Nuclear Quadrupole Resonance (NQR)—A Useful Spectroscopic Tool in Pharmacy for the Study of Polymorphism

Non-invasive authentication of mail packages using nuclear quadrupole resonance spectroscopy

The international postal network is one of the most widely used methods for correspondence throughout the world. Most postal traffic across the globe consists of legitimate interpersonal, business-consumer, and business-business communications. However, the global postal system is also utilized for criminal activity. In particular, it is often utilized to ship and distribute contraband, including illegal psychoactive drugs such as fentanyl and heroin, to consumers. Existing technological solutions are capable of identifying synthetic opioids and other illegal drugs within packages, but are accompanied by several disadvantages that make them unsuitable for large-scale authentication of international mail traffic. This paper presents a novel method for non-invasive authentication of mail packages that overcomes these challenges. The approach uses nuclear quadrupole resonance (NQR) spectroscopy to detect and quantify the presence of known active pharmaceutical ingredients (APIs) within the package. It has been experimentally demonstrated using a bench top prototype. Test results from a variety of package types demonstrate the effectiveness of the proposed authentication approach.

Investigating and Comparing the Applicability of the R3m Molecular Descriptor and Solubility Parameter Estimation Approaches in Predicting Dispersion Formation Potential of APIs in a Random Co-Polymer Polyvinylpyrrolidone Vinyl Acetate and its Homopolymer

Evaluation of different amorphous solid dispersion carrier matrices is enabled by active pharmaceutical ingredient (API) structure-based predictions. This study compares the utility of Hansen Solubility Parameters with the R3m molecular descriptor for identifying dispersion polymers based on the structure of the drug molecule. Twelve API-polymer combinations (4 APIs and 3 interrelated polymers) were used to test each approach. Co-solidified mixtures containing 75% API were prepared by melt-quenching. Phase behavior was evaluated and classified using differential scanning calorimetry, powder X-ray diffraction, polarized light microscopy, and hot stage microscopy. Observations of dispersion behavior were compared to predictions made using the Hansen Solubility Parameter and R3m. The solubility parameter approach misclassified the dispersion behavior of 1 API-polymer combination and also did not produce definite predictions in 3 out of 12 of the API-polymer combinations. In contrast, R3m classifications of dispersion behavior were correct in all but two cases, with one misclassification and one ambiguous prediction. The solubility parameters best classify dispersion behavior when specific drug-polymer intermolecular interactions are present, but may be less useful otherwise. Ultimately, these two methods are most effectively used together, as they are based on distinct features of the same molecular structure.

Interleaved NQR detection using atomic magnetometers

Interleaved Nuclear Quadrupole Resonance (NQR) detection was conducted on ammonium nitrate and potassium chlorate using two ⁸⁷Rb magnetometers, where potassium chlorate is measured during the T₁ limited recovery time of ammonium nitrate. The multi-pass magnetometers are rapidly matched to the NQR frequencies, 531 kHz and 423 kHz, with the use of a single tuning field. For ease of implementation, a double resonant tank circuit was used for excitation, but could be replaced by a broad-band transmitter. All work was done in an unshielded environment and compared to conventional coil detection. The two magnetometers were sensitive, base noise as low as 2 fT/Hz, and were shown to reduce ambient noise through signal subtraction. When an excitation pulse was introduced, however, residual ringing increased the noise floor; mitigation techniques are discussed. The two detection techniques resulted in comparable Signal-to-Noise Ratio (SNR). Interleaved detection using the atomic magnetometers took half the time of conventional detection and provided localization of the explosives.

The 1H T1 dispersion curve of fentanyl citrate to identify NQR parameters

We report the ¹H T₁ dispersion curve between 0 and 5 ​MHz for the synthetic opioid fentanyl citrate (C₂₈H₃₆N₂O₈). The structures in the curve can be used to estimate the ¹⁴N nuclear quadrupole resonance (NQR) frequencies of the material. Density functional theory predictions of the NQR parameters of several fentanyl citrate compounds are also reported. The predictions for the aniline nitrogen are consistent with structures in the observed T₁ data. To help interpret the fentanyl citrate results the T₁ dispersion curve for the explosive ammonium nitrate is also presented.

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure-property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.