A COMPUTER-CONTROLLED BIPOLAR MAGNETIC-FIELD-GRADIENT DRIVER FOR NMR ELECTROPHORETIC AND SELF-DIFFUSION MEASUREMENTS

An apparatus is described for delivering bipolar pulsed magnetic-field gradients of varying amplitude and duration for NMR self-diffusion measurements. Unlike previous designs, this gradient driver precisely regulates the total charge delivered to the gradient coil independent of coil inductance and resistance. This allows for matching of the read- and write-gradient pulses within 10 ppm, which is necessary to accurately obtain the echo maximum. Experimental and calculated normalized signals for various read-write-gradient mismatches illustrate how small mismatches can result in significant signal reduction (8% signal loss for a 33 ppm mismatch between 312 G/cm; 3 ms read-write-gradient pulses). Results and observations indicate that this instrument, contrary to others, obtains the maximum signal when the read and write gradients are matched. © 1994 by Academic Press, Inc.