Pulsed-field gradient technology

Measurement of molecular self-diffusion using pulsed field gradient technology. Used mainly in studies of molecular associations... 

Wolfender et ah, and this coupled technique is treated more comprehensively in Chapter 1. The LC-NMR technique is by nature rather insensitive however, high-field magnets and recent improvements in solvent suppression, pulse field gradients, and probe technology have made it possible to achieve useful results on various flavonoid struc-tures. The detection limit with a 60 p.1 cell in a 500 MHz instrument for a compound with a molecular weight of around 400 amu may typically be around 20 p.g, and the information provided is hitherto mainly based on NMR spectra or correlation experiments. 

Recently, the use of pulsed-field gradient (PFG) technology to obtain diffusion coefficients of molecules has been demonstrated as a useful technique for mixture analysis (53). Unlike any other 2D experiment, size-resolved or diffusion-resolved NMR assigns the resonances based on the diffusion coefficient for each proton (or other spin) in the molecule and therefore can be used to distinguish resonances arising from different molecules (63-70) (Fig. 22). A method that involves the use of PFG and TOCSY, called diffusion-encoded spectroscopy (DECODES), simplifies mixture analysis by NMR (71). The combination of PFG and TOCSY decodes the spin systems, allowing individual components in complicated mixtures to be assigned. A typical DECODES spectrum obtained in this manner is shown in Fig. 23. The use of TOCSY aids the calculation of the diffusion coefficient and determination of molecular identity. 

Lead, J. R., K. J. Wilkinson, E. Balnois, B. J. Cutak, C. K. Larive, S. Assemi, and R. Beckett. 2000. Diffusion coefficients and polydispersities of the Suwannee River fulvic acid comparison of fluorescence correlation spectroscopy, pulsed-field gradient nuclear magnetic resonance, and flow field-flow fractionation. Environmental Science and Technology 34, no. 16 3508-3513. 

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