NQR Detection of Sodium Nitrite Recrystallized in Wood

Abstract

Cylindrical pieces of wood which can be placed in 10 mm o.d. NMR sample tubes have been impregnated by an aqueous solution of sodium nitrite (NaNO2). They were subsequently dried and examined by ¹⁴N quadrupole resonance. To our surprise, we were able to observe a signal at the right frequency (4.64 MHz, the highest frequency NaNO₂ line). This implies that the material has been properly embedded but also that sodium nitrite was properly recrystallized. This latter point was more than speculative. Anyway, we could observe that it works for spruce, beech, ash, maritime pine, but not for oak (which is known to be difficult to impregnate). Instrumental parameters have been optimized for reducing the duration of the experiment. If the first measurements required around 4 h, we obtain at the present time acceptable results in about 2 min or less. In addition, we have designed a simple NLLS (Non Linear Least Squares) algorithm by which the spectral parameters of the NQR signal can be retrieved even if the peak is hardly visible in the frequency domain. Extensive measurements have been performed on NaNO₂ recrystallized in spruce. They show, among other things, that the totality of NaNO₂ which has penetrated the material has actually recrystallized and seemingly forever. Various assays have been carried out as a function of impregnation and drying conditions. The evolution of the width of the NaNO₂ line reflects defects in the crystal lattice of NaNO₂ due probably to alteration of the wood structure upon drying.