Time Domain NMR

By far the most widespread use of NMR in an on-line production environment is the utilization of downhole exploration tools by petroleum service companies such as Schlumberger, Halliburton, and Baker Hughes. Articles on these unilateral NMR systems are found in the patent databases, academic literature, and on-line resources provided by the exploration companies. The references provided here are just a few examples in a very prolific field. The technique is applied in high-temperature and pressure situations and currently is used down to a depth of about 10 km (6 miles) to produce a picture of water/oil content in the adjacent rock formations as well as to derive permeability, diffusivity, and hydrocarbon chemistry information. Mobile unilateral NMR systems such as the NMR-MOUSE are also being developed in order to take benchtop NMR systems into the field to perform analysis of geological core samples at the drill site. NMR analyzers are also being developed to determine the bitumen content and properties in tar sand production and processing.  

There is a class of physically small, benchtop NMR instruments available, useful for dedicated quantitative analysis. These instruments are pulsed, time-domain NMRs (TD-NMRs). TD-NMR is also called relaxometry. TD-NMR does not deal with spectroscopy or magnetic resonance (MR) images. The TD-NMR instrument is usually a small benchtop or handheld, low-resolution and low-field analyzer designed to detect hydrogen or fluorine nuclei. TD-NMR analysis is quantitative and rapid (normally within seconds or minutes). It is also nondestructive and noninvasive. Thanks to these advantages and its ease of use, it is widely used for routine analysis in agriculture, food science, polymer, chemical, petroleum, and pharmaceutical and medical industries. 

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Figure 1.22 Relationship of (a) the magnetization vector s position with (b) the signal time-domain NMR signal and (c) the frequency-domain NMR signal.

Figure 6.2 Pulse sequences for some common 3D time-domain NMR techniques. Nonselective pulses are indicated by filled bars. Nonselective pulses of variable flip angle are shown by the flip angle )8. Frequency-selective pulses are drawn with diagonal lines in the bars. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al, 14, copyright (1989), with permission from Academic Press, Inc.)...

Since there are two time variables, i and h, to be incremented in a 3D experiment (in comparison to one time variable to increment in the 2D experiment), such experiments require a considerable data storage space in the computer and also consume much time. It is therefore practical to limit such experiments to certain limited frequency domains of interest. Some common pulse sequences used in 3D time-domain NMR spectroscopy are shown in Fig. 6.2. 

G. Guthausen, H. Todt, W. Burk, D. Schmalbein, and A. Kamlowski, Time-domain NMR in quality control more advanced methods, in Modem Magnetic Resonance, G.A. Webb (ed.). Springer, Netherlands, 2006. 

E. Trezza, A.M. Haidnc, G.J.W. Gondappel and J.RM. van Duynboven, Rapid pbase-compositional assessment of lipid-based food products by time domain NMR, Magn. Reson. Chem., 44, 1023-1030 (2006). 

Schwartz, L.J. (1988). A Step-by-Step Picture of Pulsed (Time-Domain) NMR. J. Chem. Ed. 65,752-756. 

Raw time-domain NMR data (the FID) consists of a list of numbers, usually negative and positive integers, as a function of time in equal time increments. The list is usually quite... 

It should be mentioned that DSC and NMR do not measure the same parameters, and in this way, these techniques are complementary. DSC is a dynamic method, which gives information about the transitions between different phases of lipids, whereas NMR allows quantitation of liquid and solid phases at equilibrium. Indeed, NMR and DSC methods give different values for the solid fat index (SFI) (Walker and Bosin, 1971 Norris and Taylor, 1977) NMR values are much lower than those given by DSC below 20°C. For example, for milk fat at 5°C, DSC and NMR indicate 78.1% and 43.7% solid fat, respectively. The observed difference can be explained by the presence of an amorphous phase which, due to its melting enthalpy, is seen as a solid by the DSC method. Using time-domain NMR, Le Botlan et al. (1999) showed that in milk fat samples, an intermediate component exists between the solid and liquid phases, constituting about 6% of an aged milk fat. 

The major breakthroughs, however, have come from the use of high magnetic fields and further from the use of different multiple pulse sequences to manipulate the nuclear spins in order to generate more and more information time domain NMR spectroscopy, that is used to probe molecular dynamics in solutions. The latter made it also possible to "edit" sub-spectra and to develop different two-dimensional (2D) techniques, where correlation between different NMR parameters can be made in the experiment (e.g. SH versus 813c, see later). Solid state NMR spectroscopy is used to determine the molecular structure of solids. 

F.S. DiGennaro, D. Cowburn, Parametric Estimation of time-domain NMR signals using simulated annealing. Journal of Magnetic Resonance, 96 (1992) 582. 

The NMR signal obtained from the resonating nuclei after the sample has been irradiated by the pulse is the so-called free-induction decay curve. This curve consists of peaks and valleys. The spectrometer samples the free-induction decay curve at set time intervals and records the data, which are in a time domain. NMR spectra, however, are normally given in terms of frequency and therefore the spectrum must be transformed by use of the Fourier transform pairs ... 

It is customary to analyse time-domain NMR signals / (0 in terms of their spectra relating to amplitude and phase of harmonic waves. This decomposition of /(/) is achieved by Fourier transformation. However, the harmonic waves are implicitely assumed to extend... 

Time domain NMR was used to elucidate the molecular mechanisms involved in the plasticization and mobility in starch-sorbitol films.129... 

Having said that the Fourier transform process enables us to transform information in the time domain into equivalent information in the frequency domain and vice versa, we will now consider the NMR experiment in the time domain. NMR is possible because nuclei of many atoms possess magnetic moments and angular momenta. (Electrons possess moments... 

T. C. Farrar and E. D. Becker, Pulse and Fourier Transform NMR (Academic Press, New York, 1971). This was the first book on time domain NMR. It is elementary and readable. 

A special publication of the Royal Society of Chemistry on Magnetic Resonance in Food Science discusses many aspects of the use of conventional and less conventional NMR methods in food analysis (such as oil, meat, beer, wine, moisture in biopolymers) including multivariate analysis of time domain NMR (TD-NMR) °, the use of an NMR-MOUSE (mobile universal surface explorer) for portable NMR and a review of the challenges in transferring NMR technology to the on-line industrial situation . Quantitative analysis methods of mixtures of fatty compounds by H -NMR are reviewed. Rapid simultaneous determination by H NMR of unsaturation and composition of acyl groups have also been reported in vegetable oils. The use of IR, Raman, NMR, and MS was reviewed for the analysis of polysaccharides related to food. ... 

Rutledge DN (2001) Characterization of water in agrofood products by time domain-NMR. Food Control 12 437- 45. 

Todt H, Burk W, Guthausen G, et al. (2001) Quality control with time-domain NMR. European Journal of Lipid Science and Technology 103 835-840. 

This edition has been completely updated, revised, and expanded. To achieve this, the previous approach of having each chapter be self-contained has been abandoned repetition has been reduced to a minimum so that more topics could be covered in more detail. The topics of chromatography and mass spectrometry have been greatly expanded, when compared with the sixth edition, to better reflect the predominance of chromatography and mass spectrometry instrumentation in modern laboratories. The equally important topic of NMR, expanded in the last edition to focus on ETNMR, C, and 2D NMR spectral interpretation, now includes time domain NMR (relaxometry) and an overview of low-field, benchtop, and miniature instrumentation. The topic of electron spin resonance spectroscopy (ESR, EPR) has been added due to the recent availability of small, low-cost ESR instrumentation and its impact on materials characterization and bioanalysis. Chapter 3 has therefore been renamed to reflect the inclusion of ESR/EPR. Eorensic science applications have been added in appropriate chapters. 

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