Tomaž Apih

Nuclear Quadrupole Double Resonance Techniques for the Detection of Explosives and Drugs

Recent work of the Ljubljana group in the nuclear quadrupole double resonance detection of explosives, mines, narcotics and drugs is reviewed with particular emphasis on enhancing low-frequency nuclear quadrupole resonance signals.

Surface-induced order and diffusion in 5CB liquid crystal confined to porous glass

Liquid crystals confined into small cavities are known to have a weak orientational order even above the nematic-isotropic transition temperature. The surface-induced order and molecular dynamics in this temperature range are studied with the aid of deuteron NMR spectra, spin relaxation times T(1) and T(2,) proton dipolar-correlation effect, and direct measurements of the effective diffusion coefficient for the liquid crystal 5CB confined to controlled-pore glasses. Our results show that an arrangement of molecules parallel to the wall is induced by local molecular interactions between the liquid crystal and solid, resulting in a weak and temperature independent surface order parameter, S(0) approximately 0.02 +/- 0.01. There is no indication of a significant slowing-down of molecular diffusion at the wall, neither rotational nor translational. In cavities of nanometer size, where the nematic order evolves gradually upon cooling, a broadening of the NMR linewidths due to dynamic effects should be taken into account.

TNT detection with 14N NQR: Multipulse sequences and matched filter

Nuclear quadrupole resonance (NQR) has a distinct potential to verify the presence of nitrogen bearing substances based on the unequivocal signatures of their spectra. Therefore, this technique is especially suitable for remote detection of illicit substances and explosives. Unfortunately, the inherent signal-to-noise of the most abundant explosive trinitrotoluene (TNT) is very low. Here we present an NQR method with improved sensitivity for estimation of the probability of TNT presence in the investigated object. The method consists of a spin-lock spin-echo (SLSE) multipulse sequence for signal excitation and a time domain matched filter for signal detection. We find that the signal-to-noise increases by shortening the pulse spacings, even though this means a decrease in spectral resolution. In our case, the decrease of the pulse spacings from the typical 2 ms to 540 micros resulted in an increase of the signal-to-noise by 14 dB. A theory describing this enhancement is presented and compared to experimental results on TNT. Issues related to temperature and polymorphism variations are also discussed.

Relaxation during spin-lock spin-echo pulse sequence in N14 nuclear quadrupole resonance

In this work, we investigate off-resonance effect on the (14)N nuclear quadrupole resonance magnetization decay during the spin-lock spin-echo pulse sequence (SLSE). The compound chosen for this study is paranitrotoluene with a single (14)N site, which represents a suitable simplified model for the explosive trinitrotoluene with six nonequivalent (14)N sites. We find that the quasi-steady state magnetization exhibits dips at particular frequency offsets and more interestingly that its decay rate T(2 eff) (-1) exhibits similar dips (slower decay) at the same frequency offsets. The coexistence of dips is very important for applications where the primary use of the SLSE sequence is to increase the signal-to-noise ratio, as longer sampling times compensate for small magnetization values. A theory explaining both observations is presented which includes homonuclear dipolar interactions and spin coupling to the lattice. We show that the homonuclear dipolar interaction contributes only 20% to the total magnetization decay rate, while spin-lattice coupling is the dominant mechanism.

Phase Transition and Ring-Puckering Motion in a Metal–Organic Perovskite [(CH2)3NH2][Zn(HCOO)3]

Phase transitions in a metal-organic perovskite with an azetidinium cation, which exhibits giant polarizability, were investigated using differential scanning calorimetry (DSC) and (1)H nuclear magnetic resonance (NMR) measurements. The DSC results indicated successive phase transitions at 254 and 299 K. The temperature dependence of the spin-lattice relaxation time T(1) determined by NMR indicated that the activation energy for cation ring-puckering motion was 25 kJ mol(-1) in phase I (T > 299 K). The potential energy at the transition state of puckering is expected to decrease when the potential for the motion becomes asymmetric with decreasing temperature in phases II and III. A possible mechanism for the onset of an extraordinarily large dielectric anomaly is discussed.

1H NMR relaxometry study of a rod-like chiral liquid crystal in its isotropic, cholesteric, TGBA*, and TGBC* phases.

The molecular dynamics of a chiral liquid crystal showing a rich variety of frustrated mesophases has been investigated by means of 1H NMR relaxometry. The interest in this lactate derivative, HZL 7/*, is related to a large range of thermal stabilities of the twist grain boundary (TGB) phases. Dispersions of the 1H spin-lattice relaxation times, T1, in the frequency range from 300 MHz to 5 kHz were measured and consistently analyzed in the isotropic, chiral nematic, TGBA*, and two TGBC* phases. In the isotropic and N* phases, a three-exponential magnetization decay was observed and assigned to three specific molecular groups of the HZL 7/* (molecular core, methyl, and methylene groups). In the TGB phases, all T1 components merge into a single one. The analysis of the T1 dispersion in the TGBA* phase shows that the translational self-diffusion relaxation mechanism dominates over a broad frequency range and that layer undulations are less relevant than the relaxation contribution associated with the diffusion process across the TGB structure. In the TGBC1* phase, the T1 dispersion presents a strong contribution of in-layer tilt direction fluctuations (T1(-1) proportional to ν(-1/2)), while, in the TGBC2* phase, the linear frequency dependence of T1 could be associated with a much stronger contribution of layer undulations than for the other TGB phases. This is at present the first molecular dynamics investigation on several TGB phases by means of 1H NMR relaxometry.

Improved N14 nuclear quadrupole resonance detection of trinitrotoluene using polarization transfer from protons to N14 nuclei

Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes N14 nuclear quadrupole resonance (NQR) in trinitrotoluene fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for all three N14 NQR lines (the case with η≠0) were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. N14 NQR signals in a 15g trinitrotoluene sample of predominantly monoclinic modification were measured in 15s in different polarization magnetic fields. The conditions for optimal proton-nitrogen level crossing were determined.

Improving 14N nuclear quadrupole resonance detection of trinitrotoluene using off-resonance effects

The off-resonance dependence of the amplitudes of the six dominant (14)N nuclear quadrupole resonance (NQR) lines in commercial polymorphic trinitrotoluene (TNT) sample were experimentally determined for a wide range of experimental parameters when irradiated with the spin-lock spin-echo (SLSE) pulse sequence. We find that the amplitudes off-resonance dependence follows a sinc-like function with an additional modulation due to the spacing between the RF pulses. This dependence can be very well modeled with expressions we have derived for a single site (14)N NQR in paranitrotoluene (PNT). The results can be immediately used for the reduction of the number of free parameters used in the robust signal processing models for the TNT NQR detectors.

Proton spin–lattice relaxation study of the hydration of self-stressed expansive cement

Abstract Proton spin–lattice relaxation, compressive strength, and water tightness measurements have been undertaken in order to study the hydration process and the mechanical properties of Portland cement modified with a calcium sulfoaluminate-based expansive additive (EXPAD). The compression strength of the modified cement was more than 70% higher while water permeability was about 60% smaller than in the unmodified cement. Magnetization–recovery curves analysis allowed for a time-resolved monitoring of both the increase of the internal surface as well as the formation of solid products during the hydration. A significant difference between the hydration kinetics of modified and unmodified cement was observed.

Two-dimensional nuclear magnetic resonance study of a hydrated porous medium: An application to white cement

Structure and dynamics of a hydrated white cement were investigated by the T1-weighted lineshape, two-dimensional (2D) exchange, and 2D separation of inhomogeneous and homogeneous lineshapes nuclear magnetic resonance (NMR) techniques. T1 weighting of the proton spectrum eliminates the strong bulk water signal from the pores so that the weaker spectrum of the solid cement matrix becomes observable. The proton spectrum of the solid cement fraction exhibits a characteristic Pake doublet shape and shows a close similarity to the powder spectra of pure calcium hydroxide and the crystalline water of gypsum. The 2D exchange NMR spectrum of white cement demonstrates the existence of slow chemical exchange processes of protons in the solid matrix on a sub-kHz frequency scale. This exchange is orders of magnitude slower than the exchange between the surface and bulk water. The 2D NMR separation of inhomogeneous and homogeneous lineshapes technique demonstrates that the absorption lines of a set white cement are in...