Elimination of signal strength dependency upon coil loading–an aid to metabolite quantitation when the sample volume changes

The phase, height, and width of NMR spectral lines from a constant number of nuclei are frequently dependent upon changes in probe Q factor and tuning, caused by motion of the sample and/or changes in its electrical conductivity and size. Thus quantitation of metabolites in biological samples can be difficult. When probe tuning and matching are rendered independent by the use of a tuned coupling coil for matching, use of a very low input impedance preamplifier can virtually eliminate such dependencies in the received signal. Likewise, by reciprocity, the use of a low output impedance transmitter greatly reduces the dependence of pulse flip angle upon sample characteristics. Experimental results verifying these statements are presented, and the time course of signals from a swelling, perfused pig heart and an external reference are plotted.