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Raman spectroscopy is one of many tools available to verify the molecular composition of an analyte. Due to its non-destructive nature and its ability to accurately discern differences in closely related molecular structures, it has become invaluable in many fields, including its potential for forensic gunshot residue detection. Firearm-related fatalities in the United States continue to rise and many of them remain unsolved. This necessitates tools that are better equipped to aid in the investigation of firearm-related crimes, capable of high-throughput analysis yet remaining sensitive to give accurate and valuable information. The comparative downside of Raman spectroscopy to neighboring techniques like mass spectrometry and nuclear magnetic resonance is its lower sensitivity. Surface-enhanced Raman spectroscopy allows for the benefits of Raman spectroscopy alongside the added lower limit of detection with the simple application of nanoparticles, typically gold. Unfortunately, these benefits have seen little on-site application due to the difficulty of translating methods from conventional tabletop Raman spectrometers to point-and-shoot portable Raman spectrometers which have even lower sensitivities (higher limits of detection). Herein, we outline a versatile methodology for the detection of 6 organic gunshot residue components (diphenylamine (DPA), ethyl centralite (EC), 2,4-dinitrotoluene (2,4-DNT), 2-nitrodiphenylamine (2-nDPA), 4-nitrodiphenylamine (4-nDPA), and N-nitrosodiphenylamine (N-nDPA)) in liquid-phase that allows us to detect millimolar concentrations of these analytes. Furthermore, we report calculated vibrational assignments for these 6 analytes in solution, alongside detailed peak-by-peak analyses on a portable instrument. We showed signal enhancement and lower LODs through our data processing as well as a proof-of-concept SERS enhancement in a complex liquid-phase matrix, with an increased sensitivity of 700% when using SERS. [Display omitted] • Detailed vibrational spectra for 6 gunpowder residues using a portable Raman. • Computational vibrational analyses of 2- and 4-nitrodiphenylamine. • Homogenous detection methodology for organic gunshot residue analytes. • Improved detection limit with pre-processing using a portable Raman and SERS. [ABSTRACT FROM AUTHOR]

Fentanyl and its analogs have been at the center of the opioid epidemic currently wreaking havoc in the United States. One major element in the opioid crisis is the growing number of clandestine fentanyl labs being reported by enforcement agencies. The development of new analytical methods for detecting and identifying fentanyl and its congeners is among the useful tools in our goal to limit the use of this dangerous family of narcotics. Herein we describe an analytical technique using surface-enhanced Raman spectroscopy (SERS) and a microfluidic device, for detecting fentanyl and two of its chemical precursors, despropionylfentanyl (4ANPP) and N-phenethyl-4-piperidinone (NPP). The vibrational spectra of this family of analytes are very similar, making them difficult to distinguish by traditional means. In addition to taking advantage of the sensitivity provided by SERS, we developed a chemometric approach utilizing a hierarchical partial least squares-discriminant analysis algorithm that allowed us to distinguish spectra that possess many similar features. [ABSTRACT FROM AUTHOR]

The structure, equation of state, mechanical and vibrational properties of single crystals of ices II, VIII, XI and ice in a previously predicted phase bct-4 are studied within the Hartree-Fock method using the density functional theory (B3LYP hybrid functional) with localized Gaussian atomic orbitals as implemented in the "CRYSTAL14" software package. Calculated lattice parameters, elastic moduli of single crystals, and mechanical parameters of polycrystalline samples are shown to meet the criteria of mechanical stability. Parameters of the Vinet equation of state are obtained. Energy band widths and Mulliken atomic charges are determined using obtained energy and spatial distribution of electrons. The frequencies of long-wave vibrations are calculated and vibrational spectra are constructed. [ABSTRACT FROM AUTHOR]