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

Solution (31)P 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 (T(1)) of (31)P nucleus for model P compounds. In this study, we for the first time determined the T(1) 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., Fe(3+) and Mn(2+)) on the reduction of the T(1) values of these compounds. Without the addition of paramagnetic ions, the T(1) 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 T(1) 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 (31)P NMR analysis via addition of paramagnetic ion. Results show that, after an addition of 50 mg L(−1) paramagnetic ions, (31)P NMR measurement can be 3 times more efficient, attributed to the reduced T(1) and the corresponding recycle delay.