Quantitative Analysis of Low-Field NMR Signals in the Time Domain

Abstract

Two novel methods are described for direct quantitative analysis of NMR free induction decay (FID) signals. The methods use adaptations of the generalized rank annihilation method (GRAM) and the direct exponential curve resolution algorithm (DECRA). With FID-GRAM, the Hankel matrix of the sample signal is compared with that of a reference mixture to obtain quantitative data about the components. With FID-DECRA, a single-sample FID matrix is split into two matrices, allowing quantitative recovery of decay constants and the individual signals in the FID. Inaccurate results were obtained with FID-GRAM when there were differences between the frequency or transverse relaxation time of signals for the reference and test samples. This problem does not arise with FID-DECRA, because comparison with a reference signal is unnecessary. Application of FID-DECRA to 19F NMR data, which contained overlapping signals from three components, gave concentrations comparable to those derived from partial least squares (PLS) analysis of the Fourier transformed spectra. However, the main advantage of FID-DECRA was that accurate (<5% error) and precise (2.3% RSD) results were obtained using only one calibration sample, whereas with PLS, a training set of 10 standard mixtures was used to give comparable accuracy and precision.

Ultra-low field NMR measurements of liquids and gases with short relaxation times

“…The NMR signal parameters, fA k ; a k ; x k g m 1 can be estimated using, for example, the direct exponential curve resolution algorithm (DECRA) [18]. The DECRA method constructs a Hankel matrix from a single FID which is partitioned into two submatrices and then the decay constants and the individual signals in the FID are recovered by solving a generalized eigenvalue-eigenvector problem for these matrices.…”

Medium Resolution NMR at 20 MHz: Possibilities and Challenges

“…An alternative could be MR-NMR with its inherent high chemical selectivity. First examples are described in literature 17,[20][21][22] . In this work the modified minispec mq20 19 was used to monitor the reaction progress in an emulsion polymerization 26 .…”

“…However, there have been very few investigations on the ability of low field instruments delivering spectral information, concerning chemical shifts and J-coupling of the studied sample. In contrast to relaxometry methods for determination of water and oil content [12][13][14][15][16][17] , prerequisite for frequency resolved NMR proton spectra is a sufficient spectral resolution of the NMR spectrometer in the range of 0.2 ppm full width half maximum (FWHM) 18 and frequency or B 0 field stability. The general feasibility of NMR spectroscopy with adequate resolution at low magnetic fields have already been shown by different groups [19][20][21][22] .…”

Simultaneous 19F–1H medium resolution NMR spectroscopy for online reaction monitoring

“…Examples for the application of PLS-R to NMR data are available for high-field [25] (and references therein) as well as for MR-NMR spectroscopy [18], including comparisons with classical data analysis. In contrast to Fourier-transformed spectra, the direct exponential curve resolution algorithm (DECRA) was applied to time-domain data [26]. An interesting approach is to reduce the calibration effort, which could be explored also for NMR data [27].…”