonons, which afford very fast and sensitive detection systems. Because of these difficulties, the direct detection of the temporal evolution of Raman signals by Raman spectroscopy or CARS (coherent anti-Stokes Raman scattering) [266,268,271-273] is often not used, but indirect methods, in which the vibrational dynamics can be observed as a decaying modulation of the differential transmission in pump/probe experiments or of the transient four-wave mixing (TFWM) signal are used. 

As evidenced from the above discussion, vibrational line shapes provide information mostly about intermolecular structure. Transient hole burning and more recently echo experiments, on the other hand, can provide information about the dynamics of spectral diffusion. The first echo experiments on the HOD/ D2O system involved two excitation pulses, and the signal was detected either by integrating the intensity [20] or by heterodyning [22]. The experiments were analyzed with the standard model assuming Gaussian frequency fluctuations. The data were consistent with a spectral diffusion TCF that was bi-exponential, involving fast and slow times of about 100 fs and 1 ps, respectively. 

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