Motivated by the lack of efficient detection techniques for metal-free trinitrotoluene (TNT) containing landmines, 1H-14N cross-relaxation experiments on TNT have been performed using field-cycling spectroscopy in an electronically switchable volume coil. Using 1H NMR detection (indirect method), the 14N quadrupole spectra in several small (about 0.5 g) TNT samples of different producers are determined. The experiment is considered as a step toward a double resonance TNT landmine detection scheme using surface coils.

A general theoretical description of polarization transfer processes in multi-spin systems containing dipole as well as quadrupole spins is formulated on the background of the Liouville–von Neumann equation. The density operator formalism is used to describe the evolution of an arbitrary spin system due to quadrupole, Zeeman and dipole–dipole interactions. This approach is applied to interpret previously published 1H–14N cross-relaxation NMR experiments for measuring the 14N quadrupole coupling constants of paranitrotoluene (PNT) and trinitrotoluene (TNT) (Nolte et al 2002 J. Phys. D: Appl. Phys. 35 939) and new experiments on urea and urotropine. It is demonstrated that according to the complexity of the analysed spin system an appropriate number of spins has to be taken into consideration for a correct description of the cross-relaxation spectra. The work is a part of an extended project aiming for a method which should permit detection of TNT explosive in anti-personnel landmines.

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