Reference convolution of low resolution FIDs in the presence of poor magnetic field homogeneity

Biological systems often display multiexponential transverse relaxation times, diagnostic of the presence of different proton pools within the sample. A reliable representation of sample multiexponentiality can be calculated through the inversion of CPMG magnetization decays via the UPEN protocol. However, if a sample contains both liquid- and solid-like protons (as for example in muscle tissue or protein gels) CPMG cannot be used, as the solid-like signal may decay before the first echo is recorded. In these cases signal quantification is more simply attempted in the time domain, by fitting the FID to a discrete number of suitable decaying functions, provided no frequency difference exists between the signal pools. Low field spectrometers (0.47 T or less) are useful in this respect because FIDs obtained at low resolution do not usually carry frequency information. On the other side, they may suffer from very poor magnetic field homogeneity, owing to the fact that they are typically built around a permanent magnet with no shimming facilities, thus producing highly distorted FIDs whose shape can be hardly approximated by meaningful fitting functions. In this work we show that a technique known under the name of reference deconvolution in high resolution NMR can be slightly modified and used successfully in low resolution NMR for fitting highly distorted FIDs.