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TECHNICAL REPORTS

Organic Compounds in the Environment

An Examination of Spin–Lattice Relaxation Times for Analysis of Soil and Manure Extracts by Liquid State Phosphorus-31 Nuclear Magnetic Resonance Spectroscopy

^a AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, Otago, New Zealand
^b Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
^c Department of Geological and Environmental Science, Stanford University, Building 320, Room 118, Stanford, CA 94305-2115

^* Corresponding author (richard.mcdowell{at}agresearch.co.nz)

Received for publication July 26, 2005. 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 (T₁) to ensure adequate delays between pulses. This paper determined T₁ 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 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 T₁ 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, T₁ 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 ³¹P NMR, delay times should be three to five times the T₁ 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 T₁, provided this does not increase line-broadening too much. A regression relationship was obtained between orthophosphate T₁ values and the ratio of P concentration to Fe and Mn concentration on a w/v basis (r² = 0.97, P < 0.001), and between the T₁ for all other compound classes and the ratio of P to Fe and Mn (r² = 0.70, P < 0.01). By combining measurement of Fe, Mn, and P in the reconstituted extract and these relationships, T₁ can be estimated and the appropriate delay time used. If T₁ 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.

Abbreviations: NMR, nuclear magnetic resonance • P/(Fe + Mn), ratio of phosphorus concentration to the concentrations of iron and manganese • T₁, spin–lattice relaxation rate

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