NMR spectroscopy in the milli-Tesla regime: Measurement of 1H chemical-shift differences below the line width

NMR spectroscopy for chemical analysis at high field employs precision measurements of resonance frequencies governed by chemical shift and nuclear spin interactions. At low field and in the absence of hetero-nuclear J-couplings the natural line width limits the chemical shift measurements. We have performed chemical-shift resolved proton NMR spectra in the milli-Tesla regime, and found that in the presence of hetero-nuclear J-coupling, proton chemical-shift differences smaller than the line width can be measured down to nuclear Larmor frequencies of 41 kHz. The measurement of J-couplings and chemical shifts with simple NMR devices operating in the milli-Tesla regime may develop as an attractive tool for chemical analysis.