An Overview of the COSY Experiment

The pulse sequence for a COSY experiment contains a variable delay time, fi, as well as an acquisition time, f2- The experiment is repeated with different values of fi, and the data collected during f2 are stored in the computer. The value of ti is increased by regular, small intervals for each experiment, so that the data that are collected consist of a series of FID patterns collected during f2, each with a different value of ti. 

To identify which protons couple to each other, the coupling interaction is allowed to take place during ti. During the same period, the individual nuclear magnetization vectors spread as a result of spin-coupling interactions. These interactions modify the signal that is observed during t2. Unfortunately, the mechanism of the interaction of spins in a COSY experiment is too complex to be described completely in a simple manner. A pictorial description must suffice. 

Consider a system in which two protons are coupled to each other. 

Since each axis spans the entire chemical shift range, something on the order of a thousand individual FID patterns, each incremented in ti, must be recorded. With instruments operating at a high spectrometer frequency (high-field instruments), even more FID patterns must be collected. As a result, a typical COSY experiment may require about a half hour to be completed. Furthermore, since each FID pattern must be stored in a separate memory block in the computer, this type of experiment requires a computer with a large available memory. Nevertheless, most modem instraments are capable of performing COSY experiments routinely. 

Nuclear M netic Resonance Spectroscopy Part Five Advanced NMR Techniques 

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In natural products analysis, most frequently the stop-flow mode is chosen to acquire H spectra of the compounds of interest, or if further structural information is required to perform two-dimensional H NMR spectra, such as COSY, TOCSY, NOESY or ROESY. In many cases an on-flow NMR chromatogram (usually at flow rates between 0.3 and 1ml min-1) is recorded beforehand, either to screen for the presence of particular groups of compounds or to gain a general overview on the sample composition. (Heteronuclear LC-NMR experiments, such as HSQC and HMBC of a natural product, have been reported in the literature once [9] however, this was of a highly enriched fraction.) More recently, time-sliced stop-flow [14,16] and on-flow approaches at low flow rates [34,35] have been applied to natural product extracts in order to combine the advantages of both on-flow (a ready overview on the entire sample) and stop-flow (sufficient acquisition time for minor compounds) modes. 

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