Multidimensional signal, chemical

The basic detection concepts can be presented for the "zerodimensional case where detection decisions and detection limits are established simply from the characteristics of the chemical signal (instrument response), without giving detailed attention to other dimensions such as time, wavelength, analyte concentration, etc. Actually, higher dimensional situations (multiparameter separations or detector responses) reduce to this case either through sequential classification schemes or via algorithms which operate directly on the multidimensional data. 

A comparably simple approach uses perturbations of the target s chemical shifts to confirm binding. Multidimensional spectra are usually used, as signal overlap obscures the analysis in ID spectra. In these spectra, the displacement of peaks due to the binding event is monitored. Obviously, the use of 2D- or 3D spectra results in an increased experimental time. As an additional feature of this technique, the dissociation constant may be extracted from the experiment. This analysis is possible if the displacement is related to the concentration of the ligand via a titration. Of course, these experiments require a reference spectrum without the ligand. 

The basic idea behind the DOSY concept is similar to the one behind multidimensional NMR. In 2-D NMR, a modulation in the phase or signal intensity with respect to a known time increment is recovered by inverse FT. In a DOSY experiment, the diffusion coefficient is recovered from the signal decay as a function of a diffusion increment by an ILT. In fact, the approximate ILT of the signal amplitude with respect to q, where q is defined as ygSf (t), yields the second dimension of the spectrum which correlates the chemical shift with the diffusion coefficient. Therefore, it was termed diffusion ordered spectroscopy (DOSY). However, unlike the FT of the time domain signal that yields a unique solution, ILT does not yield a unique solution. Therefore, several software packages were developed to overcome this problem. Readers interested in more details concerning the DOSY techniques can consult a recent extensive review on the subject [17]. 

One of the most important features of DOSY is its ability to separate signals of compounds within a mixture, based on their diffusion coefficients which, in fact, reflect their size and shape, thus providing a means for virtual separation . Here, the information is spread over the entire plan thus also simplifying peaks assignment. However, it was commented that for separating peaks of different compounds, which happen to have the same chemical shift, a large difference in the diffusion coefficients (a ratio of about 3) is needed for DOSY to provide the accurate numbers. An important fact to remember is that diffusion sequences, in fact, act as filters and can therefore be imbedded or coupled to nearly any NMR sequence, including most of the multidimensional NMR sequences. 

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