Nuclear Quadrupole Resonance (NQR)Trontelj, Zvonko (Avtor) Pirnat, Janez (Avtor) Jazbinšek, Vojko (Avtor) Lužnik, Janko (Avtor) Srčič, Stanko (Avtor) Lavrič, Zoran (Avtor) Beguš, Samo (Avtor) Apih, Tomaž (Avtor) Žagar, Veselko (Avtor) Seliger, Janez (Avtor) NQRNuclear Quadrupole Resonance spectroscopy$^{14}$N NQRrelaxationdrug polymorphismcrystal polymorphismX-ray powder diffraction (XRPD)powder mixture quantificationNQR with optically pumped magnetometerNuclear Quadrupole Resonance (NQR) spectroscopy has been known for 70 years. It is suitable for the study of measured (poly)crystalline chemical compounds containing quadrupole nuclei (nuclei with spin I ≥ 1) where the characteristic NQR frequencies represent the fingerprints of these compounds. In several cases, $^{14}$N NQR can distinguish between the polymorphic crystalline phases of active pharmaceutical ingredients (APIs). In order to further stimulate $^{14}$N NQR studies, we review here several results of API polymorphism studies obtained in Ljubljana laboratories: (a) In sulfanilamide, a clear distinction between three known polymorphs (α, β, γ) was demonstrated. (b) In famotidine, the full spectra of all seven different nitrogen positions were measured; two polymorphs were distinguished. (c) In piroxicam, the $^{14}$N NQR data helped in confirming the new polymorphic form V. (d) The compaction pressure in the tablet production of paracetamol, which is connected with linewidth change, can be used to distinguish between producers of paracetamol. We established that paracetamol in the tablets of six different manufacturers can be identified by $^{14}$N NQR linewidth. (e) Finally, in order to get an extremely sensitive $^{14}$N NQR spectrometer, the optical detection of the $^{14}$N NQR signal is mentioned.20202021-12-06 12:24:59Članek v reviji133626UDK: 53:543.42ISSN pri članku: 2073-4352DOI: 10.3390/cryst10060450COBISS_ID: 17705475sl