Difference spectra dynamic range

Transient IR spectroscopy in the range of the amide I band is a direct tool to follow the structural dynamics of the peptide moiety. IR difference spectra on the bicyclic molecule bc-AMPB are plotted in Fig. 5. Shortly after excitation the absorption is dominated by a red shift. Such a red shift is expected for a strong vibrational excitation of the molecule. On the time-scale of a few picosecond this red shift decays to a large extent and is replaced by a dispersive feature of opposite sign at tD = 20 ps. At later delay times this feature changes details of its shape, it sharpens up and some substructure appears around 1680 cm 1. After 1.7 ns the shape is similar, but not completely identical to the difference spectrum recorded with stationary FTIR spectroscopy. This time dependence shows that the dominant structural change responsible for the IR difference spectrum occurs on the 20 ps time-scale and that minor structural changes continue until nanoseconds and even later times. 

Dynamic range describes the ratio of the highest to lowest intensity feature in a mass spectrum. The lowest intensity feature needs to provide meaningful information on an ion, such as with the S/N ratio > 3. Dynamic range is an important factor when mass spectrometers are used to analyze samples with large concentration differences for example, when one of the sample molecules has an abundance several thousand times lower than others. Dynamic range is distinct from detection limit of a mass spectrometer because detection limit concerns the smallest detectable sample quantity without considering the interference of other species. 

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