Detection of Fast Transient Signals

Transient signals are typically obtained in atomic spectrometry when samples are introduced by flow injection techniques or when the spectrometer is used as an element-specific detector in hyphenated techniques. Inductively coupled plasma mass spectrometry has nowadays become the detection technique of choice for multielement-specific detection in speciation as it allows multielemental 

Hyphenation between a chromatographic separation system and an ICP mass spectrometer often leads to transient signals of very short duration (from 

However, the main limitation of ICP-TOFMS at present is, perhaps, its comparatively low sensitivity detection limits for ICP-TOFMS are roughly one 

As discussed above, ICP-IDMS is becoming a highly valuable method for trace element and element-speciation analysis. For ICP-IDMS the highly precise and accurate isotope ratio measurements that are currently required can be made by resorting to multicollector-ICP-MS. 

All isotope ratio measurements have to be corrected for instrumental mass bias by normalising to an invariant isotope of the same element (internal correction) or, whenever the internal approach cannot be applied, to a well-characterised isotope standard material (external correction). However, the external correction method requires the mass discrimination of an element being identical for the sample and the standard, which is not always the case. A large benefit of the hyphenated chromatography-ICP-MS system is that all measurements of standards and real samples can be carried out with exactly the same matrix - the eluent of the HPLC system. 

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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. 

The photolytic flash must have enough energy to prepare, in a very short time, a detectable concentration of transient species. The lowest detectable concentration depends on the probe technique, and here the methods of UV/VIS/near IR absorption and emission spectroscopy are the best. Their drawback is that they provide very little structural information about the nature of the transient species. IR and Raman spectra are much more informative, but they present many problems in fast reaction kinetics because of the weakness of the signals. 

From comparison of Table 8.2 with Table 7.1 (or of Eq. (7.20) with Eq. (7.10)), i.e. of transient NOE or NOESY vs. steady state NOE intensities, it appears that the latter are superior under any circumstance. This superiority is striking if the intrinsic asymmetry of the steady state NOE with respect to the symmetry of transient NOE and NOESY experiments (Section 7.4) can be exploited, as in the case of irradiation of fast relaxing nuclei to detect NOE to slow relaxing nuclei. Of course, NOE experiments are advantageous over NOESY experiments if one is looking for dipolar connectivities from only a few specific 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... 

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