An examination of spin-lattice relaxation times for analysis of soil and manure extracts by liquid state phosphorus-31 nuclear magnetic resonance spectroscopy

Phosphorous (P)-31 nuclear magnetic resonance (NMR) spectroscopy is used in the analysis of P fortus in extracts of soils and manures for environmental and agronomic purposes. Quantitative spectra require knowledge about spin-lattice relaxation times (T-1) to ensure adequate delays between pulses. This paper determined T-1 values of P forms in reconstituted (0.2 g in 0.7 mL(-1)) samples of freeze-dried 0.25 M NaOH plus 50 mM EDTA extracts of eight diverse soils (Aquept, Dystrochrept X 2, Hapludand, Rendoll, Udand, Haplostoll., and Orthod), three different manures (dairy cattle, deer, and sheep), and one epiphyte moss. Total concentrations in the reconstituted samples ranged front 5 to 175 mg Fe mL(-1), 2 to 62 mg Mn mL(-1), and 72 to 837 mg P mL(-1). Values of T-1 for orthophosphate monoesters, orthophosphate diesters, and pyrophosphate varied from 0.42 to 1.69 s in soils and from 0.89 to 2.59 s in manures and the epiphyte. In contrast, T-1 for orthophosphate varied front 0.78 to 1.94 s in soils and 1.45 to 5.82 s in manures and the epiphyte. For quantitative P-31 NMR, delay times should be three to five times the T-1 value, translating to delays of 3 to 5 s for soils and tip to 25 s for manures. If the required delay is too long then strategies such as adding paramagnetics could shorten T-1, provided this does not increase line-broadening too much. A regression relationship was obtained between orthophosphate T-1 values and the ratio of P concentration to Fe and Mn concentration on a w/v basis (r(2) = 0.97, P < 0.001), and between the T-1 for all other compound classes and the ratio of P to Fe and Mn (r(2) = 0.70, P < 0.01). By combining measurement of Fe, Mn, and P in the reconstituted extract and these relationships, T-1 can be estimated and the appropriate delay time used. If T-1 is not considered and the delay time is too short, some peaks will be under- or over-represented and the relative distribution of P forms not quantitative.