Ultraviolet Absorption Coefficient Research Articles

Spectral characteristics of dissolved organic matter in Plateau Lakes: Identifying eutrophication indicators in Southwest China

Dissolved organic matter (DOM) acts as a chemical intermediary between terrestrial and lacustrine ecosystems and significantly affects the structure and function of lakes. The trophic state of lakes, driven by terrestrial input and phytoplankton biomass, alters the optical properties of DOM. From November 2018 to July 2019, we collected 119 water samples from the Erhai watershed and analyzed them using UV–Vis and EEM-PARAFAC to study the optical properties of DOM in relation to the trophic conditions. Our result indicated that the tyrosine-like protein (C1), tryptophan-like protein (C2), and humic-like compounds (C3) were among the mostly autochthonous components of the DOM. The percentage of the C3 was higher in eutrophic lakes than in mesotrophic and light-eutrophic lakes. The ultraviolet absorption coefficients at 254 nm (aCDOM(254)) and fluorescence intensity at 355 nm (Fn(355)) increased significantly (p < 0.01) with an increased trophic state. Our findings indicate that the influence of nutrients and environmental factors (such as pH and water temperature) on DOM varies with the trophic state. The development of novel predictive models for trophic state assessment was largely based on the significant correlations between TSI and aCDOM(254) (R2 = 0.762, p < 0.01) and Fn(355) (R2 = 0.705, p < 0.01). This neural network model facilitates the creation of a novel fast assessment tool by highlighting the connection between DOM features and the trophic state index. By enabling swift experimental measurements, this model offers a high-resolution monitoring solution for tracking the eutrophication of plateau lakes and rivers.

Assessing spatiotemporal variability in the concentration and composition of dissolved organic matter and its impact on iron solubility in tropical freshwater systems through a machine learning approach

Dissolved organic matter (DOM) plays important roles not only in maintaining the productivity and functioning of aquatic ecosystems but also in the global carbon cycle, although the sources and biogeochemical functions of terrestrially derived DOM have not been fully elucidated, particularly in the tropics and subtropics. This study aimed to evaluate the factors influencing spatiotemporal variability in (i) the concentration and composition of DOM, including dissolved organic carbon (DOC), ultraviolet absorption coefficient at 254-nm wavelength (a254), and components identified by fluorescence excitation–emission matrix coupled with parallel factor analysis (EEM-PARAFAC), and (ii) the concentration of dissolved iron (DFe) across freshwater systems (rivers, forested streams, and dam reservoirs) on a tropical island (Ishigaki Island, Japan) based on the results of water quality monitoring at 2-month intervals over a 2-year period. Random forests (RF) machine learning algorithm was employed, with the catchment characteristics (land use, soil type) and water temperature as the predictor variables for DOM and the composition of DOM (EEM-PARAFAC components) and hydrochemistry (water temperature, pH, and concentrations of divalent cations) as the predictor variables for DFe. The RF models for DOC, a254, and three humic-like components exhibited excellent predictive performance, indicating that these DOM properties are not only seasonally variable but also strongly influenced by the compositions of land uses and soil types in the upstream watershed. Poorly drained riparian lowland soil (Gleyic Fluvisols) was identified as the most important catchment parameter that positively influences these DOM variables. The RF model also explained a large portion of the variation in DFe, while terrestrial humic-like components were the most important parameters, emphasizing their significance as organic ligands for iron. These results improve our understanding of the impacts of terrestrial DOM and iron loadings on tropical and subtropical coastal ecosystems as well as on regional and global carbon budgets.

Tracking the early signals of crude oil in seawater and plankton after a major oil spill in the Red Sea

Understanding the immediate impacts of oil spills is essential to recognizing their long-term consequences on the marine environment. In this study, we traced the early (within one week) signals of crude oil in seawater and plankton after a major oil spill in October 2019 in the Red Sea. At the time of sampling, the plume had moved eastward, but we detected significant signs of incorporation of oil carbon into the dissolved organic carbon pool, resulting in a 10–20% increase in the ultraviolet (UV) absorption coefficient (a254) of chromophoric dissolved organic matter (CDOM), elevated oil fluorescence emissions, and depletion of the carbon isotope composition (δ13C) of the seawater. The abundance of the picophytoplankton Synechococcus was not affected, but the proportion of low nucleic acid (LNA) bacteria was significantly higher. Moreover, specific bacterial genera (Alcanivorax, Salinisphaera, and Oleibacter) were enriched in the seawater microbiome. Metagenome-assembled genomes (MAGs) suggested that such bacteria presented pathways for growing on oil hydrocarbons. Traces of polycyclic aromatic hydrocarbons (PAHs) were also detected in zooplankton tissues, revealing the rapid entry of oil pollutants into the pelagic food web. Our study emphasizes the early signs of short-lived spills as an important aspect of the prediction of long-term impacts of marine oil spills.

Impact of M Dwarfs Ultraviolet Radiation on Prebiotic Chemistry: The Case of Adenine.

To date, several exoplanets have been found to orbit within the habitable zone of main sequence M stars (M dwarfs). These stars exhibit different levels of chromospheric activity that produces ultraviolet (UV) radiation. UV may be harmful to life, but it can also trigger reactions of prebiotic importance on the surface of a potentially habitable planet (PHP). We created a code to obtain the adenine yield for a known adenine synthesis route from diaminomaleonitrile (DAMN). We used computational methods to calculate the reaction coefficient rates (photolysis rate J and rate constant K) for the intermediate molecules DAMN, diaminofumaronitrile (DAFN), and 4-aminoimidazole-5-carbonitrile (AICN) of the adenine synthesis route. We used stellar UV sources and a mercury lamp to compare the theoretical results with experiments performed with lamps. The surface UV flux of planets in the habitable zone of two active M dwarfs (Proxima Centauri and AD Leonis) and the prebiotic Earth was calculated using the photochemical model ATMOS, considering a CO2-N2-H2O atmosphere. We obtained UV absorption coefficients for DAMN and DAFN and thermodynamic parameters that are useful for prebiotic chemistry studies. According to our results, experiments using UV lamps may underestimate the photolysis production of molecules of prebiotic importance. Our results indicate that photolysis reactions are fast with a yield of 50% of AICN in 10 s for the young Sun and ∼1 h for Proxima Centauri b. Planets around active M dwarfs may provide the most favorable environment for UV-mediated production of compounds relevant to the origins of life. The kinetic reaction AICN + HCN  adenine is the bottleneck of the pathway with reaction rates <10-22 L/(mol·s).

Effects of reducing atmosphere and iron content on UV transmission property of alkali-silicate and alkali-borosilicate glasses

The effects of the reducing atmosphere during glass melting and the iron content in glass on the ultraviolet (UV) transmission property of borosilicate and silicate glasses were studied. The reducing atmosphere is controlled by the amount of citric acid in each glass batch. The effects of reducing atmosphere on the glass UV transmittance were examined in terms of the distribution of trigonal boron (BO3) and tetrahedral boron (BO4) units and iron reduction index (IRI). Our study reveals that the reducing atmospheres not only had a significant effect on the boron-oxygen structure of alkali-borosilicate glass, but also on the UV transmittance. In addition, IRI has a significant effect on the UV transmittance at a wavelength of 254 nm, with a relative change up to 461%. The iron oxidation state was found to affect the UV absorption coefficient of ferric ion (Fe(III) has about four times greater than that of ferrous ion (Fe(II)) at 254 nm.

Bio- and photo-lability of dissolved organic matter in the Pearl River (Zhujiang) estuary

We investigated the bio- and photo-lability of dissolved organic matter (DOM) from the head, mixing zone, and mouth of the Pearl River estuary. At all three sites, bio- and photo-refractory dissolved organic carbon (DOC) and biorefractory chromophoric DOM (CDOM) dominated over the corresponding bio- and photo-labile constituents, while photolabile CDOM dominated over photo-refractory CDOM. Relative to the mixing-zone and mouth waters, the headwater was enriched with bio- and photo-labile DOC and photolabile CDOM and depleted with biolabile CDOM. Biolabile DOC was richer than photolabile DOC in the headwater, while photolabile CDOM was richer than biolabile CDOM at all three sites. Pre-biotransformation inhibited, stimulated, or had little impact on DOM photodegradation, depending on site. Ultra-violet absorption coefficients are indicators of bio- and photo-refractory DOC. The relative proportions of transparent and chromophoric DOM control the turnover of biolabile DOC and the effect of pre-biotransformation on DOM photodegradation.

Two-dimensional wide-bandgap GeSe2 vertical ultraviolet photodetectors with high responsivity and ultrafast response speed.

Germanium selenide (GeSe2), as a typical member of 2D wide bandgap semiconductors (WBSs), shows great potential in ultraviolet (UV) optoelectronics due to its excellent flexibility, superior environmental stability, competitive UV absorption coefficient, and significant spectral selectivity. However, the GeSe2-based UV photodetector suffers from high operation voltages and low photocurrent, which prevents its practical imaging applications. In this work, we report an elevated photocurrent generation in a vertical stacking graphene/GeSe2/graphene heterostructure with low operation voltage and low power consumption. Efficient collection of photoexcited carriers in GeSe2 through graphene electrodes results in outstanding UV detection properties, including a pronounced responsivity of 37.1 A W-1, a specific detectivity of 8.83 × 1011 Jones, and an ultrahigh on/off ratio (∼105) at 355 nm. In addition, building a Schottky barrier between GeSe2 and graphene and reducing the channel length can increase the photoresponse speed to ∼300 μs. These accomplishments set the stage for future optoelectronic applications of vertical 2D WBS heterostructure UV photodetectors.

Kinetic and Continuum Modeling of High-Temperature Oxygen and Nitrogen Binary Mixtures

The present paper provides a comprehensive comparative analysis of thermochemistry models of various fidelity levels developed in leading research groups around the world. Fully kinetic, hybrid kinetic–continuum, and fully continuum approaches are applied to analyze parameters of hypersonic flows starting from the revision of single-temperature rate constants up to the application in 1-D postshock conditions. Comparison of state-specific and two-temperature approaches shows there are very significant and often qualitative differences in the time-dependent nonequilibrium reaction rates and their ratio to the corresponding single-temperature rates. A major impact of the vibration–dissociation coupling on the temporal relaxation of gas properties is shown. For instance, the legacy Park’s model has a strongly nonlinear behavior of nonequilibrium reaction rate with vibrational temperature, while a nearly linear shape exists for all state-specific approaches. Analysis of vibrational level populations in the nonequilibrium region shows a profound impact of the numerical approach and the model on the population ratios, and thus vibrational temperatures inferred from such ratios. The difference in the ultraviolet absorption coefficients, calculated by a temperature-based spectral code using vibrational populations from state-specific and kinetic approaches, is found to exceed an order of magnitude.

Laser-induced fluorescence and its effect on the damage resistance of fluoride-containing phosphate-based glasses

A series of multi-component fluoride-containing phosphate-based glasses prepared in a reducing atmosphere showed improved resistance to high-energy ultraviolet (UV) laser-induced damage and strong laser-induced fluorescence (LIF) within the glass bulk. The UV optical absorption, photoluminescence, and fluorescence decay properties of these glasses were investigated to explore the defect-related LIF mechanism and its underlying effect on the glasses’ laser-induced damage threshold (LIDT). Seven laser wavelengths ranging from 253 nm to 532 nm were used to excite the LIF, and two characteristic fluorescence bands peaking at approximately 414 nm and 780–800 nm occurred in all three types of glasses. The LIF band at 414 nm was attributed to PO3-EC defects in the second harmonic frequency (2ω) absorptive glass and third harmonic frequency (3ω) transparent glass, but Fe2+ ions in the fundamental frequency (1ω) absorptive glass. A later fluorescence band at 780–800 nm occurred due to POHC defects in the 2ω absorptive and 3ω transparent glasses and Fe3+ ions in the 1ω absorptive glass. A detailed study on the dynamic decay processes of two additional dominant fluorescence peaks at 450 nm and 780 nm under 266 nm excitation revealed the potential effect of LIF on LIDT improvement. The relatively longer LIF lifetime, higher LIF intensity, and larger LIF peak area corresponded with and contributed to a higher LIDT, especially in the 3ω transparent glass with a low UV absorption coefficient. This study provides strong evidence for the prior hypothesis between strong LIF and LIDT.

Large-Scale Ultrathin 2D Wide-Bandgap BiOBr Nanoflakes for Gate-Controlled Deep-Ultraviolet Phototransistors.

Ternary two-dimensional (2D) semiconductors with controllable wide bandgap, high ultraviolet (UV) absorption coefficient, and critical tuning freedom degree of stoichiometry variation have a great application prospect for UV detection. However, as-reported ternary 2D semiconductors often possess a bandgap below 3.0 eV, which must be further enlarged to achieve comprehensively improved UV, especially deep-UV (DUV), detection capacity. Herein, sub-one-unit-cell 2D monolayer BiOBr nanoflakes (≈0.57 nm) with a large size of 70 µm are synthesized for high-performance DUV detection due to the large bandgap of 3.69 eV. Phototransistors based on the 2D ultrathin BiOBr nanoflakes deliver remarkable DUV detection performance including ultrahigh photoresponsivity (Rλ , 12739.13 A W-1 ), ultrahigh external quantum efficiency (EQE, 6.46 × 106 %), and excellent detectivity (D*, 8.37 × 1012 Jones) at 245 nm with a gate voltage (Vg ) of 35 V attributed to the photogating effects. The ultrafast response (τrise = 102 µs) can be achieved by utilizing photoconduction effects at Vg of -40 V. The combination of photocurrent generation mechanisms for BiOBr-based phototransistors controlled by Vg can pave a way for designing novel 2D optoelectronic materials to achieve optimal device performance.

Structural and optical investigation of pure and Al doped ZnO annealed at different temperatures

The structural and optical properties of ZnO and Zn0.9Al0.1O (AlZnO) samples annealed in air at different temperatures of (400–900 °C) for 12 h are investigated. It is found that cell parameters, average crystalline diameter and average grain size are affected by both Al and annealing. The ultra violet (UV) absorption coefficient (α) is increased by Al and can be represented by the relation ;(α (AlZnO)~2α (ZnO), but it is generally decreased by annealing. Two different types of band gap are obtained for each sample in which their values are higher for AlZnO than that of ZnO. The first band gap is called fundamental energy gap (Egh) and its value above 3 eV, while the second is called optical energy gap (EgL) and its value below 2.75 eV. In addition, the values of EgL (ZnO), Egh (AlZnO) and EgL (AlZnO) are increased by annealing up to 700 °C, followed by a decrease at 900 °C, while the values of Egh (ZnO) are increased by annealing up to 900 °C. Furthermore, a unique energy gap of 3.33 eV is obtained at 312 °C for ZnO and AlZnO. On the other hand, the lattice dielectric constant (eL), free carrier concentration (N/m*), dielectric loss (tan δ), optical conductivity (σopt) and electrical conductivity (σele) are obtained for both series. It is found that eL is increased by annealing up to 900 °C for both series, while (N/m*) is increased by annealing for ZnO, but it is decreased for AlZnO. But their values are higher for ZnO than that of AlZnO. Further, the value of eL (11.26) of the as-prepared ZnO is nearly similar to the value of eL (11.76) for AlZnO at 900 °C. Although tan δ is similar for both series, it is decreased by annealing up to 900 °C for ZnO, and increased for AlZnO up to 700 °C followed by decrease at 900 °C. Similar behavior is obtained for σopt, while the vice is versa for σele. The obtained results are interpreted in terms of the variance of carrier concentration, oxygen vacancies and interatomic spacing against annealing temperature and Al content. The present results confirm the semiconducting nature as well as assure that the high photo-response of the investigated samples.

Structural variation of coloured dissolved organic matter during summer and winter seasons in a tropical estuary: A case study.

The diurnal variations in water quality and optical properties of organic matter were studied in the Gautami-Godavari estuary during two contrasting seasons. Dissolved inorganic nitrogen (DIN) and silicate showed similar patterns with the tide during summer, whereas in winter contrasting trends were noticed. Three-folds higher N to P ratio was recorded in winter than in summer. The spectral slope ratio (SR) and specific ultra violet absorption coefficient (SUVA) peaked during summer (1.28 ± 0.09 and 3.95 ± 0.2) followed by winter (1.10 ± 0.18 and 1.91 ± 0.35). The parallel factor (PARAFAC) analysis of excitation emission matrix (EEM) fluorescence spectra was extracted three humic (C1, C2, C3) and one protein-like (tryptophan (C4)) fluorophore components. Humic like fluorophores inversely correlated with the tide in both the seasons, due to influence of seawater. In summer, the Chlorophyll a (Chl a) and dissolved organic carbon (DOC) showed positive correlations with humic like and C4 fluorophores, suggesting the insitu organic matter production.

Enhance the Optical Properties of the Synthesis PEG/Graphene- Based Nanocomposite films using GO nanosheets

Graphene is one of the most interesting and attracting nanofillers. The investigation focused on the effect of two significant factors using graphene nanosheets and polymer molecular weights (Mw) on the optical properties of polymer graphene-based nanocomposites. New sonication-mixing-aquatic methods were applied using the three Mw, 4k, 8k and 20k of polyethylene glycol (PEG), as a polymer model, with low loading ratio graphene oxide nanosheets (GONSs) to synthesise the nanocomposites. Fine distribution and good homogeneity of GONSs were successfully presented in the PEG matrix as examined applying the optical microscope (OM). The results presented an enhance in the most optical properties, which shows significantly in the ultraviolet region (∼300 nm in wavelength), such as, absorbance, absorption coefficient, real and imaginary dielectric constants up to 71%, 355%, 37% and 41% after increasing the Mw, except the allowed and forbidden indirect optical energy gap were reduced to 18% and 29%, respectively. Moreover, the contribution of GO with Mw of PEG exhibited a notable improvement of the optical properties up to 100%, 440%, 48% and 61%, whereas the allowed and forbidden indirect optical energy gap were reduced to 43% and 86%. These results illustrated significant roles of the effect of MW and GO in the optical properties that give rise to better photovoltaic performances of heterojunction solar cells and may use as filters and antireflection coating in the substantial applications.

A predictive artificial neural network model as a simulator of the extra virgin olive oil elaboration process

Nine neural models were created to predict the characteristics of the extra virgin olive oil developed as a quality objective and by-products. These models are designed with the help of data of process variables from physical sensors such as temperature, flows, current intensity, etc. and physicochemical ones like the near infrared spectrum of the olive mass. The results obtained for the extractability of the process (fatty content and moisture) were highly significant correlations (r2≥0.90) and with similar prediction errors (root mean of squared error of prediction) relative to other analysis techniques which measure the by-product directly. For prediction the models gave correlations above 0.94, with the exception of ultraviolet absorption coefficients (0.72–0.84), with small prediction errors and the quality indicator relative error range with values above the optimal 10. The set of developed artificial neural networks models constitute the basis of the global ‘simulator’ tool of the extra virgin olive oil process. This simulator can perform a predictive optimization of the process to pre-adjust the process variables according to the goals marked in productivity or quality, from an near infrared spectral database or by real-time scanning. This simulator could be integrated into a control system that performs the function of a ‘virtual plant’ that allows the said system to adjust in real time the appropriate variables to meet the objectives.

The removal kinetics of dissolved organic matter and the optical clarity of groundwater

Concentrations of dissolved organic matter (DOM) and ultraviolet/visible light absorbance decrease systematically as groundwater moves through the unsaturated zones overlying aquifers and along flowpaths within aquifers. These changes occur over distances of tens of meters (m) implying rapid removal kinetics of the chromophoric DOM that imparts color to groundwater. A one-compartment input-output model was used to derive a differential equation describing the removal of DOM from the dissolved phase due to the combined effects of biodegradation and sorption. The general solution to the equation was parameterized using a 2-year record of dissolved organic carbon (DOC) concentration changes in groundwater at a long-term observation well. Estimated rates of DOC loss were rapid and ranged from 0.093 to 0.21 micromoles per liter per day (μM d−1), and rate constants for DOC removal ranged from 0.0021 to 0.011 per day (d−1). Applying these removal rate constants to an advective-dispersion model illustrates substantial depletion of DOC over flow-path distances of 200 m or less and in timeframes of 2 years or less. These results explain the low to moderate DOC concentrations (20–75 μM; 0.26–1 mg L−1) and ultraviolet absorption coefficient values (a 254 < 5 m−1) observed in groundwater produced from 59 wells tapping eight different aquifer systems of the United States. The nearly uniform optical clarity of groundwater, therefore, results from similarly rapid DOM-removal kinetics exhibited by geologically and hydrologically dissimilar aquifers.

Synthesis and properties of feruloyl corn bran arabinoxylan esters.

To enhance the antioxidant activity and UV absorption coefficient of corn bran arabinoxylan (CAX), ferulic acid (FA) with this physiological activity is used to modify CAX. Corn bran arabinoxylan was extracted from corn bran according to alkaline hydrogen peroxide (AHP) method. FA was covalently linked to CAX by esterification in a two-step feasible synthesis to generate ferulic acid arabinoxylan esters (FA-CAX). The structure and molecular weight of FA-CAX were characterized by NMR and HPSEC, the degrees of substitution (DS) was determined by HPLC, and the ultraviolet (UV) coefficient of FA-CAX was tested by UV spectroscopy. The antioxidant activity of FA-CAX was investigated on the basis of 2,2'-diphenyl-1-picrylhydrazyl (DPPH) radical assay, and protecting ability of FA-CAX to UV-induced oxidative damage was tested using linolenic acid dispersion as stratum corneum lipid model. The results demonstrated that FA was attached to CAX successfully, and the inherent structure of CAX would not be broken during the process of the synthesis. FA-CAX-1 and FA-CAX-2 contained different amount of FA, with DS at 0.33 and 1.25, respectively, can absorb UV both at UVA and UVB. Moreover, FA-CAX-2 exhibited better antioxidant activity than FA-CAX-1 based on the two test methods. Ferulic acid modified CAX had significant antioxidant ability and UV absorption coefficient. And higher amount of FA leads to higher antioxidant activity and stronger UV absorption and stability. With increasing amount of FA attached to CAX, the antioxidant activities were better and the UV absorption was stronger and more durable.

The effect of hydration on the UV absorption coefficient of intact melanosomes

The physical properties of melanosomes have been shown to depend on water content. Herein, the ultraviolet absorption coefficient at λ = 244 nm for intact bovine choroidal melanosomes is determined from photoemission electron microscopy images recorded as a function of vacuum exposure. The dehydration of the melanosome under ultra-high vacuum manifests itself by a decrease in the absorption coefficient to about 60% of its initial value, and a concomitant increase in its image brightness. This change in the absorption of the melanosome is consistent with the influence of solvent polarity on the UV absorption coefficient of model systems for the pigment eumelanin, the predominant UV absorber contained in the choroid melanosomes.

Effect of different coagulants on the ultraviolet light intensity attenuation

Abstract The effect of different coagulants added to deionized water on ultraviolet (UV) light intensity at the wavelength of 253.7 nm was investigated in this study. It was demonstrated that iron salts had a greater negative effect on the UV transmittance (UVT) of the solution, but aluminum salts had little effect on the UVT of the solution except polyaluminium chloride (PAC). Polyferric sulfate (PFS) and PAC, as highly effective inorganic-polymer coagulants, had a greater negative effect on the UVT of the solution than the related metal salts. UV light attenuation of iron salts and aluminum salts existing in the solution were in good agreement with the Lambert-Beer law, except PAC and PFS. The UV absorption coefficients of different coagulants were obtained and compared below: PFS>FeCl3>Fe2(SO4)3>FeSO4>PAC>AlCl3>Al2(SO4)3. If the coagulation pretreatment with PFS was added before UV disinfection of water, it would lead to stronger UV light attenuation than that with PAC. It means that more UV light can ...

Liquid-immersion laser micromachining of GaN grown on sapphire

Liquid-immersion nanosecond-pulsed laser micromachining is introduced as an efficient way for device isolation and rapid prototyping on GaN-on-sapphire wafer. Using deionized water as an ambient medium for laser micromachining, smooth trenches that are free from redeposition can be formed in the GaN layer. Coupled with the large difference between the ablation thresholds and ultraviolet absorption coefficients of GaN and sapphire, the GaN/sapphire interface can be left undamaged after the ablation process. This technique overcomes the limitation of heat accumulation in nanosecond-pulse regime, and offers a cost-effective alternative to ultrashort-pulse laser micromachining. In this report, the advantages offered by liquid immersion are elucidated in terms of improved heat conduction, increased plasma-induced recoil pressure due to water confinement, weakened plasma shielding effect in water, and the collapse of cavitation bubbles. Simulation results show that the reduced fluctuation of temperature profile over time in water could be correlated with the reduced redeposition of Ga from thermal decomposition at the trench sidewalls.

Ultraviolet Absorption Coefficients of Melanosomes Containing Eumelanin As Related to the Relative Content of DHI and DHICA

Central to understanding the photochemical function(s) of melano- somes is the determination of their absorption properties and an understanding of how the absorption varies with the molecular composition of melanin. Herein, the absorption coefficients for melanosomes containing predominantly eumelanin, a polymeric pigment derived from the molecular precursors 5,6-dihydroxyindole (DHI)and5,6-dihydroxyindole-2-carboxylicacid(DHICA),arereportedfor λ=244nm. The absorption coefficient varies with the DHICA/DHI ratio, determined from chemical degradationanalyses.WithincreasingDHICAcontent,theabsorptioncoefficientof the melanosome increases. This observation is consistent with reported extinction coefficients, which reveal that at 244 nm, the extinction coefficient of DHICA is a factor of ∼2.1 greater than that of DHI. The melanosome absorption coefficients are compared to predicted values based on a linear combination of the absorption of the constituent monomers.