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An Examination of Spin–Lattice Relaxation Times for Analysis of Soil and Manure Extracts by Liquid State Phosphorus‐31 Nuclear Magnetic Resonance Spectroscopy
Author(s) -
McDowell R. W.,
Stewart I.,
CadeMenun B. J.
Publication year - 2006
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2005.0285
Subject(s) - soil water , chemistry , nuclear magnetic resonance spectroscopy , phosphorus , spectroscopy , soil test , manure , analytical chemistry (journal) , nuclear chemistry , environmental chemistry , agronomy , soil science , environmental science , biology , physics , organic chemistry , quantum mechanics
Phosphorous (P)‐31 nuclear magnetic resonance (NMR) spectroscopy is used in the analysis of P forms 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 m M EDTA extracts of eight diverse soils (Aquept, Dystrochrept × 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 from 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 from 0.78 to 1.94 s in soils and 1.45 to 5.82 s in manures and the epiphyte. For quantitative 31 P NMR, delay times should be three to five times the T 1 value, translating to delays of 3 to 5 s for soils and up 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.