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Archive for April, 2009

A LEAP IN SENSITIVITY

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NMR spectroscopy has always been tagged as a method plagued with low sensitivity. This is no longer true, thanks to the recent methodological and technological developments, which have pushed sensitivity of NMR to new levels. Recent technological developments include new generation cryogenic probes affording more than four-fold gain in sensitivity (see http://www.varianinc.com/cgi-bin/nav?products/nmr/probes/liquids/cold_probes/index&cid=KMQKOINJFN and http://www.bruker-biospin.com/cryoprobe_micro.html). However, what has caught attention lately is the demonstration of high levels of sensitivity achieved using Dynamic Nuclear Polarization (DNP) (Ardenkjær-Larsen et al. Proc. Natl. Acad. Sci. USA 100, 10158-10163 (2003)). This method belongs to the general class of hyperpolarization methods, which were proposed more than five decades ago. DNP has been in use in solid-state NMR, but its demonstration in solution state and for MRI has opened new avenues for rapid data collection.

DNP is based on the idea of transferring the high electron spin polarization (600 times that of nucleus) onto nuclear spins. The mechanism requires unpaired electrons, which are added to the sample (such as a free radical). In order for the DNP process to be effective, the radical must be distributed homogenously throughout the sample. The high electron spin polarization is transferred to the nuclear spins by microwave irradiation over a long period of time (a few hours). This is carried out at low magnetic field strengths (3.5 T) and low temperatures (1-2 K) wherein the sample is in a solid form. After polarization the sample is rapidly dissolved in a solvent and immediately transferred to another magnet for high-resolution NMR measurements. A very high sensitivity (~700 times) arising from the increased polarization has been demonstrated which allows (1) the use of much smaller amounts of sample material and (2) faster data acquisition. Other applications include ability to acquire 13C spectra of molecules directly with high sensitivity, using the enhanced signal to carry out chemical kinetic experiments. Commercialization of this method has already begun with Oxford instruments coming out with a DNP polarizer called “Hypersense” (http://www.oxford-instruments.com/products/dnp/hyperSense/Pages/hyperSense.aspx)

Currently, a few constraints limit the widespread use of the method for multidimensional NMR spectroscopy. First, the dead time for the transfer of the sample from the polarizer to the high field magnet required for recording the spectra should be less than the T1 relaxation of the sample. Carrying out the polarization and the subsequent desired NMR experiment in the same magnet could minimize this dead time. Second, during signal averaging or multiple scans as required for acquiring multidimensional NMR spectra, the polarization decays rapidly. One elegant means to overcome this limitation is the use of ultrafast single-scan NMR method (Frydman and Blazina, Nature Physics 3, 415-419 (2007)). Further developments are required to achieve progress in this direction. Taken together, the DNP method will emerge in the forefront in coming years.

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Written by Hanudatta Atreya

April 3rd, 2009 at 10:28 am

Posted in NMR