Nuclear quadrupole resonance (NQR) enhancement by polarization transfer and its limitation due to relaxation
TL;DR: In this article, the authors examined the potential and limitations of the polarization transfer enhancement of 14N nuclear quadrupole resonance (NQR) for the detection of explosives with low NQR frequencies.
Abstract: Aiming for polarization transfer enhancement of 14N nuclear quadrupole resonance (NQR) for the detection of explosives with low NQR frequencies, we examine the potential and limitations of this method. As illustrative sample materials two non-explosive compounds, urotropine (C6H12N4) and urea (CON2H4) with NQR frequencies of 3.3 MHz and 2.8 MHz, respectively, have been chosen. In both substances the NQR signal can be easily seen. In urotropine no signal enhancement has been detected. The reason is a 14N spin-lattice relaxation time being much shorter than the 1H–14N polarization transfer time. Although in urea the signal enhancement is significant there is, because of the long 1H polarization time, still no effective gain as compared with the pure NQR signal accumulated during the same time interval. To estimate the expected NQR signal enhancement, a polarization enhancement factor has been derived in terms of a simplified theoretical treatment, neglecting spin-lattice relaxation. The substantial influence of relaxation effects on the signal enhancement has been discussed in a qualitative manner in connection with the experiments performed for urea and urotropine.
Citations
More filters
TL;DR: With improved instrumentation and with the help of relaxation theory, field cycling NMR instrumentation gets access to interesting new applications such as ionic motion in solid electrolytes, structure determination in molecular crystals, ultraslow polymer dynamics and rotational resonance phenomena.
89 citations
Cites background from "Nuclear quadrupole resonance (NQR) ..."
...22) for the enhancement of the (14)N polarization we present our zero field (14)N NMR signal amplitudes in urea (CON2H4) measured as a function of the evolution field [96] (Fig....
[...]
01 Jan 2014
11 citations
Cites background from "Nuclear quadrupole resonance (NQR) ..."
...It is another sample that is often adopted for NQR experimental studies [117, 118]....
[...]
01 Jan 2014
4 citations
Cites background from "Nuclear quadrupole resonance (NQR) ..."
...Cross-polarization techniques o↵er particular promise for enhancing NQR signals [42, 59, 63, 71, 78]....
[...]
01 Jan 2009
2 citations
References
More filters
Book•
01 Jan 1969
6,280 citations
TL;DR: The design, construction, and performance of a low-inductance solenoidal coil with high B(0) homogeneity for fast-field-cycling NMR is presented and has shown good stability and reliability.
65 citations
TL;DR: In this article, the possibilities of detecting nuclear quadrupole resonance (NQR) signals in explosives and drugs are considered. Direct and indirect NQR techniques for searching substances are described and the potentialities of various experimental methods are compared.
Abstract: Possibilities of detecting nuclear quadrupole resonance (NQR) signals in explosives and drugs are considered. Direct and indirect NQR techniques for searching substances are described and the potentialities of various experimental methods are compared.
59 citations
55 citations
TL;DR: In this paper, cross-relaxation spectroscopy was used as a sensitive method of detecting 14 N quadrupole-resonance signals in hydrogen-containing solids.
Abstract: Cross-relaxation spectroscopy can be used as a sensitive method of detecting 14 N quadrupole-resonance signals in hydrogen-containing solids. The 1 H spin system is polarized in a high magnetic field that is then reduced adiabatically to a much lower value satisfying the level-crossing condition, when the 1 H Zeeman splitting matches one of the 14 N quadrupole splittings. If the 14 N spin–lattice relaxation time is much shorter than that of the 1 H nuclei, a drastic loss of 1 H polarization occurs that is measured by recording the residual 1 H magnetic resonance signal after the sample has been returned to the higher field. The experimental cycle can be run in several different ways according to the relative values of the 1 H spin–lattice relaxation times ( T 1 ) in high and low field, the 14 N spin–lattice relaxation ( T 1Q ) and cross-polarization times ( T CP ), all of which can markedly influence the spectra. The line shapes are broadened by the presence of the magnetic field and Zeeman shifts of the peak frequencies also occur, for which simple corrections may be derived. The methods used have high sensitivity, particularly if the ratio T 1 / T 1Q is large. They have the advantage over other double-resonance techniques in that long proton T 1 values are not necessary for the success of an experiment; it is also possible to select conditions in which the recovered 1 H signal is directly proportional to the relative numbers of 14 N nuclei present and the magnitude of the cross-relaxation field. Multi-proton relaxation jumps also give rise to signals at subharmonics of the fundamental, whose relative intensities reflect the extent to which the 14 N and 1 H relaxation is coupled via their dipole–dipole interactions, which are not completely quenched in the finite magnetic fields necessary in cross-relaxation spectroscopy. These conclusions are illustrated in a number of 14 N spectra of compounds in which quadrupole-resonance signals have not previously been recorded.
53 citations