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Forland BM, Hughey KD, Wilhelm MJ, Williams ON, Cappello BF, Gaspar CL, Myers TL, Sharpe SW, Johnson TJ. Optimal Spectral Resolution for Infrared Studies of Solids and Liquids. APPLIED SPECTROSCOPY 2024; 78:486-503. [PMID: 38404070 DOI: 10.1177/00037028241231601] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/27/2024]
Abstract
Due to a legacy originating in the limited capability of early computers, the spectroscopic resolution used in Fourier transform infrared spectroscopy and other systems has largely been implemented using only powers of two for more than 50 years. In this study, we investigate debunking the spectroscopic lore of, e.g., using only 2, 4, 8, or 16 cm-1 resolution and determine the optimal resolution in terms of both (i) a desired signal-to-noise ratio and (ii) efficient use of acquisition time. The study is facilitated by the availability of solids and liquids reference spectral data recorded at 2.0 cm-1 resolution and is based on an examination in the 4000-400 cm-1 range of 61 liquids and 70 solids spectra, with a total analysis of 4237 peaks, each of which was also examined for being singlet/multiplet in nature. Of the 1765 liquid bands examined, only 27 had widths <5 cm-1. Of the 2472 solid bands examined, only 39 peaks have widths <5 cm-1. For both the liquid and solid bands, a skewed distribution of peak widths was observed: For liquids, the mean peak width was 24.7 cm-1 but the median peak width was 13.7 cm-1, and, similarly, for solids, the mean peak width was 22.2 cm-1 but the median peak width was 11.2 cm-1. While recognizing other studies may differ in scope and limiting the analysis to only room temperature data, we have found that a resolution to resolve 95% of all bands is 5.7 cm-1 for liquids and 5.3 cm-1 for solids; such a resolution would capture the native linewidth (not accounting for instrumental broadening) for 95% of all the solids and liquid bands, respectively. After decades of measuring liquids and solids at 4, 8, or 16 cm-1 resolution, we suggest that, when accounting only for intrinsic linewidths, an optimized resolution of 6.0 cm-1 will capture 91% of all condensed-phase bands, i.e., broadening of only 9% of the narrowest of bands, but yielding a large gain in signal-to-noise with minimal loss of specificity.
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Affiliation(s)
- Brenda M Forland
- Pacific Northwest National Laboratory, Richland, Washington, USA
| | - Kendall D Hughey
- Pacific Northwest National Laboratory, Richland, Washington, USA
| | | | | | | | - Connor L Gaspar
- Pacific Northwest National Laboratory, Richland, Washington, USA
| | - Tanya L Myers
- Pacific Northwest National Laboratory, Richland, Washington, USA
| | - Steven W Sharpe
- Pacific Northwest National Laboratory, Richland, Washington, USA
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Peterson KA, Francis RM, Banach CA, Bradley AM, Burton SD, Erickson JD, Lockwood SP, Jensen KL, Yokosuk MO, Johnson TJ, Myers TL. Method to derive the infrared complex refractive indices n(λ) and k(λ) for organic solids from KBr pellet absorption measurements. APPLIED OPTICS 2024; 63:1553-1565. [PMID: 38437368 DOI: 10.1364/ao.514661] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/29/2023] [Accepted: 01/23/2024] [Indexed: 03/06/2024]
Abstract
Obtaining the complex refractive index vectors n(ν~) and k(ν~) allows calculation of the (infrared) reflectance spectrum that is obtained from a solid in any of its many morphological forms. We report an adaptation to the KBr pellet technique using two gravimetric dilutions to derive quantitative n(ν~)/k(ν~) for dozens of powders with greater repeatability. The optical constants of bisphenol A and sucrose are compared to those derived by other methods, particularly for powdered materials. The variability of the k values for bisphenol A was examined by 10 individual measurements, showing an average coefficient of variation for k peak heights of 5.6%. Though no established standards exist, the pellet-derived k peak values of bisphenol A differ by 11% and 31% from their single-angle- and ellipsometry-derived values, respectively. These values provide an initial estimate of the precision and accuracy of complex refractive indices that can be derived using this method. Limitations and advantages of the method are discussed, the salient advantage being a more rapid method to derive n/k for those species that do not readily form crystals or specular pellets.
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Azam MS, Ranson MD, Hore DK. Temperature-Dependent Infrared Refractive Index of Polymers from a Calibrated Attenuated Total Reflection Infrared Measurement. APPLIED SPECTROSCOPY 2022; 76:1254-1262. [PMID: 35354313 PMCID: PMC9549256 DOI: 10.1177/00037028221094598] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/31/2021] [Accepted: 03/16/2022] [Indexed: 06/14/2023]
Abstract
We demonstrate a straightforward method by which a commonly available reference sample such as water can be used to calibrate an attenuated total internal reflection infrared absorbance measurement in order to account for the polarization of the beam incident on the internal reflecting element, and the spread of angles about the nominal angle of incidence. This enables quantitative comparison of attenuated total reflection-derived absorbance data with spectra calculated from optical constants. We then apply this calibration to the measurement of temperature-dependent absorption spectra of a polydimethylsiloxane sample. We illustrate that the extracted optical constants scale with the temperature-dependent changes in the polymer density better than the raw absorbance values on vibrational resonance.
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Affiliation(s)
- Md S. Azam
- Department of Chemistry,
University
of Victoria, Victoria, BC, Canada
| | - Malcolm D. Ranson
- Department of Chemistry,
University
of Victoria, Victoria, BC, Canada
| | - Dennis K. Hore
- Department of Chemistry,
University
of Victoria, Victoria, BC, Canada
- Department of Computer Science,
University
of Victoria, Victoria, BC, Canada
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Johnson TJ, Baker TJ, Bradley AM, Yokosuk MO, Myers TL. Twice-Modulated Light in Fourier Transform Infrared (FT-IR) Spectrometers from Reflective Samples: Avoiding Distorted Intensity Values. APPLIED SPECTROSCOPY 2022; 76:620-624. [PMID: 35220757 DOI: 10.1177/00037028211066336] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
We document an artifact associated with the back reflection from samples or sampling accessories in Fourier transform spectrometers. Samples oriented normal to the incoming modulated beam can reflect light back to the interferometer and this light (the percentage dependent on the sample's refractive index) is modulated by the interferometer a second time resulting in light erroneously appearing at twice its true frequency. The phenomenon occurs across the spectrum but is most apparent when positive-going narrow absorption peaks at 1f display as negative-going peaks at 2f. We have redressed the artifact by implementing a rotation stage directly beneath the sample holder: As the stage is rotated through small angles relative to beam normal, the back-reflected light does not enter the interferometer and the artifact disappears. The observation is relevant to several IR sampling methods: gas/liquid cells, alkali halide pellets, reflectance accessories, etc.
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Affiliation(s)
| | - Tracy J Baker
- 6865Pacific Northwest National Laboratory, Richland, WA, USA
| | | | | | - Tanya L Myers
- 6865Pacific Northwest National Laboratory, Richland, WA, USA
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Di Pietro SA, Emerson HP, Katsenovich YP, Johnson TJ, Francis RM, Mason HE, Marple MA, Sawvel AM, Szecsody JE. Solid phase characterization and transformation of illite mineral with gas-phase ammonia treatment. JOURNAL OF HAZARDOUS MATERIALS 2022; 424:127657. [PMID: 34785437 DOI: 10.1016/j.jhazmat.2021.127657] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/10/2021] [Revised: 10/07/2021] [Accepted: 10/27/2021] [Indexed: 06/13/2023]
Abstract
In situ remediation applications of ammonia (NH3) gas have potential for sequestration of subsurface contamination. Ammonia gas injections initially increase the pore water pH leading to mineral dissolution followed by formation of secondary precipitates as the pH is neutralized. However, there is a lack of understanding of fundamental alteration processes due to NH3 treatment. In these batch studies, phyllosilicate minerals (illite and montmorillonite) were exposed to NH3 gas with subsequent aeration to simulate in situ remediation. Following treatments, solids were characterized using a variety of techniques, including X-ray diffraction, N2 adsorption-desorption analysis for surface area, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and microscopy methods to investigate physicochemical transformations. Results indicate that, at high pH, the clays are altered as observed by differences in morphology and particle size via microscopy. However, the two clays interact differently with NH3. While montmorillonite interlayers collapsed due to intercalation, illite layers were unaffected as confirmed by FTIR analysis. Further, structural changes in silicate ([SiO4]n-) and aluminol (Al-OH) groups were identified by NMR and FTIR. This research showed that mineral alteration processes occur during and after NH3 gas treatment which may be used to remove radionuclides from the aqueous phase through sorption, co-precipitation, and coating with secondary phyllosilicate alteration products.
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Affiliation(s)
- Silvina A Di Pietro
- Applied Research Center, Florida International University, 10555 W. Flagler St., Miami, FL 33174, United States.
| | - Hilary P Emerson
- Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99354, United States
| | - Yelena P Katsenovich
- Applied Research Center, Florida International University, 10555 W. Flagler St., Miami, FL 33174, United States
| | - Timothy J Johnson
- Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99354, United States
| | - Ryan M Francis
- Department of Chemical Engineering University of Washington, Seattle, WA 98195, United States
| | - Harris E Mason
- Center for Nuclear Magnetic Resonance Spectroscopy, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States
| | - Maxwell A Marple
- Center for Nuclear Magnetic Resonance Spectroscopy, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States
| | - April M Sawvel
- Center for Nuclear Magnetic Resonance Spectroscopy, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States
| | - James E Szecsody
- Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99354, United States
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Fortin G. Calculation of Spectral Optical Constants Using Combined Ellipsometric and Reflectance Methods for Smooth and Rough Bulk Samples. APPLIED SPECTROSCOPY 2021; 75:1449-1460. [PMID: 34637350 DOI: 10.1177/00037028211047898] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Spectra of the optical constants n and k of a substance are often deduced from spectroscopic measurements, performed on a thick and homogeneous sample, and from a model used to simulate these measurements. Spectra obtained for n and k using the ellipsometric method generally produce polarized reflectance simulations in strong agreement with the experimental measurements, but they sometimes introduce significant discrepancies over limited spectral ranges, whereas spectra of n and k obtained with the single-angle reflectance method require a perfectly smooth sample surface to be viable. This paper presents an alternative method to calculate n and k. The method exploits both ellipsometric measurements and s-polarized specular reflectance measurements, and compensates for potential surface scattering effects with the introduction of a specularity factor. It is applicable to bulk samples having either a smooth or a rough surface. It provides spectral optical constants that are consistent with s-polarized reflectance measurements. Demonstrations are performed in the infrared region using a glass slide (smooth surface) and a pellet of compressed ammonium sulfate powder (rough surface).
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Affiliation(s)
- Gilles Fortin
- Defence Research and Development Canada, Valcartier Research Centre, Québec, Canada
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Yokosuk MO, Tiwald TE, Saunders DL, Blake TA, Myers TL. Combining spectroscopic techniques to determine the optical constants of powdered lactose. APPLIED OPTICS 2021; 60:2412-2421. [PMID: 33690346 DOI: 10.1364/ao.414107] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/10/2020] [Accepted: 02/20/2021] [Indexed: 06/12/2023]
Abstract
A method for deriving the optical constants (n/k) of organic powdered materials using pressed pellets in the mid-infrared spectral range is introduced that combines variable angle spectroscopic ellipsometry and transmission spectroscopy. The approach is applied to anhydrous lactose, in which three different forms of pellets were pressed and measured: a pure lactose pellet and a mixed lactose/potassium bromide (KBr) pellet with a large analyte percentage were used for ellipsometric measurements, and a KBr transmission pellet with only a small analyte percentage was used for transmission measurements. The transmittance data provide an initial set of oscillators and improve the spectral fitting of weak absorption features (k<0.01). Ellipsometric data for the pure and mixed pellets are then fit simultaneously to derive the final n/k values for lactose from 6000-400cm-1. An alternative method just using the ellipsometric data from the mixed pellet and the transmittance data is also presented and shows good agreement with the multi-sample analysis, providing a simpler method for powders that do not press easily into pure pellets. Finally, the derived optical constants were used to model the reflectance data, demonstrating a good match with the measured reflectance spectra if non-idealities are included.
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