Miniature, lightweight nuclear-magnetic-resonance (NMR) spectrometers suitable for characterizing ferromagnetic minerals in the field are undergoing development. In previously developed miniature NMR spectrometers, more than half the weight is contributed by permanent magnets. The present developmental miniature NMR spectrometers can be made much lighter because they do not contain permanent magnets: Unlike other NMR spectrometers, they are designed to operate without applied magnetic fields; instead, they exploit the natural magnetic fields of the mineral phases to be studied.

Figure 1. This Miniature NMR Spectrometer has a mass of only 65 g (this includes the battery) and consumes a power of only 0.2 W.
Figure 2. These NMR Spectra of specimens of magnetite and hematite were obtained by use of the instrument shown in Figure 1.
Figure 1 shows a prototype instrument of this type designed to exploit the natural magnetic fields in minerals that contain iron. These fields give rise to nuclear magnetic resonance of the isotope 57Fe at frequencies in the approximate range of 60 to 74 MHz. Resonances occur at specific frequencies associated with mineral phases of interest. For example, magnetite (Fe304) exhibits a resonance at 63.8 MHz for 57Fe3 ions in octahedral sites (coordination with oxygen atoms) and another resonance at 67.6 for 57Fe3 at tetrahedral sites.

This instrument includes a marginal oscillator, the frequency of which is determined mainly by tuning capacitors, two varactors, and the NMR sample coil, in which a mineral specimen is placed. During operation, the frequency is swept slowly by use of one varactor, and is modulated at a rate of 110 Hz by use of the other varactor. The instrument also includes a digital-to-analog and an analog-to-digital converter and a microprocessor that communicates with an external laptop computer, generates frequency-sweep and modulation signals, samples the output from the oscillator, and performs synchronous detection.

The instrument is connected with the computer via a telephone (RS232) cable. Such parameters as the scan range, scan rate, number of averages, and time constant can be set through the computer keyboard. Spectral data can be displayed on the computer screen. The software in the computer includes routines for processing data to estimate concentrations of mineral phases of interest.

The performance of the instrument was demonstrated in NMR measurement experiments on two mineral specimens: one that contained magnetite in chlorite schist mixed with magnetically inert particles and one made of hematite (α–Fe2O3). The spectra obtained from these specimens (see Figure 2) show the expected magnetite resonances and a hematite resonance at about 70.8 MHz.

This work was done by Soon Sam Kim, Narayan Mysoor, and Christopher Ulmer of Caltech for NASA’s Jet Propulsion Laboratory.


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This article first appeared in the June, 2002 issue of NASA Tech Briefs Magazine.

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