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NMR quantitation: influence of RF inhomogeneity
Author(s) -
Mo Huaping,
Harwood John,
Raftery Daniel
Publication year - 2011
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.2812
Subject(s) - chemistry , excitation , flip angle , radiation damping , homogeneity (statistics) , analyte , calibration , signal (programming language) , excitation function , calibration curve , computational physics , analytical chemistry (journal) , radio frequency , pulse sequence , nuclear magnetic resonance , physics , detection limit , chromatography , medicine , telecommunications , statistics , mathematics , particle physics , quantum mechanics , computer science , magnetic resonance imaging , radiology , programming language
The NMR peak integral is ideally linearly dependent on the sine of excitation angle (θ), which has provided unsurpassed flexibility in quantitative NMR by allowing the use of a signal of any concentration as the internal concentration reference. Controlling the excitation angle is particularly critical for solvent proton concentration referencing to minimize the negative impact of radiation damping, and to reduce the risk of receiver gain compression. In practice, due to the influence of RF inhomogeneity for any given probe, the observed peak integral is not exactly proportional to sin θ. To evaluate the impact quantitatively, we introduce a RF inhomogeneity factor I (θ) as a function of the nominal pulse excitation angle and propose a simple calibration procedure. Alternatively, I (θ) can be calculated from the probe's RF profile, which can be readily obtained as a gradient image of an aqueous sample. Our results show that without consideration of I (θ), even for a probe with good RF homogeneity, up to 5% error can be introduced due to different excitation pulse angles used for the analyte and the reference. Hence, a simple calibration of I (θ) can eliminate such errors and allow an accurate description of the observed NMR signal's dependence on the excitation angle in quantitative analysis. Copyright © 2011 John Wiley & Sons, Ltd.