Hannah R. Morris

Imaging Spectrometers for Fluorescence and Raman Microscopy: Acousto-Optic and Liquid Crystal Tunable Filters

Acousto-optic tunable filters (AOTF) and liquid crystal tunable filters (LCTF) are evaluated for their suitability as fluorescence microscopy imaging spectrometers. AOTFs are solid-state birefringent crystals that provide an electronically tunable spectral notch passband in response to an applied acoustic field. LCTFs also provide a notch passband that can be controlled by incorporating liquid crystal waveplate retarders within a Lyot birefringent filter. In this paper, spectroscopic performance and imaging quality are contrasted by evaluation of model systems. Studies include transmission imaging of standard resolution targets, multispectral fluorescence emission imaging of tagged polystyrene microspheres, and immunofluorescence imaging of neurotransmitters within rat-brainstem thin sections. In addition, the first use of LCTFs for Raman microscopy is demonstrated. Raman microscopy is a noninvasive spectral imaging technique that can provide chemically significant image contrast complementary to fluorescence microscopy without the use of stains or tags.

Liquid Crystal Tunable Filter Raman Chemical Imaging

A Lyot-type liquid crystal tunable filter (LCTF) suitable for high-definition Raman chemical imaging has been developed. The LCTF has been incorporated into an efficient Raman imaging system that provides significant performance advantages relative to any previous approach to Raman microscopy. The LCTF and associated optical path is physically compact, which accommodates integration of the LCTF within an infinity-corrected optical microscope. The LCTF simultaneously provides diffraction-limited spatial resolution and 7.6-cm-1 spectral bandpass across the full free spectral range of the imaging spectrometer. The LCTF Raman microscope successfully integrates, in a facile manner, the utility of optical microscopy and the analytical capabilities of Raman spectroscopy. In this paper the LCTF Raman imaging system is described in detail, as well as results of initial studies of polymer and corrosion product model systems.

Hydrolysis of the Amorphous Cellulose in Cotton-Based Paper

Hydrolysis of cellulose in Whatman no. 42 cotton-based paper was studied using gel permeation chromatography (GPC), electrospray ionization-mass spectrometry (ESI-MS), and uniaxial tensile testing to understand the course and kinetics of the reaction. GPC results suggested that scission reactions passed through three stages. Additionally, the evolution of soluble oligomers in the ESI-MS data and the steady course of strength loss showed that the hydrolysis reaction occurred at a constant rate. These findings are explained with a more detailed description of the cellulose hydrolysis, which includes multiple chain scissions on amorphous segments. The breaks occur with increasing frequency near the ends of amorphous segments, where chains protrude from crystalline domains. Oligomers unattached to crystalline domains are eventually created. Late-stage reactions near the ends of amorphous segments produce a kinetic behavior that falsely suggests that hydrolysis had ceased. Monte Carlo simulations of cellulose degradation corroborated the experimental findings.

Solid dosage form analysis by near infrared spectroscopy: comparison of reflectance and transmittance measurements including the determination of effective sample mass

Investigations into the near infrared analysis of intact tablets allow comparison of the reflectance and transmittance methodologies. The studies involved estimation of the effective sample mass and the effect of sample inhomogeneities. An empirical method for determining effective sample mass used tablets of varying thickness and constant compression force. Tablets containing a range of cimetidine concentrations were used to study the effect of drug concentration on effective sample mass as a function of wavelength of observation. Effective sampling depth was between 1.9 and 2.7 mm for reflectance measurements. With transmittance measurements, signal was completely attenuated when tablet thickness was greater than 3.4–4.9 mm, depending on the wavelength of observation. Multi-layered tablets provided a means of probing the effect of sample inhomogeneity. Both reflectance and transmittance measurements may be sensitive to sample inhomogeneity. Transmittance measurements are sensitive to pathlength variations.

Minimally invasive monitoring of cellulose degradation by desorption electrospray ionization and laser ablation electrospray ionization mass spectrometry.

Minimally invasive desorption electrospray ionization-mass spectrometry (DESI-MS) and laser ablation electrospray ionization-MS (LAESI-MS) were used to look for soluble cellulose degradation products produced by accelerated aging in unsized cotton paper. Soluble extracts from papers aged 144 to 26,856 hours were first analyzed in solution using traditional electrospray ionization-MS (ESI-MS). Results were compared to those from direct analysis of condensed phase degradation products extracted from the absorbent paper substrate using DESI-MS and LAESI-MS. ESI-MS results showed evidence of oligosaccharide degradation products ranging from cellobiose to cellononaose; using DESI-MS and LAESI-MS, products from cellobiose to cellodecaose and glucose to cellooctaose, respectively, were observed. As degradation proceeded, increased quantities of both low and high molecular weight oligosaccharides were observed. The analytical approaches developed in the control study were applied for the detection of degradation products in two naturally-aged books dating from the 19th century, both made from cotton and linen. Oligosaccharides ranging from glucose to cellopentaose were observed.

EVALUATION OF APPEARANCE AND FADING OF DAYLIGHT FLUORESCENT WATERCOLORS

Abstract Twelve daylight fluorescent watercolors from the Dr. Ph. Martin Radiant Concentrated Water Color line of products were studied to evaluate their appearance and light sensitivity under various lighting conditions—high and low correlated-color-temperature sources, simulated daylight sources with ultraviolet light both included and excluded, and black light. The appearance tests showed that all watercolors experienced a significant reduction in fluorescent emission when a low correlated-color-temperature light source was used compared to a high correlated-color-temperature source. Only a slight change in appearance was experienced by five of the watercolors studied when UV wavelengths were excluded from a simulated daylight source, while others experienced no change at all. Nine of the watercolors showed a significant reduction in fading rate during prolonged exposure to simulated daylight from which the UV content was excluded. Black light exposure produced fading patterns similar to those found with simulated daylight exposure. However, some of the watercolors were relatively stable when exposed to black light compared with their fading rates when exposed to simulated daylight. The conservation issues related to fluorescent materials, including difficulties with matching color and inferring original appearance, were also examined. Six of the 12 watercolors showed promising results for the possibility of using dilutions of the original fluorescent material to match faded versions of that watercolor.

The Light Bleaching of Discolored Films of an Acrylic Artists' Medium

Abstract Films of Liquitex gloss medium on gesso that had been discolored either by dark storage of the original product or from addition of colored matter extracted from linen canvas were bleached by brief exposure to high-intensity visible light. After 20 hours of exposure to a UV-filtered xenon lamp, an exposure dose equivalent to four days under UV-filtered daylight fluorescent lights, the intrinsic yellow color of the naturally aged films (initial Yellow Index of 23) was found to bleach significantly (about 40% of the original color remained after bleaching). For films that were discolored from material extracted from linen canvas (initial Yellow Index of 37), only a small fraction of the yellow discoloration was bleached in the exposure (about 70% of the original color remained after bleaching). The bleaching of the discolored films was not linear with light dose, and during the exposure of both the intrinsically discolored and canvas-discolored films the color decreased rapidly at first and more slowly as the bleaching proceeded. In subsequent dark storage of the bleached films, the original levels of discoloration returned. However, when bleached films were subsequently exposed to periodic, moderate light levels simulating gallery conditions, the films retained much of the improved appearance that had been gained from the bleaching treatment.

Assessing the Risks of Alkaline Damage During Deacidification Treatments of Oxidized Paper

Abstract The sensitivity of oxidized cellulose to damaging beta-elimination reactions was studied to define safe pH conditions and treatment times beyond which damage from alkaline treatment may occur; whether damage caused by alkaline treatment was affected by the type or amount of oxidation was also investigated. Cotton paper was oxidized using UV-A radiation, a hydrogen peroxide solution, or a sodium metaperiodate solution. Following oxidation, samples were soaked at room temperature in carbon-dioxide–free calcium hydroxide baths, at pH 8.5, 10.0, or 12.5, for 20 or 60 minutes. Changes to the molecular weight of the cellulose as a function of the amount of oxidation, treatment bath pH, and treatment time, were monitored to understand which conditions cause degradation to occur. The UV-A and hydrogen peroxide oxidized samples, simulating naturally oxidized papers, showed minimal sensitivity to beta-elimination reactions in baths up to pH 10.0; slight degradation was observed during treatment at pH 12.5. By contrast, sodium metaperiodate oxidized samples, which did not mimic naturally oxidized papers, were markedly degraded in pH 10.0 and 12.5 baths, with more degradation occurring at higher pH. For the oxidized papers found to be sensitive to beta-elimination reactions, longer treatment times led to more degradation.

Silver Nanoparticle Films as Sulfide Gas Sensors in Oddy Tests

The preparation and performance of a silver nanoparticle-based sensor for use in Oddy tests are reported. A suspension of spherical silver nanoparticles (Ag NPs) (mean diameter of 30 nm, absorption of surface plasmon resonance (SPR) at 428 nm) in methanol was synthesized and the Ag NPs were self-assembled into monolayer films on glass slides, using polyethylenimine as a linking agent. UV-vis spectrophotometry was employed to measure the SPR intensity of the Ag NP films in order to evaluate the extent of reaction. It was observed that the Ag NP films were quite stable under Oddy test conditions in a blank test, after a brief alteration of the spectrum due to particle dispersal, with no significant decrease in the SPR intensity after 1.5 months at 60°C and 100% RH. The sensitivity of Ag NP films to sulfide gases emitted from a test wool fabric in the Oddy test was investigated. UV-vis spectra taken after the Oddy tests showed the disappearance of the Ag NP SPR peak and the growth of the UV absorption due to Ag 2 S. Elemental analysis with energy dispersive x-ray spectroscopy confirmed that sulfur had been incorporated into the Ag NP film. Ag NP assemblies of lower NP density were created that indicated the presence of sulfide gases prior to significant tarnishing of a Ag foil. The results demonstrate that the Ag NP films can be used as sensitive, quantitative optical sensors to replace Ag foils in the Oddy test system.

“Virtual Fading” of Art Objects: Simulating the Future Fading of Artifacts by Visualizing Micro-Fading Test Results

Abstract The authors examine the importance of visualizing the color changes of colorants exposed to visible light, and use this information to create a “virtually faded” art object. The color information collected during lightfastness measurement (as CIE L*a*b* values) with a micro-fading tester can be transformed to RGB values, and color swatches representing the original and faded colors displayed on a computer monitor. Such demonstration of the appearance changes can help to visualize the degree of color change and the precise nature of that change, which may not be simple lightening of the color but could also include hue or chroma changes. Judging the visual impact of a light-induced color change requires viewing the altered color in its context within a particular image. Digital renderings of paintings were created with Adobe Photoshop and Matlab, using a number of fugitive colors, the fading characteristics of which had been recorded with prolonged microfading tests of 100–1920 minutes in duration depending on the light dosage required for the color to no longer be changing. Based on this information describing the appearance of each of the colors as it changes with light exposure, simulations of the “virtually faded” painting were generated, representing the image appearance with incremental light exposure. Such virtually faded simulations can offer insight into the severity of particular light-induced color changes, allow targeting of crucial color areas that might warrant tracking in a color monitoring program, and inform discussions about the “end of exhibition life” of an object and appropriate exhibition rotation policies.