Background subtraction is a general problem in spectroscopy often addressed with application-specific techniques, or methods that introduce a variety of implementation barriers such as having to specify peak-free regions of the spectrum. An iterative dual-tree complex wavelet transform-based background subtraction method (DTCWT-IA) was recently developed for the analysis of ultrafast electron diffraction patterns. The method was designed to require minimal user intervention, to support streamlined analysis of many diffraction patterns with complex overlapping peaks and time-varying backgrounds, and is implemented in an open-source computer program. We examined the performance of DTCWT-IA for the analysis of spectra acquired by a range of optical spectroscopies including ultraviolet–visible spectroscopy (UV–Vis), X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman spectroscopy (SERS). A key benefit of the method is that the user need not specify regions of the spectrum where no peaks are expected to occur. SER spectra were used to investigate the robustness of DTCWT-IA to signal-to-noise levels in the spectrum and to user operation, specifically to two of the algorithm parameter settings: decomposition level and iteration number. The single, general DTCWT-IA implementation performs well in comparison to the different conventional approaches to background subtraction for UV–Vis, XPS, and SERS, while requiring minimal input. The method thus holds the same potential for optical spectroscopy as for ultrafast electron diffraction, namely streamlined analysis of spectra with complex distributions of peaks and varying signal levels, thus supporting real-time spectral analysis or the analysis of data acquired from different sources.
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Kernel principle component analysis applied to Raman spectra to differentiate drugs administered to rabbit cornea in blind study
Scanning confocal Raman spectroscopy was applied for detecting and identifying topically applied ocular pharmaceuticals on rabbit corneal tissue. Raman spectra for Cyclosporin A, Difluprednate, and Dorzolamide were acquired together with Raman spectra from rabbit corneas with an unknown amount of applied drug. Kernel principle component analysis (KPCA) was then used to explore a transform that can describe the acquired set of Raman spectra. Using this transform, we observe some spectral similarity between cornea spectra and Cyclosporin A, with little similarity to Dorzolamide and Difluprednate. Further investigation is needed to identify why these differences occur.
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- Award ID(s):
- 1810995
- NSF-PAR ID:
- 10173485
- Date Published:
- Journal Name:
- Kernel principle component analysis applied to Raman spectra to differentiate drugs administered to rabbit cornea in blind study
- Page Range / eLocation ID:
- 6
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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A spatial heterodyne Raman spectrometer (SHRS), constructed using a modular optical cage and lens tube system, is described for use with a commercial silica and a custom single-crystal (SC) sapphire fiber Raman probe. The utility of these fiber-coupled SHRS chemical sensors is demonstrated using 532 nm laser excitation for acquiring Raman measurements of solid (sulfur) and liquid (cyclohexane) Raman standards as well as real-world, plastic-bonded explosives (PBX) comprising 1,3,5- triamino- 2,4,6- trinitrobenzene (TATB) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) energetic materials. The SHRS is a fixed grating-based dispersive interferometer equipped with an array detector. Each Raman spectrum was extracted from its corresponding fringe image (i.e., interferogram) using a Fourier transform method. Raman measurements were acquired with the SHRS Littrow wavelength set at the laser excitation wavelength over a spectral range of ∼1750 cm−1with a spectral resolution of ∼8 cm−1for sapphire and ∼10 cm−1for silica fiber probes. The large aperture of the SHRS allows much larger fiber diameters to be used without degrading spectral resolution as demonstrated with the larger sapphire collection fiber diameter (330 μm) compared to the silica fiber (100 μm). Unlike the dual silica fiber Raman probe, the dual sapphire fiber Raman probe did not include filtering at the fiber probe tip nearest the sample. Even so, SC sapphire fiber probe measurements produced less background than silica fibers allowing Raman measurements as close as ∼85 cm−1to the excitation laser. Despite the short lengths of sapphire fiber used to construct the sapphire probe, well-defined, sharp sapphire Raman bands at 420, 580, and 750 cm−1were observed in the SHRS spectra of cyclohexane and the highly fluorescent HMX-based PBX. SHRS measurements of the latter produced low background interference in the extracted Raman spectrum because the broad band fluorescence (i.e., a direct current, or DC, component) does not contribute to the interferogram intensity (i.e., the alternating current, or AC, component). SHRS spectral resolution, throughput, and signal-to-noise ratio are also discussed along with the merits of using sapphire Raman bands as internal performance references and as internal wavelength calibration standards in Raman measurements.
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Infrared and Raman spectroscopy techniques were applied to investigate the drying and aggregation behavior of Nafion ionomer particles dispersed in aqueous solution. Gravimetric measurements aided the identification of gel-phase development within a series of time-resolved spectra that tracked transformations of a dispersion sample during solvent evaporation. A spectral band characteristic of ionomer sidechain end group vibration provided a quantitative probe of the dispersion-to-gel change. For sets of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra, adherence to Beer’s law was attributed to the relatively constant refractive index in the frequency region of hydrated -SO3 - group vibrations as fluorocarbon-rich ionomer regions aggregate in forming the structural framework of membranes and thin films. Although vibrational bands associated with ionomer backbone CF2 stretching vibrations were affected by distortion characteristic of wavelength-dependent refractive index change within a sample, the onset of band distortion signaled gel formation and coincided with ionomer mass % values just below the critical gelation point for Nafion aqueous dispersions. Similar temporal behavior was observed in confocal Raman microscopy experiments that monitored the formation of a thin ionomer film from an individual dispersion droplet. For the ATR FTIR spectroscopy and confocal Raman microscopy techniques, intensity in the water H-O-H bending vibrational band dropped sharply at the ionomer critical gelation point and displayed a time dependence consistent with changes in water content derived from gravimetric measurements. The reported studies lay groundwork for examining the impact of dispersing solvents and above-ambient temperatures on fluorinated ionomer transformations that influence structural properties of dispersion-cast membranes and thin films.more » « less
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Abstract Stand‐off Raman spectroscopy combined with chemometrics was demonstrated to have great potential for detecting and identifying biological stains from a distance for forensic purposes. Bloodstain spectra were collected using a handheld Raman spectrometer equipped with a stand‐off attachment and compared with spectra acquired with a benchtop Raman microscope. The two sets of spectra were visually similar, supporting the potential use of a handheld Raman spectrometer with stand‐off attachment for blood detection at the scene of crime. In addition, the stand‐off Raman spectra of bloodstains were correctly identified using a previously built SVM‐DA model for the identification of all main body fluids (Muro et al., Forensic Chemistry, 2016). Stand‐off Raman spectroscopy opens a new potential opportunity for further development of Raman spectroscopy as a universal tool for detecting and identifying body fluid traces for forensic purposes.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1117/12.2546431
