Improvement of quantitative solution 31P NMR analysis of soil organic P: a study of spin–lattice relaxation responding to paramagnetic ions

Solution ³¹P nuclear magnetic resonance (NMR) spectroscopy has been widely applied to analyze the speciation of soil organic P; however, this time-consuming technique suffers from a low analytical efficiency, because of the lack of fundamental information such as the spin–lattice relaxation (T₁) of ³¹P nucleus for model P compounds. In this study, we for the first time determined the T₁ values of twelve typical soil organic P compounds using the inversion recovery method. Furthermore, we examined the effect of co-existing paramagnetic ions (e.g., Fe³⁺ and Mn²⁺) on the reduction of the T₁ values of these compounds. Without the addition of paramagnetic ions, the T₁ values of twelve model P compounds ranged from 0.61?s for phytic acid to 9.65?s for orthophosphate. In contrast, the presence of paramagnetic ion significantly shortened the T₁ values of orthophosphate, pyrophosphate, and phytic acid to 1.29, 1.26, and 0.07?s, respectively, except that of deoxyribonucleic acid (DNA) remaining unchanged. Additionally, we evaluated the feasibility of improving the efficiency of quantitative ³¹P NMR analysis via addition of paramagnetic ion. Results show that, after an addition of 50?mg L^?1 paramagnetic ions, ³¹P NMR measurement can be 3 times more efficient, attributed to the reduced T₁ and the corresponding recycle delay.